externals: add `attrs` for new `jsonschema`

Updating `jsonschema` to 3.2.0 requires `attrs`. Add it to externals.

16 files changed, 4928 insertions, 0 deletions

diff --git a/COPYRIGHT b/COPYRIGHT
index 9dfc1ee081..c0550caf95 100644
--- a/COPYRIGHT
+++ b/COPYRIGHT
@@ -38,6 +38,10 @@ PackageName: argparse
 PackageHomePage: https://pypi.python.org/pypi/argparse
 PackageLicenseDeclared: Python-2.0
 
+PackageName: attrs
+PackageHomePage: https://github.com/python-attrs/attrs
+PackageLicenseDeclared: MIT
+
 PackageName: ctest_log_parser
 PackageHomePage: https://github.com/Kitware/CMake
 PackageLicenseDeclared: BSD-3-Clause
diff --git a/lib/spack/external/__init__.py b/lib/spack/external/__init__.py
index aa615ba4f0..833aa47b8d 100644
--- a/lib/spack/external/__init__.py
+++ b/lib/spack/external/__init__.py
@@ -24,6 +24,13 @@ argparse
   vendored copy ever needs to be updated again:
   https://github.com/spack/spack/pull/6786/commits/dfcef577b77249106ea4e4c69a6cd9e64fa6c418
 
+attrs
+----------------
+
+* Homepage: https://github.com/python-attrs/attrs
+* Usage: Needed by jsonschema.
+* Version: 21.2.0 (83d3cd70f90a3f4d19ee8b508e58d1c58821c0ad)
+
 ctest_log_parser
 ----------------
 
diff --git a/lib/spack/external/attr/LICENSE b/lib/spack/external/attr/LICENSE
new file mode 100644
index 0000000000..7ae3df9309
--- /dev/null
+++ b/lib/spack/external/attr/LICENSE
@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2015 Hynek Schlawack
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/lib/spack/external/attr/__init__.py b/lib/spack/external/attr/__init__.py
new file mode 100644
index 0000000000..b1ce7fe248
--- /dev/null
+++ b/lib/spack/external/attr/__init__.py
@@ -0,0 +1,78 @@
+from __future__ import absolute_import, division, print_function
+
+import sys
+
+from functools import partial
+
+from . import converters, exceptions, filters, setters, validators
+from ._cmp import cmp_using
+from ._config import get_run_validators, set_run_validators
+from ._funcs import asdict, assoc, astuple, evolve, has, resolve_types
+from ._make import (
+    NOTHING,
+    Attribute,
+    Factory,
+    attrib,
+    attrs,
+    fields,
+    fields_dict,
+    make_class,
+    validate,
+)
+from ._version_info import VersionInfo
+
+
+__version__ = "21.2.0"
+__version_info__ = VersionInfo._from_version_string(__version__)
+
+__title__ = "attrs"
+__description__ = "Classes Without Boilerplate"
+__url__ = "https://www.attrs.org/"
+__uri__ = __url__
+__doc__ = __description__ + " <" + __uri__ + ">"
+
+__author__ = "Hynek Schlawack"
+__email__ = "hs@ox.cx"
+
+__license__ = "MIT"
+__copyright__ = "Copyright (c) 2015 Hynek Schlawack"
+
+
+s = attributes = attrs
+ib = attr = attrib
+dataclass = partial(attrs, auto_attribs=True)  # happy Easter ;)
+
+__all__ = [
+    "Attribute",
+    "Factory",
+    "NOTHING",
+    "asdict",
+    "assoc",
+    "astuple",
+    "attr",
+    "attrib",
+    "attributes",
+    "attrs",
+    "cmp_using",
+    "converters",
+    "evolve",
+    "exceptions",
+    "fields",
+    "fields_dict",
+    "filters",
+    "get_run_validators",
+    "has",
+    "ib",
+    "make_class",
+    "resolve_types",
+    "s",
+    "set_run_validators",
+    "setters",
+    "validate",
+    "validators",
+]
+
+if sys.version_info[:2] >= (3, 6):
+    from ._next_gen import define, field, frozen, mutable
+
+    __all__.extend((define, field, frozen, mutable))
diff --git a/lib/spack/external/attr/_cmp.py b/lib/spack/external/attr/_cmp.py
new file mode 100644
index 0000000000..b747b603f1
--- /dev/null
+++ b/lib/spack/external/attr/_cmp.py
@@ -0,0 +1,152 @@
+from __future__ import absolute_import, division, print_function
+
+import functools
+
+from ._compat import new_class
+from ._make import _make_ne
+
+
+_operation_names = {"eq": "==", "lt": "<", "le": "<=", "gt": ">", "ge": ">="}
+
+
+def cmp_using(
+    eq=None,
+    lt=None,
+    le=None,
+    gt=None,
+    ge=None,
+    require_same_type=True,
+    class_name="Comparable",
+):
+    """
+    Create a class that can be passed into `attr.ib`'s ``eq``, ``order``, and
+    ``cmp`` arguments to customize field comparison.
+
+    The resulting class will have a full set of ordering methods if
+    at least one of ``{lt, le, gt, ge}`` and ``eq``  are provided.
+
+    :param Optional[callable] eq: `callable` used to evaluate equality
+        of two objects.
+    :param Optional[callable] lt: `callable` used to evaluate whether
+        one object is less than another object.
+    :param Optional[callable] le: `callable` used to evaluate whether
+        one object is less than or equal to another object.
+    :param Optional[callable] gt: `callable` used to evaluate whether
+        one object is greater than another object.
+    :param Optional[callable] ge: `callable` used to evaluate whether
+        one object is greater than or equal to another object.
+
+    :param bool require_same_type: When `True`, equality and ordering methods
+        will return `NotImplemented` if objects are not of the same type.
+
+    :param Optional[str] class_name: Name of class. Defaults to 'Comparable'.
+
+    See `comparison` for more details.
+
+    .. versionadded:: 21.1.0
+    """
+
+    body = {
+        "__slots__": ["value"],
+        "__init__": _make_init(),
+        "_requirements": [],
+        "_is_comparable_to": _is_comparable_to,
+    }
+
+    # Add operations.
+    num_order_functions = 0
+    has_eq_function = False
+
+    if eq is not None:
+        has_eq_function = True
+        body["__eq__"] = _make_operator("eq", eq)
+        body["__ne__"] = _make_ne()
+
+    if lt is not None:
+        num_order_functions += 1
+        body["__lt__"] = _make_operator("lt", lt)
+
+    if le is not None:
+        num_order_functions += 1
+        body["__le__"] = _make_operator("le", le)
+
+    if gt is not None:
+        num_order_functions += 1
+        body["__gt__"] = _make_operator("gt", gt)
+
+    if ge is not None:
+        num_order_functions += 1
+        body["__ge__"] = _make_operator("ge", ge)
+
+    type_ = new_class(class_name, (object,), {}, lambda ns: ns.update(body))
+
+    # Add same type requirement.
+    if require_same_type:
+        type_._requirements.append(_check_same_type)
+
+    # Add total ordering if at least one operation was defined.
+    if 0 < num_order_functions < 4:
+        if not has_eq_function:
+            # functools.total_ordering requires __eq__ to be defined,
+            # so raise early error here to keep a nice stack.
+            raise ValueError(
+                "eq must be define is order to complete ordering from "
+                "lt, le, gt, ge."
+            )
+        type_ = functools.total_ordering(type_)
+
+    return type_
+
+
+def _make_init():
+    """
+    Create __init__ method.
+    """
+
+    def __init__(self, value):
+        """
+        Initialize object with *value*.
+        """
+        self.value = value
+
+    return __init__
+
+
+def _make_operator(name, func):
+    """
+    Create operator method.
+    """
+
+    def method(self, other):
+        if not self._is_comparable_to(other):
+            return NotImplemented
+
+        result = func(self.value, other.value)
+        if result is NotImplemented:
+            return NotImplemented
+
+        return result
+
+    method.__name__ = "__%s__" % (name,)
+    method.__doc__ = "Return a %s b.  Computed by attrs." % (
+        _operation_names[name],
+    )
+
+    return method
+
+
+def _is_comparable_to(self, other):
+    """
+    Check whether `other` is comparable to `self`.
+    """
+    for func in self._requirements:
+        if not func(self, other):
+            return False
+    return True
+
+
+def _check_same_type(self, other):
+    """
+    Return True if *self* and *other* are of the same type, False otherwise.
+    """
+    return other.value.__class__ is self.value.__class__
diff --git a/lib/spack/external/attr/_compat.py b/lib/spack/external/attr/_compat.py
new file mode 100644
index 0000000000..6939f338da
--- /dev/null
+++ b/lib/spack/external/attr/_compat.py
@@ -0,0 +1,242 @@
+from __future__ import absolute_import, division, print_function
+
+import platform
+import sys
+import types
+import warnings
+
+
+PY2 = sys.version_info[0] == 2
+PYPY = platform.python_implementation() == "PyPy"
+
+
+if PYPY or sys.version_info[:2] >= (3, 6):
+    ordered_dict = dict
+else:
+    from collections import OrderedDict
+
+    ordered_dict = OrderedDict
+
+
+if PY2:
+    from collections import Mapping, Sequence
+
+    from UserDict import IterableUserDict
+
+    # We 'bundle' isclass instead of using inspect as importing inspect is
+    # fairly expensive (order of 10-15 ms for a modern machine in 2016)
+    def isclass(klass):
+        return isinstance(klass, (type, types.ClassType))
+
+    def new_class(name, bases, kwds, exec_body):
+        """
+        A minimal stub of types.new_class that we need for make_class.
+        """
+        ns = {}
+        exec_body(ns)
+
+        return type(name, bases, ns)
+
+    # TYPE is used in exceptions, repr(int) is different on Python 2 and 3.
+    TYPE = "type"
+
+    def iteritems(d):
+        return d.iteritems()
+
+    # Python 2 is bereft of a read-only dict proxy, so we make one!
+    class ReadOnlyDict(IterableUserDict):
+        """
+        Best-effort read-only dict wrapper.
+        """
+
+        def __setitem__(self, key, val):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise TypeError(
+                "'mappingproxy' object does not support item assignment"
+            )
+
+        def update(self, _):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError(
+                "'mappingproxy' object has no attribute 'update'"
+            )
+
+        def __delitem__(self, _):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise TypeError(
+                "'mappingproxy' object does not support item deletion"
+            )
+
+        def clear(self):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError(
+                "'mappingproxy' object has no attribute 'clear'"
+            )
+
+        def pop(self, key, default=None):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError(
+                "'mappingproxy' object has no attribute 'pop'"
+            )
+
+        def popitem(self):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError(
+                "'mappingproxy' object has no attribute 'popitem'"
+            )
+
+        def setdefault(self, key, default=None):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError(
+                "'mappingproxy' object has no attribute 'setdefault'"
+            )
+
+        def __repr__(self):
+            # Override to be identical to the Python 3 version.
+            return "mappingproxy(" + repr(self.data) + ")"
+
+    def metadata_proxy(d):
+        res = ReadOnlyDict()
+        res.data.update(d)  # We blocked update, so we have to do it like this.
+        return res
+
+    def just_warn(*args, **kw):  # pragma: no cover
+        """
+        We only warn on Python 3 because we are not aware of any concrete
+        consequences of not setting the cell on Python 2.
+        """
+
+
+else:  # Python 3 and later.
+    from collections.abc import Mapping, Sequence  # noqa
+
+    def just_warn(*args, **kw):
+        """
+        We only warn on Python 3 because we are not aware of any concrete
+        consequences of not setting the cell on Python 2.
+        """
+        warnings.warn(
+            "Running interpreter doesn't sufficiently support code object "
+            "introspection.  Some features like bare super() or accessing "
+            "__class__ will not work with slotted classes.",
+            RuntimeWarning,
+            stacklevel=2,
+        )
+
+    def isclass(klass):
+        return isinstance(klass, type)
+
+    TYPE = "class"
+
+    def iteritems(d):
+        return d.items()
+
+    new_class = types.new_class
+
+    def metadata_proxy(d):
+        return types.MappingProxyType(dict(d))
+
+
+def make_set_closure_cell():
+    """Return a function of two arguments (cell, value) which sets
+    the value stored in the closure cell `cell` to `value`.
+    """
+    # pypy makes this easy. (It also supports the logic below, but
+    # why not do the easy/fast thing?)
+    if PYPY:
+
+        def set_closure_cell(cell, value):
+            cell.__setstate__((value,))
+
+        return set_closure_cell
+
+    # Otherwise gotta do it the hard way.
+
+    # Create a function that will set its first cellvar to `value`.
+    def set_first_cellvar_to(value):
+        x = value
+        return
+
+        # This function will be eliminated as dead code, but
+        # not before its reference to `x` forces `x` to be
+        # represented as a closure cell rather than a local.
+        def force_x_to_be_a_cell():  # pragma: no cover
+            return x
+
+    try:
+        # Extract the code object and make sure our assumptions about
+        # the closure behavior are correct.
+        if PY2:
+            co = set_first_cellvar_to.func_code
+        else:
+            co = set_first_cellvar_to.__code__
+        if co.co_cellvars != ("x",) or co.co_freevars != ():
+            raise AssertionError  # pragma: no cover
+
+        # Convert this code object to a code object that sets the
+        # function's first _freevar_ (not cellvar) to the argument.
+        if sys.version_info >= (3, 8):
+            # CPython 3.8+ has an incompatible CodeType signature
+            # (added a posonlyargcount argument) but also added
+            # CodeType.replace() to do this without counting parameters.
+            set_first_freevar_code = co.replace(
+                co_cellvars=co.co_freevars, co_freevars=co.co_cellvars
+            )
+        else:
+            args = [co.co_argcount]
+            if not PY2:
+                args.append(co.co_kwonlyargcount)
+            args.extend(
+                [
+                    co.co_nlocals,
+                    co.co_stacksize,
+                    co.co_flags,
+                    co.co_code,
+                    co.co_consts,
+                    co.co_names,
+                    co.co_varnames,
+                    co.co_filename,
+                    co.co_name,
+                    co.co_firstlineno,
+                    co.co_lnotab,
+                    # These two arguments are reversed:
+                    co.co_cellvars,
+                    co.co_freevars,
+                ]
+            )
+            set_first_freevar_code = types.CodeType(*args)
+
+        def set_closure_cell(cell, value):
+            # Create a function using the set_first_freevar_code,
+            # whose first closure cell is `cell`. Calling it will
+            # change the value of that cell.
+            setter = types.FunctionType(
+                set_first_freevar_code, {}, "setter", (), (cell,)
+            )
+            # And call it to set the cell.
+            setter(value)
+
+        # Make sure it works on this interpreter:
+        def make_func_with_cell():
+            x = None
+
+            def func():
+                return x  # pragma: no cover
+
+            return func
+
+        if PY2:
+            cell = make_func_with_cell().func_closure[0]
+        else:
+            cell = make_func_with_cell().__closure__[0]
+        set_closure_cell(cell, 100)
+        if cell.cell_contents != 100:
+            raise AssertionError  # pragma: no cover
+
+    except Exception:
+        return just_warn
+    else:
+        return set_closure_cell
+
+
+set_closure_cell = make_set_closure_cell()
diff --git a/lib/spack/external/attr/_config.py b/lib/spack/external/attr/_config.py
new file mode 100644
index 0000000000..8ec920962d
--- /dev/null
+++ b/lib/spack/external/attr/_config.py
@@ -0,0 +1,23 @@
+from __future__ import absolute_import, division, print_function
+
+
+__all__ = ["set_run_validators", "get_run_validators"]
+
+_run_validators = True
+
+
+def set_run_validators(run):
+    """
+    Set whether or not validators are run.  By default, they are run.
+    """
+    if not isinstance(run, bool):
+        raise TypeError("'run' must be bool.")
+    global _run_validators
+    _run_validators = run
+
+
+def get_run_validators():
+    """
+    Return whether or not validators are run.
+    """
+    return _run_validators
diff --git a/lib/spack/external/attr/_funcs.py b/lib/spack/external/attr/_funcs.py
new file mode 100644
index 0000000000..fda508c5c4
--- /dev/null
+++ b/lib/spack/external/attr/_funcs.py
@@ -0,0 +1,395 @@
+from __future__ import absolute_import, division, print_function
+
+import copy
+
+from ._compat import iteritems
+from ._make import NOTHING, _obj_setattr, fields
+from .exceptions import AttrsAttributeNotFoundError
+
+
+def asdict(
+    inst,
+    recurse=True,
+    filter=None,
+    dict_factory=dict,
+    retain_collection_types=False,
+    value_serializer=None,
+):
+    """
+    Return the ``attrs`` attribute values of *inst* as a dict.
+
+    Optionally recurse into other ``attrs``-decorated classes.
+
+    :param inst: Instance of an ``attrs``-decorated class.
+    :param bool recurse: Recurse into classes that are also
+        ``attrs``-decorated.
+    :param callable filter: A callable whose return code determines whether an
+        attribute or element is included (``True``) or dropped (``False``).  Is
+        called with the `attr.Attribute` as the first argument and the
+        value as the second argument.
+    :param callable dict_factory: A callable to produce dictionaries from.  For
+        example, to produce ordered dictionaries instead of normal Python
+        dictionaries, pass in ``collections.OrderedDict``.
+    :param bool retain_collection_types: Do not convert to ``list`` when
+        encountering an attribute whose type is ``tuple`` or ``set``.  Only
+        meaningful if ``recurse`` is ``True``.
+    :param Optional[callable] value_serializer: A hook that is called for every
+        attribute or dict key/value.  It receives the current instance, field
+        and value and must return the (updated) value.  The hook is run *after*
+        the optional *filter* has been applied.
+
+    :rtype: return type of *dict_factory*
+
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    ..  versionadded:: 16.0.0 *dict_factory*
+    ..  versionadded:: 16.1.0 *retain_collection_types*
+    ..  versionadded:: 20.3.0 *value_serializer*
+    """
+    attrs = fields(inst.__class__)
+    rv = dict_factory()
+    for a in attrs:
+        v = getattr(inst, a.name)
+        if filter is not None and not filter(a, v):
+            continue
+
+        if value_serializer is not None:
+            v = value_serializer(inst, a, v)
+
+        if recurse is True:
+            if has(v.__class__):
+                rv[a.name] = asdict(
+                    v,
+                    True,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
+                )
+            elif isinstance(v, (tuple, list, set, frozenset)):
+                cf = v.__class__ if retain_collection_types is True else list
+                rv[a.name] = cf(
+                    [
+                        _asdict_anything(
+                            i,
+                            filter,
+                            dict_factory,
+                            retain_collection_types,
+                            value_serializer,
+                        )
+                        for i in v
+                    ]
+                )
+            elif isinstance(v, dict):
+                df = dict_factory
+                rv[a.name] = df(
+                    (
+                        _asdict_anything(
+                            kk,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
+                        ),
+                        _asdict_anything(
+                            vv,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
+                        ),
+                    )
+                    for kk, vv in iteritems(v)
+                )
+            else:
+                rv[a.name] = v
+        else:
+            rv[a.name] = v
+    return rv
+
+
+def _asdict_anything(
+    val,
+    filter,
+    dict_factory,
+    retain_collection_types,
+    value_serializer,
+):
+    """
+    ``asdict`` only works on attrs instances, this works on anything.
+    """
+    if getattr(val.__class__, "__attrs_attrs__", None) is not None:
+        # Attrs class.
+        rv = asdict(
+            val,
+            True,
+            filter,
+            dict_factory,
+            retain_collection_types,
+            value_serializer,
+        )
+    elif isinstance(val, (tuple, list, set, frozenset)):
+        cf = val.__class__ if retain_collection_types is True else list
+        rv = cf(
+            [
+                _asdict_anything(
+                    i,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
+                )
+                for i in val
+            ]
+        )
+    elif isinstance(val, dict):
+        df = dict_factory
+        rv = df(
+            (
+                _asdict_anything(
+                    kk, filter, df, retain_collection_types, value_serializer
+                ),
+                _asdict_anything(
+                    vv, filter, df, retain_collection_types, value_serializer
+                ),
+            )
+            for kk, vv in iteritems(val)
+        )
+    else:
+        rv = val
+        if value_serializer is not None:
+            rv = value_serializer(None, None, rv)
+
+    return rv
+
+
+def astuple(
+    inst,
+    recurse=True,
+    filter=None,
+    tuple_factory=tuple,
+    retain_collection_types=False,
+):
+    """
+    Return the ``attrs`` attribute values of *inst* as a tuple.
+
+    Optionally recurse into other ``attrs``-decorated classes.
+
+    :param inst: Instance of an ``attrs``-decorated class.
+    :param bool recurse: Recurse into classes that are also
+        ``attrs``-decorated.
+    :param callable filter: A callable whose return code determines whether an
+        attribute or element is included (``True``) or dropped (``False``).  Is
+        called with the `attr.Attribute` as the first argument and the
+        value as the second argument.
+    :param callable tuple_factory: A callable to produce tuples from.  For
+        example, to produce lists instead of tuples.
+    :param bool retain_collection_types: Do not convert to ``list``
+        or ``dict`` when encountering an attribute which type is
+        ``tuple``, ``dict`` or ``set``.  Only meaningful if ``recurse`` is
+        ``True``.
+
+    :rtype: return type of *tuple_factory*
+
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    ..  versionadded:: 16.2.0
+    """
+    attrs = fields(inst.__class__)
+    rv = []
+    retain = retain_collection_types  # Very long. :/
+    for a in attrs:
+        v = getattr(inst, a.name)
+        if filter is not None and not filter(a, v):
+            continue
+        if recurse is True:
+            if has(v.__class__):
+                rv.append(
+                    astuple(
+                        v,
+                        recurse=True,
+                        filter=filter,
+                        tuple_factory=tuple_factory,
+                        retain_collection_types=retain,
+                    )
+                )
+            elif isinstance(v, (tuple, list, set, frozenset)):
+                cf = v.__class__ if retain is True else list
+                rv.append(
+                    cf(
+                        [
+                            astuple(
+                                j,
+                                recurse=True,
+                                filter=filter,
+                                tuple_factory=tuple_factory,
+                                retain_collection_types=retain,
+                            )
+                            if has(j.__class__)
+                            else j
+                            for j in v
+                        ]
+                    )
+                )
+            elif isinstance(v, dict):
+                df = v.__class__ if retain is True else dict
+                rv.append(
+                    df(
+                        (
+                            astuple(
+                                kk,
+                                tuple_factory=tuple_factory,
+                                retain_collection_types=retain,
+                            )
+                            if has(kk.__class__)
+                            else kk,
+                            astuple(
+                                vv,
+                                tuple_factory=tuple_factory,
+                                retain_collection_types=retain,
+                            )
+                            if has(vv.__class__)
+                            else vv,
+                        )
+                        for kk, vv in iteritems(v)
+                    )
+                )
+            else:
+                rv.append(v)
+        else:
+            rv.append(v)
+
+    return rv if tuple_factory is list else tuple_factory(rv)
+
+
+def has(cls):
+    """
+    Check whether *cls* is a class with ``attrs`` attributes.
+
+    :param type cls: Class to introspect.
+    :raise TypeError: If *cls* is not a class.
+
+    :rtype: bool
+    """
+    return getattr(cls, "__attrs_attrs__", None) is not None
+
+
+def assoc(inst, **changes):
+    """
+    Copy *inst* and apply *changes*.
+
+    :param inst: Instance of a class with ``attrs`` attributes.
+    :param changes: Keyword changes in the new copy.
+
+    :return: A copy of inst with *changes* incorporated.
+
+    :raise attr.exceptions.AttrsAttributeNotFoundError: If *attr_name* couldn't
+        be found on *cls*.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    ..  deprecated:: 17.1.0
+        Use `evolve` instead.
+    """
+    import warnings
+
+    warnings.warn(
+        "assoc is deprecated and will be removed after 2018/01.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+    new = copy.copy(inst)
+    attrs = fields(inst.__class__)
+    for k, v in iteritems(changes):
+        a = getattr(attrs, k, NOTHING)
+        if a is NOTHING:
+            raise AttrsAttributeNotFoundError(
+                "{k} is not an attrs attribute on {cl}.".format(
+                    k=k, cl=new.__class__
+                )
+            )
+        _obj_setattr(new, k, v)
+    return new
+
+
+def evolve(inst, **changes):
+    """
+    Create a new instance, based on *inst* with *changes* applied.
+
+    :param inst: Instance of a class with ``attrs`` attributes.
+    :param changes: Keyword changes in the new copy.
+
+    :return: A copy of inst with *changes* incorporated.
+
+    :raise TypeError: If *attr_name* couldn't be found in the class
+        ``__init__``.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    ..  versionadded:: 17.1.0
+    """
+    cls = inst.__class__
+    attrs = fields(cls)
+    for a in attrs:
+        if not a.init:
+            continue
+        attr_name = a.name  # To deal with private attributes.
+        init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
+        if init_name not in changes:
+            changes[init_name] = getattr(inst, attr_name)
+
+    return cls(**changes)
+
+
+def resolve_types(cls, globalns=None, localns=None, attribs=None):
+    """
+    Resolve any strings and forward annotations in type annotations.
+
+    This is only required if you need concrete types in `Attribute`'s *type*
+    field. In other words, you don't need to resolve your types if you only
+    use them for static type checking.
+
+    With no arguments, names will be looked up in the module in which the class
+    was created. If this is not what you want, e.g. if the name only exists
+    inside a method, you may pass *globalns* or *localns* to specify other
+    dictionaries in which to look up these names. See the docs of
+    `typing.get_type_hints` for more details.
+
+    :param type cls: Class to resolve.
+    :param Optional[dict] globalns: Dictionary containing global variables.
+    :param Optional[dict] localns: Dictionary containing local variables.
+    :param Optional[list] attribs: List of attribs for the given class.
+        This is necessary when calling from inside a ``field_transformer``
+        since *cls* is not an ``attrs`` class yet.
+
+    :raise TypeError: If *cls* is not a class.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class and you didn't pass any attribs.
+    :raise NameError: If types cannot be resolved because of missing variables.
+
+    :returns: *cls* so you can use this function also as a class decorator.
+        Please note that you have to apply it **after** `attr.s`. That means
+        the decorator has to come in the line **before** `attr.s`.
+
+    ..  versionadded:: 20.1.0
+    ..  versionadded:: 21.1.0 *attribs*
+
+    """
+    try:
+        # Since calling get_type_hints is expensive we cache whether we've
+        # done it already.
+        cls.__attrs_types_resolved__
+    except AttributeError:
+        import typing
+
+        hints = typing.get_type_hints(cls, globalns=globalns, localns=localns)
+        for field in fields(cls) if attribs is None else attribs:
+            if field.name in hints:
+                # Since fields have been frozen we must work around it.
+                _obj_setattr(field, "type", hints[field.name])
+        cls.__attrs_types_resolved__ = True
+
+    # Return the class so you can use it as a decorator too.
+    return cls
diff --git a/lib/spack/external/attr/_make.py b/lib/spack/external/attr/_make.py
new file mode 100644
index 0000000000..a1912b1233
--- /dev/null
+++ b/lib/spack/external/attr/_make.py
@@ -0,0 +1,3052 @@
+from __future__ import absolute_import, division, print_function
+
+import copy
+import inspect
+import linecache
+import sys
+import threading
+import uuid
+import warnings
+
+from operator import itemgetter
+
+from . import _config, setters
+from ._compat import (
+    PY2,
+    PYPY,
+    isclass,
+    iteritems,
+    metadata_proxy,
+    new_class,
+    ordered_dict,
+    set_closure_cell,
+)
+from .exceptions import (
+    DefaultAlreadySetError,
+    FrozenInstanceError,
+    NotAnAttrsClassError,
+    PythonTooOldError,
+    UnannotatedAttributeError,
+)
+
+
+if not PY2:
+    import typing
+
+
+# This is used at least twice, so cache it here.
+_obj_setattr = object.__setattr__
+_init_converter_pat = "__attr_converter_%s"
+_init_factory_pat = "__attr_factory_{}"
+_tuple_property_pat = (
+    "    {attr_name} = _attrs_property(_attrs_itemgetter({index}))"
+)
+_classvar_prefixes = (
+    "typing.ClassVar",
+    "t.ClassVar",
+    "ClassVar",
+    "typing_extensions.ClassVar",
+)
+# we don't use a double-underscore prefix because that triggers
+# name mangling when trying to create a slot for the field
+# (when slots=True)
+_hash_cache_field = "_attrs_cached_hash"
+
+_empty_metadata_singleton = metadata_proxy({})
+
+# Unique object for unequivocal getattr() defaults.
+_sentinel = object()
+
+
+class _Nothing(object):
+    """
+    Sentinel class to indicate the lack of a value when ``None`` is ambiguous.
+
+    ``_Nothing`` is a singleton. There is only ever one of it.
+
+    .. versionchanged:: 21.1.0 ``bool(NOTHING)`` is now False.
+    """
+
+    _singleton = None
+
+    def __new__(cls):
+        if _Nothing._singleton is None:
+            _Nothing._singleton = super(_Nothing, cls).__new__(cls)
+        return _Nothing._singleton
+
+    def __repr__(self):
+        return "NOTHING"
+
+    def __bool__(self):
+        return False
+
+    def __len__(self):
+        return 0  # __bool__ for Python 2
+
+
+NOTHING = _Nothing()
+"""
+Sentinel to indicate the lack of a value when ``None`` is ambiguous.
+"""
+
+
+class _CacheHashWrapper(int):
+    """
+    An integer subclass that pickles / copies as None
+
+    This is used for non-slots classes with ``cache_hash=True``, to avoid
+    serializing a potentially (even likely) invalid hash value. Since ``None``
+    is the default value for uncalculated hashes, whenever this is copied,
+    the copy's value for the hash should automatically reset.
+
+    See GH #613 for more details.
+    """
+
+    if PY2:
+        # For some reason `type(None)` isn't callable in Python 2, but we don't
+        # actually need a constructor for None objects, we just need any
+        # available function that returns None.
+        def __reduce__(self, _none_constructor=getattr, _args=(0, "", None)):
+            return _none_constructor, _args
+
+    else:
+
+        def __reduce__(self, _none_constructor=type(None), _args=()):
+            return _none_constructor, _args
+
+
+def attrib(
+    default=NOTHING,
+    validator=None,
+    repr=True,
+    cmp=None,
+    hash=None,
+    init=True,
+    metadata=None,
+    type=None,
+    converter=None,
+    factory=None,
+    kw_only=False,
+    eq=None,
+    order=None,
+    on_setattr=None,
+):
+    """
+    Create a new attribute on a class.
+
+    ..  warning::
+
+        Does *not* do anything unless the class is also decorated with
+        `attr.s`!
+
+    :param default: A value that is used if an ``attrs``-generated ``__init__``
+        is used and no value is passed while instantiating or the attribute is
+        excluded using ``init=False``.
+
+        If the value is an instance of `Factory`, its callable will be
+        used to construct a new value (useful for mutable data types like lists
+        or dicts).
+
+        If a default is not set (or set manually to `attr.NOTHING`), a value
+        *must* be supplied when instantiating; otherwise a `TypeError`
+        will be raised.
+
+        The default can also be set using decorator notation as shown below.
+
+    :type default: Any value
+
+    :param callable factory: Syntactic sugar for
+        ``default=attr.Factory(factory)``.
+
+    :param validator: `callable` that is called by ``attrs``-generated
+        ``__init__`` methods after the instance has been initialized.  They
+        receive the initialized instance, the `Attribute`, and the
+        passed value.
+
+        The return value is *not* inspected so the validator has to throw an
+        exception itself.
+
+        If a `list` is passed, its items are treated as validators and must
+        all pass.
+
+        Validators can be globally disabled and re-enabled using
+        `get_run_validators`.
+
+        The validator can also be set using decorator notation as shown below.
+
+    :type validator: `callable` or a `list` of `callable`\\ s.
+
+    :param repr: Include this attribute in the generated ``__repr__``
+        method. If ``True``, include the attribute; if ``False``, omit it. By
+        default, the built-in ``repr()`` function is used. To override how the
+        attribute value is formatted, pass a ``callable`` that takes a single
+        value and returns a string. Note that the resulting string is used
+        as-is, i.e. it will be used directly *instead* of calling ``repr()``
+        (the default).
+    :type repr: a `bool` or a `callable` to use a custom function.
+
+    :param eq: If ``True`` (default), include this attribute in the
+        generated ``__eq__`` and ``__ne__`` methods that check two instances
+        for equality. To override how the attribute value is compared,
+        pass a ``callable`` that takes a single value and returns the value
+        to be compared.
+    :type eq: a `bool` or a `callable`.
+
+    :param order: If ``True`` (default), include this attributes in the
+        generated ``__lt__``, ``__le__``, ``__gt__`` and ``__ge__`` methods.
+        To override how the attribute value is ordered,
+        pass a ``callable`` that takes a single value and returns the value
+        to be ordered.
+    :type order: a `bool` or a `callable`.
+
+    :param cmp: Setting *cmp* is equivalent to setting *eq* and *order* to the
+        same value. Must not be mixed with *eq* or *order*.
+    :type cmp: a `bool` or a `callable`.
+
+    :param Optional[bool] hash: Include this attribute in the generated
+        ``__hash__`` method.  If ``None`` (default), mirror *eq*'s value.  This
+        is the correct behavior according the Python spec.  Setting this value
+        to anything else than ``None`` is *discouraged*.
+    :param bool init: Include this attribute in the generated ``__init__``
+        method.  It is possible to set this to ``False`` and set a default
+        value.  In that case this attributed is unconditionally initialized
+        with the specified default value or factory.
+    :param callable converter: `callable` that is called by
+        ``attrs``-generated ``__init__`` methods to convert attribute's value
+        to the desired format.  It is given the passed-in value, and the
+        returned value will be used as the new value of the attribute.  The
+        value is converted before being passed to the validator, if any.
+    :param metadata: An arbitrary mapping, to be used by third-party
+        components.  See `extending_metadata`.
+    :param type: The type of the attribute.  In Python 3.6 or greater, the
+        preferred method to specify the type is using a variable annotation
+        (see `PEP 526 <https://www.python.org/dev/peps/pep-0526/>`_).
+        This argument is provided for backward compatibility.
+        Regardless of the approach used, the type will be stored on
+        ``Attribute.type``.
+
+        Please note that ``attrs`` doesn't do anything with this metadata by
+        itself. You can use it as part of your own code or for
+        `static type checking <types>`.
+    :param kw_only: Make this attribute keyword-only (Python 3+)
+        in the generated ``__init__`` (if ``init`` is ``False``, this
+        parameter is ignored).
+    :param on_setattr: Allows to overwrite the *on_setattr* setting from
+        `attr.s`. If left `None`, the *on_setattr* value from `attr.s` is used.
+        Set to `attr.setters.NO_OP` to run **no** `setattr` hooks for this
+        attribute -- regardless of the setting in `attr.s`.
+    :type on_setattr: `callable`, or a list of callables, or `None`, or
+        `attr.setters.NO_OP`
+
+    .. versionadded:: 15.2.0 *convert*
+    .. versionadded:: 16.3.0 *metadata*
+    .. versionchanged:: 17.1.0 *validator* can be a ``list`` now.
+    .. versionchanged:: 17.1.0
+       *hash* is ``None`` and therefore mirrors *eq* by default.
+    .. versionadded:: 17.3.0 *type*
+    .. deprecated:: 17.4.0 *convert*
+    .. versionadded:: 17.4.0 *converter* as a replacement for the deprecated
+       *convert* to achieve consistency with other noun-based arguments.
+    .. versionadded:: 18.1.0
+       ``factory=f`` is syntactic sugar for ``default=attr.Factory(f)``.
+    .. versionadded:: 18.2.0 *kw_only*
+    .. versionchanged:: 19.2.0 *convert* keyword argument removed.
+    .. versionchanged:: 19.2.0 *repr* also accepts a custom callable.
+    .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
+    .. versionadded:: 19.2.0 *eq* and *order*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.3.0 *kw_only* backported to Python 2
+    .. versionchanged:: 21.1.0
+       *eq*, *order*, and *cmp* also accept a custom callable
+    .. versionchanged:: 21.1.0 *cmp* undeprecated
+    """
+    eq, eq_key, order, order_key = _determine_attrib_eq_order(
+        cmp, eq, order, True
+    )
+
+    if hash is not None and hash is not True and hash is not False:
+        raise TypeError(
+            "Invalid value for hash.  Must be True, False, or None."
+        )
+
+    if factory is not None:
+        if default is not NOTHING:
+            raise ValueError(
+                "The `default` and `factory` arguments are mutually "
+                "exclusive."
+            )
+        if not callable(factory):
+            raise ValueError("The `factory` argument must be a callable.")
+        default = Factory(factory)
+
+    if metadata is None:
+        metadata = {}
+
+    # Apply syntactic sugar by auto-wrapping.
+    if isinstance(on_setattr, (list, tuple)):
+        on_setattr = setters.pipe(*on_setattr)
+
+    if validator and isinstance(validator, (list, tuple)):
+        validator = and_(*validator)
+
+    if converter and isinstance(converter, (list, tuple)):
+        converter = pipe(*converter)
+
+    return _CountingAttr(
+        default=default,
+        validator=validator,
+        repr=repr,
+        cmp=None,
+        hash=hash,
+        init=init,
+        converter=converter,
+        metadata=metadata,
+        type=type,
+        kw_only=kw_only,
+        eq=eq,
+        eq_key=eq_key,
+        order=order,
+        order_key=order_key,
+        on_setattr=on_setattr,
+    )
+
+
+def _compile_and_eval(script, globs, locs=None, filename=""):
+    """
+    "Exec" the script with the given global (globs) and local (locs) variables.
+    """
+    bytecode = compile(script, filename, "exec")
+    eval(bytecode, globs, locs)
+
+
+def _make_method(name, script, filename, globs=None):
+    """
+    Create the method with the script given and return the method object.
+    """
+    locs = {}
+    if globs is None:
+        globs = {}
+
+    _compile_and_eval(script, globs, locs, filename)
+
+    # In order of debuggers like PDB being able to step through the code,
+    # we add a fake linecache entry.
+    linecache.cache[filename] = (
+        len(script),
+        None,
+        script.splitlines(True),
+        filename,
+    )
+
+    return locs[name]
+
+
+def _make_attr_tuple_class(cls_name, attr_names):
+    """
+    Create a tuple subclass to hold `Attribute`s for an `attrs` class.
+
+    The subclass is a bare tuple with properties for names.
+
+    class MyClassAttributes(tuple):
+        __slots__ = ()
+        x = property(itemgetter(0))
+    """
+    attr_class_name = "{}Attributes".format(cls_name)
+    attr_class_template = [
+        "class {}(tuple):".format(attr_class_name),
+        "    __slots__ = ()",
+    ]
+    if attr_names:
+        for i, attr_name in enumerate(attr_names):
+            attr_class_template.append(
+                _tuple_property_pat.format(index=i, attr_name=attr_name)
+            )
+    else:
+        attr_class_template.append("    pass")
+    globs = {"_attrs_itemgetter": itemgetter, "_attrs_property": property}
+    _compile_and_eval("\n".join(attr_class_template), globs)
+    return globs[attr_class_name]
+
+
+# Tuple class for extracted attributes from a class definition.
+# `base_attrs` is a subset of `attrs`.
+_Attributes = _make_attr_tuple_class(
+    "_Attributes",
+    [
+        # all attributes to build dunder methods for
+        "attrs",
+        # attributes that have been inherited
+        "base_attrs",
+        # map inherited attributes to their originating classes
+        "base_attrs_map",
+    ],
+)
+
+
+def _is_class_var(annot):
+    """
+    Check whether *annot* is a typing.ClassVar.
+
+    The string comparison hack is used to avoid evaluating all string
+    annotations which would put attrs-based classes at a performance
+    disadvantage compared to plain old classes.
+    """
+    annot = str(annot)
+
+    # Annotation can be quoted.
+    if annot.startswith(("'", '"')) and annot.endswith(("'", '"')):
+        annot = annot[1:-1]
+
+    return annot.startswith(_classvar_prefixes)
+
+
+def _has_own_attribute(cls, attrib_name):
+    """
+    Check whether *cls* defines *attrib_name* (and doesn't just inherit it).
+
+    Requires Python 3.
+    """
+    attr = getattr(cls, attrib_name, _sentinel)
+    if attr is _sentinel:
+        return False
+
+    for base_cls in cls.__mro__[1:]:
+        a = getattr(base_cls, attrib_name, None)
+        if attr is a:
+            return False
+
+    return True
+
+
+def _get_annotations(cls):
+    """
+    Get annotations for *cls*.
+    """
+    if _has_own_attribute(cls, "__annotations__"):
+        return cls.__annotations__
+
+    return {}
+
+
+def _counter_getter(e):
+    """
+    Key function for sorting to avoid re-creating a lambda for every class.
+    """
+    return e[1].counter
+
+
+def _collect_base_attrs(cls, taken_attr_names):
+    """
+    Collect attr.ibs from base classes of *cls*, except *taken_attr_names*.
+    """
+    base_attrs = []
+    base_attr_map = {}  # A dictionary of base attrs to their classes.
+
+    # Traverse the MRO and collect attributes.
+    for base_cls in reversed(cls.__mro__[1:-1]):
+        for a in getattr(base_cls, "__attrs_attrs__", []):
+            if a.inherited or a.name in taken_attr_names:
+                continue
+
+            a = a.evolve(inherited=True)
+            base_attrs.append(a)
+            base_attr_map[a.name] = base_cls
+
+    # For each name, only keep the freshest definition i.e. the furthest at the
+    # back.  base_attr_map is fine because it gets overwritten with every new
+    # instance.
+    filtered = []
+    seen = set()
+    for a in reversed(base_attrs):
+        if a.name in seen:
+            continue
+        filtered.insert(0, a)
+        seen.add(a.name)
+
+    return filtered, base_attr_map
+
+
+def _collect_base_attrs_broken(cls, taken_attr_names):
+    """
+    Collect attr.ibs from base classes of *cls*, except *taken_attr_names*.
+
+    N.B. *taken_attr_names* will be mutated.
+
+    Adhere to the old incorrect behavior.
+
+    Notably it collects from the front and considers inherited attributes which
+    leads to the buggy behavior reported in #428.
+    """
+    base_attrs = []
+    base_attr_map = {}  # A dictionary of base attrs to their classes.
+
+    # Traverse the MRO and collect attributes.
+    for base_cls in cls.__mro__[1:-1]:
+        for a in getattr(base_cls, "__attrs_attrs__", []):
+            if a.name in taken_attr_names:
+                continue
+
+            a = a.evolve(inherited=True)
+            taken_attr_names.add(a.name)
+            base_attrs.append(a)
+            base_attr_map[a.name] = base_cls
+
+    return base_attrs, base_attr_map
+
+
+def _transform_attrs(
+    cls, these, auto_attribs, kw_only, collect_by_mro, field_transformer
+):
+    """
+    Transform all `_CountingAttr`s on a class into `Attribute`s.
+
+    If *these* is passed, use that and don't look for them on the class.
+
+    *collect_by_mro* is True, collect them in the correct MRO order, otherwise
+    use the old -- incorrect -- order.  See #428.
+
+    Return an `_Attributes`.
+    """
+    cd = cls.__dict__
+    anns = _get_annotations(cls)
+
+    if these is not None:
+        ca_list = [(name, ca) for name, ca in iteritems(these)]
+
+        if not isinstance(these, ordered_dict):
+            ca_list.sort(key=_counter_getter)
+    elif auto_attribs is True:
+        ca_names = {
+            name
+            for name, attr in cd.items()
+            if isinstance(attr, _CountingAttr)
+        }
+        ca_list = []
+        annot_names = set()
+        for attr_name, type in anns.items():
+            if _is_class_var(type):
+                continue
+            annot_names.add(attr_name)
+            a = cd.get(attr_name, NOTHING)
+
+            if not isinstance(a, _CountingAttr):
+                if a is NOTHING:
+                    a = attrib()
+                else:
+                    a = attrib(default=a)
+            ca_list.append((attr_name, a))
+
+        unannotated = ca_names - annot_names
+        if len(unannotated) > 0:
+            raise UnannotatedAttributeError(
+                "The following `attr.ib`s lack a type annotation: "
+                + ", ".join(
+                    sorted(unannotated, key=lambda n: cd.get(n).counter)
+                )
+                + "."
+            )
+    else:
+        ca_list = sorted(
+            (
+                (name, attr)
+                for name, attr in cd.items()
+                if isinstance(attr, _CountingAttr)
+            ),
+            key=lambda e: e[1].counter,
+        )
+
+    own_attrs = [
+        Attribute.from_counting_attr(
+            name=attr_name, ca=ca, type=anns.get(attr_name)
+        )
+        for attr_name, ca in ca_list
+    ]
+
+    if collect_by_mro:
+        base_attrs, base_attr_map = _collect_base_attrs(
+            cls, {a.name for a in own_attrs}
+        )
+    else:
+        base_attrs, base_attr_map = _collect_base_attrs_broken(
+            cls, {a.name for a in own_attrs}
+        )
+
+    attr_names = [a.name for a in base_attrs + own_attrs]
+
+    AttrsClass = _make_attr_tuple_class(cls.__name__, attr_names)
+
+    if kw_only:
+        own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
+        base_attrs = [a.evolve(kw_only=True) for a in base_attrs]
+
+    attrs = AttrsClass(base_attrs + own_attrs)
+
+    # Mandatory vs non-mandatory attr order only matters when they are part of
+    # the __init__ signature and when they aren't kw_only (which are moved to
+    # the end and can be mandatory or non-mandatory in any order, as they will
+    # be specified as keyword args anyway). Check the order of those attrs:
+    had_default = False
+    for a in (a for a in attrs if a.init is not False and a.kw_only is False):
+        if had_default is True and a.default is NOTHING:
+            raise ValueError(
+                "No mandatory attributes allowed after an attribute with a "
+                "default value or factory.  Attribute in question: %r" % (a,)
+            )
+
+        if had_default is False and a.default is not NOTHING:
+            had_default = True
+
+    if field_transformer is not None:
+        attrs = field_transformer(cls, attrs)
+    return _Attributes((attrs, base_attrs, base_attr_map))
+
+
+if PYPY:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        if isinstance(self, BaseException) and name in (
+            "__cause__",
+            "__context__",
+        ):
+            BaseException.__setattr__(self, name, value)
+            return
+
+        raise FrozenInstanceError()
+
+
+else:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        raise FrozenInstanceError()
+
+
+def _frozen_delattrs(self, name):
+    """
+    Attached to frozen classes as __delattr__.
+    """
+    raise FrozenInstanceError()
+
+
+class _ClassBuilder(object):
+    """
+    Iteratively build *one* class.
+    """
+
+    __slots__ = (
+        "_attr_names",
+        "_attrs",
+        "_base_attr_map",
+        "_base_names",
+        "_cache_hash",
+        "_cls",
+        "_cls_dict",
+        "_delete_attribs",
+        "_frozen",
+        "_has_pre_init",
+        "_has_post_init",
+        "_is_exc",
+        "_on_setattr",
+        "_slots",
+        "_weakref_slot",
+        "_has_own_setattr",
+        "_has_custom_setattr",
+    )
+
+    def __init__(
+        self,
+        cls,
+        these,
+        slots,
+        frozen,
+        weakref_slot,
+        getstate_setstate,
+        auto_attribs,
+        kw_only,
+        cache_hash,
+        is_exc,
+        collect_by_mro,
+        on_setattr,
+        has_custom_setattr,
+        field_transformer,
+    ):
+        attrs, base_attrs, base_map = _transform_attrs(
+            cls,
+            these,
+            auto_attribs,
+            kw_only,
+            collect_by_mro,
+            field_transformer,
+        )
+
+        self._cls = cls
+        self._cls_dict = dict(cls.__dict__) if slots else {}
+        self._attrs = attrs
+        self._base_names = set(a.name for a in base_attrs)
+        self._base_attr_map = base_map
+        self._attr_names = tuple(a.name for a in attrs)
+        self._slots = slots
+        self._frozen = frozen
+        self._weakref_slot = weakref_slot
+        self._cache_hash = cache_hash
+        self._has_pre_init = bool(getattr(cls, "__attrs_pre_init__", False))
+        self._has_post_init = bool(getattr(cls, "__attrs_post_init__", False))
+        self._delete_attribs = not bool(these)
+        self._is_exc = is_exc
+        self._on_setattr = on_setattr
+
+        self._has_custom_setattr = has_custom_setattr
+        self._has_own_setattr = False
+
+        self._cls_dict["__attrs_attrs__"] = self._attrs
+
+        if frozen:
+            self._cls_dict["__setattr__"] = _frozen_setattrs
+            self._cls_dict["__delattr__"] = _frozen_delattrs
+
+            self._has_own_setattr = True
+
+        if getstate_setstate:
+            (
+                self._cls_dict["__getstate__"],
+                self._cls_dict["__setstate__"],
+            ) = self._make_getstate_setstate()
+
+    def __repr__(self):
+        return "<_ClassBuilder(cls={cls})>".format(cls=self._cls.__name__)
+
+    def build_class(self):
+        """
+        Finalize class based on the accumulated configuration.
+
+        Builder cannot be used after calling this method.
+        """
+        if self._slots is True:
+            return self._create_slots_class()
+        else:
+            return self._patch_original_class()
+
+    def _patch_original_class(self):
+        """
+        Apply accumulated methods and return the class.
+        """
+        cls = self._cls
+        base_names = self._base_names
+
+        # Clean class of attribute definitions (`attr.ib()`s).
+        if self._delete_attribs:
+            for name in self._attr_names:
+                if (
+                    name not in base_names
+                    and getattr(cls, name, _sentinel) is not _sentinel
+                ):
+                    try:
+                        delattr(cls, name)
+                    except AttributeError:
+                        # This can happen if a base class defines a class
+                        # variable and we want to set an attribute with the
+                        # same name by using only a type annotation.
+                        pass
+
+        # Attach our dunder methods.
+        for name, value in self._cls_dict.items():
+            setattr(cls, name, value)
+
+        # If we've inherited an attrs __setattr__ and don't write our own,
+        # reset it to object's.
+        if not self._has_own_setattr and getattr(
+            cls, "__attrs_own_setattr__", False
+        ):
+            cls.__attrs_own_setattr__ = False
+
+            if not self._has_custom_setattr:
+                cls.__setattr__ = object.__setattr__
+
+        return cls
+
+    def _create_slots_class(self):
+        """
+        Build and return a new class with a `__slots__` attribute.
+        """
+        cd = {
+            k: v
+            for k, v in iteritems(self._cls_dict)
+            if k not in tuple(self._attr_names) + ("__dict__", "__weakref__")
+        }
+
+        # If our class doesn't have its own implementation of __setattr__
+        # (either from the user or by us), check the bases, if one of them has
+        # an attrs-made __setattr__, that needs to be reset. We don't walk the
+        # MRO because we only care about our immediate base classes.
+        # XXX: This can be confused by subclassing a slotted attrs class with
+        # XXX: a non-attrs class and subclass the resulting class with an attrs
+        # XXX: class.  See `test_slotted_confused` for details.  For now that's
+        # XXX: OK with us.
+        if not self._has_own_setattr:
+            cd["__attrs_own_setattr__"] = False
+
+            if not self._has_custom_setattr:
+                for base_cls in self._cls.__bases__:
+                    if base_cls.__dict__.get("__attrs_own_setattr__", False):
+                        cd["__setattr__"] = object.__setattr__
+                        break
+
+        # Traverse the MRO to collect existing slots
+        # and check for an existing __weakref__.
+        existing_slots = dict()
+        weakref_inherited = False
+        for base_cls in self._cls.__mro__[1:-1]:
+            if base_cls.__dict__.get("__weakref__", None) is not None:
+                weakref_inherited = True
+            existing_slots.update(
+                {
+                    name: getattr(base_cls, name)
+                    for name in getattr(base_cls, "__slots__", [])
+                }
+            )
+
+        base_names = set(self._base_names)
+
+        names = self._attr_names
+        if (
+            self._weakref_slot
+            and "__weakref__" not in getattr(self._cls, "__slots__", ())
+            and "__weakref__" not in names
+            and not weakref_inherited
+        ):
+            names += ("__weakref__",)
+
+        # We only add the names of attributes that aren't inherited.
+        # Setting __slots__ to inherited attributes wastes memory.
+        slot_names = [name for name in names if name not in base_names]
+        # There are slots for attributes from current class
+        # that are defined in parent classes.
+        # As their descriptors may be overriden by a child class,
+        # we collect them here and update the class dict
+        reused_slots = {
+            slot: slot_descriptor
+            for slot, slot_descriptor in iteritems(existing_slots)
+            if slot in slot_names
+        }
+        slot_names = [name for name in slot_names if name not in reused_slots]
+        cd.update(reused_slots)
+        if self._cache_hash:
+            slot_names.append(_hash_cache_field)
+        cd["__slots__"] = tuple(slot_names)
+
+        qualname = getattr(self._cls, "__qualname__", None)
+        if qualname is not None:
+            cd["__qualname__"] = qualname
+
+        # Create new class based on old class and our methods.
+        cls = type(self._cls)(self._cls.__name__, self._cls.__bases__, cd)
+
+        # The following is a fix for
+        # https://github.com/python-attrs/attrs/issues/102.  On Python 3,
+        # if a method mentions `__class__` or uses the no-arg super(), the
+        # compiler will bake a reference to the class in the method itself
+        # as `method.__closure__`.  Since we replace the class with a
+        # clone, we rewrite these references so it keeps working.
+        for item in cls.__dict__.values():
+            if isinstance(item, (classmethod, staticmethod)):
+                # Class- and staticmethods hide their functions inside.
+                # These might need to be rewritten as well.
+                closure_cells = getattr(item.__func__, "__closure__", None)
+            elif isinstance(item, property):
+                # Workaround for property `super()` shortcut (PY3-only).
+                # There is no universal way for other descriptors.
+                closure_cells = getattr(item.fget, "__closure__", None)
+            else:
+                closure_cells = getattr(item, "__closure__", None)
+
+            if not closure_cells:  # Catch None or the empty list.
+                continue
+            for cell in closure_cells:
+                try:
+                    match = cell.cell_contents is self._cls
+                except ValueError:  # ValueError: Cell is empty
+                    pass
+                else:
+                    if match:
+                        set_closure_cell(cell, cls)
+
+        return cls
+
+    def add_repr(self, ns):
+        self._cls_dict["__repr__"] = self._add_method_dunders(
+            _make_repr(self._attrs, ns=ns)
+        )
+        return self
+
+    def add_str(self):
+        repr = self._cls_dict.get("__repr__")
+        if repr is None:
+            raise ValueError(
+                "__str__ can only be generated if a __repr__ exists."
+            )
+
+        def __str__(self):
+            return self.__repr__()
+
+        self._cls_dict["__str__"] = self._add_method_dunders(__str__)
+        return self
+
+    def _make_getstate_setstate(self):
+        """
+        Create custom __setstate__ and __getstate__ methods.
+        """
+        # __weakref__ is not writable.
+        state_attr_names = tuple(
+            an for an in self._attr_names if an != "__weakref__"
+        )
+
+        def slots_getstate(self):
+            """
+            Automatically created by attrs.
+            """
+            return tuple(getattr(self, name) for name in state_attr_names)
+
+        hash_caching_enabled = self._cache_hash
+
+        def slots_setstate(self, state):
+            """
+            Automatically created by attrs.
+            """
+            __bound_setattr = _obj_setattr.__get__(self, Attribute)
+            for name, value in zip(state_attr_names, state):
+                __bound_setattr(name, value)
+
+            # The hash code cache is not included when the object is
+            # serialized, but it still needs to be initialized to None to
+            # indicate that the first call to __hash__ should be a cache
+            # miss.
+            if hash_caching_enabled:
+                __bound_setattr(_hash_cache_field, None)
+
+        return slots_getstate, slots_setstate
+
+    def make_unhashable(self):
+        self._cls_dict["__hash__"] = None
+        return self
+
+    def add_hash(self):
+        self._cls_dict["__hash__"] = self._add_method_dunders(
+            _make_hash(
+                self._cls,
+                self._attrs,
+                frozen=self._frozen,
+                cache_hash=self._cache_hash,
+            )
+        )
+
+        return self
+
+    def add_init(self):
+        self._cls_dict["__init__"] = self._add_method_dunders(
+            _make_init(
+                self._cls,
+                self._attrs,
+                self._has_pre_init,
+                self._has_post_init,
+                self._frozen,
+                self._slots,
+                self._cache_hash,
+                self._base_attr_map,
+                self._is_exc,
+                self._on_setattr is not None
+                and self._on_setattr is not setters.NO_OP,
+                attrs_init=False,
+            )
+        )
+
+        return self
+
+    def add_attrs_init(self):
+        self._cls_dict["__attrs_init__"] = self._add_method_dunders(
+            _make_init(
+                self._cls,
+                self._attrs,
+                self._has_pre_init,
+                self._has_post_init,
+                self._frozen,
+                self._slots,
+                self._cache_hash,
+                self._base_attr_map,
+                self._is_exc,
+                self._on_setattr is not None
+                and self._on_setattr is not setters.NO_OP,
+                attrs_init=True,
+            )
+        )
+
+        return self
+
+    def add_eq(self):
+        cd = self._cls_dict
+
+        cd["__eq__"] = self._add_method_dunders(
+            _make_eq(self._cls, self._attrs)
+        )
+        cd["__ne__"] = self._add_method_dunders(_make_ne())
+
+        return self
+
+    def add_order(self):
+        cd = self._cls_dict
+
+        cd["__lt__"], cd["__le__"], cd["__gt__"], cd["__ge__"] = (
+            self._add_method_dunders(meth)
+            for meth in _make_order(self._cls, self._attrs)
+        )
+
+        return self
+
+    def add_setattr(self):
+        if self._frozen:
+            return self
+
+        sa_attrs = {}
+        for a in self._attrs:
+            on_setattr = a.on_setattr or self._on_setattr
+            if on_setattr and on_setattr is not setters.NO_OP:
+                sa_attrs[a.name] = a, on_setattr
+
+        if not sa_attrs:
+            return self
+
+        if self._has_custom_setattr:
+            # We need to write a __setattr__ but there already is one!
+            raise ValueError(
+                "Can't combine custom __setattr__ with on_setattr hooks."
+            )
+
+        # docstring comes from _add_method_dunders
+        def __setattr__(self, name, val):
+            try:
+                a, hook = sa_attrs[name]
+            except KeyError:
+                nval = val
+            else:
+                nval = hook(self, a, val)
+
+            _obj_setattr(self, name, nval)
+
+        self._cls_dict["__attrs_own_setattr__"] = True
+        self._cls_dict["__setattr__"] = self._add_method_dunders(__setattr__)
+        self._has_own_setattr = True
+
+        return self
+
+    def _add_method_dunders(self, method):
+        """
+        Add __module__ and __qualname__ to a *method* if possible.
+        """
+        try:
+            method.__module__ = self._cls.__module__
+        except AttributeError:
+            pass
+
+        try:
+            method.__qualname__ = ".".join(
+                (self._cls.__qualname__, method.__name__)
+            )
+        except AttributeError:
+            pass
+
+        try:
+            method.__doc__ = "Method generated by attrs for class %s." % (
+                self._cls.__qualname__,
+            )
+        except AttributeError:
+            pass
+
+        return method
+
+
+_CMP_DEPRECATION = (
+    "The usage of `cmp` is deprecated and will be removed on or after "
+    "2021-06-01.  Please use `eq` and `order` instead."
+)
+
+
+def _determine_attrs_eq_order(cmp, eq, order, default_eq):
+    """
+    Validate the combination of *cmp*, *eq*, and *order*. Derive the effective
+    values of eq and order.  If *eq* is None, set it to *default_eq*.
+    """
+    if cmp is not None and any((eq is not None, order is not None)):
+        raise ValueError("Don't mix `cmp` with `eq' and `order`.")
+
+    # cmp takes precedence due to bw-compatibility.
+    if cmp is not None:
+        return cmp, cmp
+
+    # If left None, equality is set to the specified default and ordering
+    # mirrors equality.
+    if eq is None:
+        eq = default_eq
+
+    if order is None:
+        order = eq
+
+    if eq is False and order is True:
+        raise ValueError("`order` can only be True if `eq` is True too.")
+
+    return eq, order
+
+
+def _determine_attrib_eq_order(cmp, eq, order, default_eq):
+    """
+    Validate the combination of *cmp*, *eq*, and *order*. Derive the effective
+    values of eq and order.  If *eq* is None, set it to *default_eq*.
+    """
+    if cmp is not None and any((eq is not None, order is not None)):
+        raise ValueError("Don't mix `cmp` with `eq' and `order`.")
+
+    def decide_callable_or_boolean(value):
+        """
+        Decide whether a key function is used.
+        """
+        if callable(value):
+            value, key = True, value
+        else:
+            key = None
+        return value, key
+
+    # cmp takes precedence due to bw-compatibility.
+    if cmp is not None:
+        cmp, cmp_key = decide_callable_or_boolean(cmp)
+        return cmp, cmp_key, cmp, cmp_key
+
+    # If left None, equality is set to the specified default and ordering
+    # mirrors equality.
+    if eq is None:
+        eq, eq_key = default_eq, None
+    else:
+        eq, eq_key = decide_callable_or_boolean(eq)
+
+    if order is None:
+        order, order_key = eq, eq_key
+    else:
+        order, order_key = decide_callable_or_boolean(order)
+
+    if eq is False and order is True:
+        raise ValueError("`order` can only be True if `eq` is True too.")
+
+    return eq, eq_key, order, order_key
+
+
+def _determine_whether_to_implement(
+    cls, flag, auto_detect, dunders, default=True
+):
+    """
+    Check whether we should implement a set of methods for *cls*.
+
+    *flag* is the argument passed into @attr.s like 'init', *auto_detect* the
+    same as passed into @attr.s and *dunders* is a tuple of attribute names
+    whose presence signal that the user has implemented it themselves.
+
+    Return *default* if no reason for either for or against is found.
+
+    auto_detect must be False on Python 2.
+    """
+    if flag is True or flag is False:
+        return flag
+
+    if flag is None and auto_detect is False:
+        return default
+
+    # Logically, flag is None and auto_detect is True here.
+    for dunder in dunders:
+        if _has_own_attribute(cls, dunder):
+            return False
+
+    return default
+
+
+def attrs(
+    maybe_cls=None,
+    these=None,
+    repr_ns=None,
+    repr=None,
+    cmp=None,
+    hash=None,
+    init=None,
+    slots=False,
+    frozen=False,
+    weakref_slot=True,
+    str=False,
+    auto_attribs=False,
+    kw_only=False,
+    cache_hash=False,
+    auto_exc=False,
+    eq=None,
+    order=None,
+    auto_detect=False,
+    collect_by_mro=False,
+    getstate_setstate=None,
+    on_setattr=None,
+    field_transformer=None,
+):
+    r"""
+    A class decorator that adds `dunder
+    <https://wiki.python.org/moin/DunderAlias>`_\ -methods according to the
+    specified attributes using `attr.ib` or the *these* argument.
+
+    :param these: A dictionary of name to `attr.ib` mappings.  This is
+        useful to avoid the definition of your attributes within the class body
+        because you can't (e.g. if you want to add ``__repr__`` methods to
+        Django models) or don't want to.
+
+        If *these* is not ``None``, ``attrs`` will *not* search the class body
+        for attributes and will *not* remove any attributes from it.
+
+        If *these* is an ordered dict (`dict` on Python 3.6+,
+        `collections.OrderedDict` otherwise), the order is deduced from
+        the order of the attributes inside *these*.  Otherwise the order
+        of the definition of the attributes is used.
+
+    :type these: `dict` of `str` to `attr.ib`
+
+    :param str repr_ns: When using nested classes, there's no way in Python 2
+        to automatically detect that.  Therefore it's possible to set the
+        namespace explicitly for a more meaningful ``repr`` output.
+    :param bool auto_detect: Instead of setting the *init*, *repr*, *eq*,
+        *order*, and *hash* arguments explicitly, assume they are set to
+        ``True`` **unless any** of the involved methods for one of the
+        arguments is implemented in the *current* class (i.e. it is *not*
+        inherited from some base class).
+
+        So for example by implementing ``__eq__`` on a class yourself,
+        ``attrs`` will deduce ``eq=False`` and will create *neither*
+        ``__eq__`` *nor* ``__ne__`` (but Python classes come with a sensible
+        ``__ne__`` by default, so it *should* be enough to only implement
+        ``__eq__`` in most cases).
+
+        .. warning::
+
+           If you prevent ``attrs`` from creating the ordering methods for you
+           (``order=False``, e.g. by implementing ``__le__``), it becomes
+           *your* responsibility to make sure its ordering is sound. The best
+           way is to use the `functools.total_ordering` decorator.
+
+
+        Passing ``True`` or ``False`` to *init*, *repr*, *eq*, *order*,
+        *cmp*, or *hash* overrides whatever *auto_detect* would determine.
+
+        *auto_detect* requires Python 3. Setting it ``True`` on Python 2 raises
+        a `PythonTooOldError`.
+
+    :param bool repr: Create a ``__repr__`` method with a human readable
+        representation of ``attrs`` attributes..
+    :param bool str: Create a ``__str__`` method that is identical to
+        ``__repr__``.  This is usually not necessary except for
+        `Exception`\ s.
+    :param Optional[bool] eq: If ``True`` or ``None`` (default), add ``__eq__``
+        and ``__ne__`` methods that check two instances for equality.
+
+        They compare the instances as if they were tuples of their ``attrs``
+        attributes if and only if the types of both classes are *identical*!
+    :param Optional[bool] order: If ``True``, add ``__lt__``, ``__le__``,
+        ``__gt__``, and ``__ge__`` methods that behave like *eq* above and
+        allow instances to be ordered. If ``None`` (default) mirror value of
+        *eq*.
+    :param Optional[bool] cmp: Setting *cmp* is equivalent to setting *eq*
+        and *order* to the same value. Must not be mixed with *eq* or *order*.
+    :param Optional[bool] hash: If ``None`` (default), the ``__hash__`` method
+        is generated according how *eq* and *frozen* are set.
+
+        1. If *both* are True, ``attrs`` will generate a ``__hash__`` for you.
+        2. If *eq* is True and *frozen* is False, ``__hash__`` will be set to
+           None, marking it unhashable (which it is).
+        3. If *eq* is False, ``__hash__`` will be left untouched meaning the
+           ``__hash__`` method of the base class will be used (if base class is
+           ``object``, this means it will fall back to id-based hashing.).
+
+        Although not recommended, you can decide for yourself and force
+        ``attrs`` to create one (e.g. if the class is immutable even though you
+        didn't freeze it programmatically) by passing ``True`` or not.  Both of
+        these cases are rather special and should be used carefully.
+
+        See our documentation on `hashing`, Python's documentation on
+        `object.__hash__`, and the `GitHub issue that led to the default \
+        behavior <https://github.com/python-attrs/attrs/issues/136>`_ for more
+        details.
+    :param bool init: Create a ``__init__`` method that initializes the
+        ``attrs`` attributes. Leading underscores are stripped for the argument
+        name. If a ``__attrs_pre_init__`` method exists on the class, it will
+        be called before the class is initialized. If a ``__attrs_post_init__``
+        method exists on the class, it will be called after the class is fully
+        initialized.
+
+        If ``init`` is ``False``, an ``__attrs_init__`` method will be
+        injected instead. This allows you to define a custom ``__init__``
+        method that can do pre-init work such as ``super().__init__()``,
+        and then call ``__attrs_init__()`` and ``__attrs_post_init__()``.
+    :param bool slots: Create a `slotted class <slotted classes>` that's more
+        memory-efficient. Slotted classes are generally superior to the default
+        dict classes, but have some gotchas you should know about, so we
+        encourage you to read the `glossary entry <slotted classes>`.
+    :param bool frozen: Make instances immutable after initialization.  If
+        someone attempts to modify a frozen instance,
+        `attr.exceptions.FrozenInstanceError` is raised.
+
+        .. note::
+
+            1. This is achieved by installing a custom ``__setattr__`` method
+               on your class, so you can't implement your own.
+
+            2. True immutability is impossible in Python.
+
+            3. This *does* have a minor a runtime performance `impact
+               <how-frozen>` when initializing new instances.  In other words:
+               ``__init__`` is slightly slower with ``frozen=True``.
+
+            4. If a class is frozen, you cannot modify ``self`` in
+               ``__attrs_post_init__`` or a self-written ``__init__``. You can
+               circumvent that limitation by using
+               ``object.__setattr__(self, "attribute_name", value)``.
+
+            5. Subclasses of a frozen class are frozen too.
+
+    :param bool weakref_slot: Make instances weak-referenceable.  This has no
+        effect unless ``slots`` is also enabled.
+    :param bool auto_attribs: If ``True``, collect `PEP 526`_-annotated
+        attributes (Python 3.6 and later only) from the class body.
+
+        In this case, you **must** annotate every field.  If ``attrs``
+        encounters a field that is set to an `attr.ib` but lacks a type
+        annotation, an `attr.exceptions.UnannotatedAttributeError` is
+        raised.  Use ``field_name: typing.Any = attr.ib(...)`` if you don't
+        want to set a type.
+
+        If you assign a value to those attributes (e.g. ``x: int = 42``), that
+        value becomes the default value like if it were passed using
+        ``attr.ib(default=42)``.  Passing an instance of `Factory` also
+        works as expected in most cases (see warning below).
+
+        Attributes annotated as `typing.ClassVar`, and attributes that are
+        neither annotated nor set to an `attr.ib` are **ignored**.
+
+        .. warning::
+           For features that use the attribute name to create decorators (e.g.
+           `validators <validators>`), you still *must* assign `attr.ib` to
+           them. Otherwise Python will either not find the name or try to use
+           the default value to call e.g. ``validator`` on it.
+
+           These errors can be quite confusing and probably the most common bug
+           report on our bug tracker.
+
+        .. _`PEP 526`: https://www.python.org/dev/peps/pep-0526/
+    :param bool kw_only: Make all attributes keyword-only (Python 3+)
+        in the generated ``__init__`` (if ``init`` is ``False``, this
+        parameter is ignored).
+    :param bool cache_hash: Ensure that the object's hash code is computed
+        only once and stored on the object.  If this is set to ``True``,
+        hashing must be either explicitly or implicitly enabled for this
+        class.  If the hash code is cached, avoid any reassignments of
+        fields involved in hash code computation or mutations of the objects
+        those fields point to after object creation.  If such changes occur,
+        the behavior of the object's hash code is undefined.
+    :param bool auto_exc: If the class subclasses `BaseException`
+        (which implicitly includes any subclass of any exception), the
+        following happens to behave like a well-behaved Python exceptions
+        class:
+
+        - the values for *eq*, *order*, and *hash* are ignored and the
+          instances compare and hash by the instance's ids (N.B. ``attrs`` will
+          *not* remove existing implementations of ``__hash__`` or the equality
+          methods. It just won't add own ones.),
+        - all attributes that are either passed into ``__init__`` or have a
+          default value are additionally available as a tuple in the ``args``
+          attribute,
+        - the value of *str* is ignored leaving ``__str__`` to base classes.
+    :param bool collect_by_mro: Setting this to `True` fixes the way ``attrs``
+       collects attributes from base classes.  The default behavior is
+       incorrect in certain cases of multiple inheritance.  It should be on by
+       default but is kept off for backward-compatability.
+
+       See issue `#428 <https://github.com/python-attrs/attrs/issues/428>`_ for
+       more details.
+
+    :param Optional[bool] getstate_setstate:
+       .. note::
+          This is usually only interesting for slotted classes and you should
+          probably just set *auto_detect* to `True`.
+
+       If `True`, ``__getstate__`` and
+       ``__setstate__`` are generated and attached to the class. This is
+       necessary for slotted classes to be pickleable. If left `None`, it's
+       `True` by default for slotted classes and ``False`` for dict classes.
+
+       If *auto_detect* is `True`, and *getstate_setstate* is left `None`,
+       and **either** ``__getstate__`` or ``__setstate__`` is detected directly
+       on the class (i.e. not inherited), it is set to `False` (this is usually
+       what you want).
+
+    :param on_setattr: A callable that is run whenever the user attempts to set
+        an attribute (either by assignment like ``i.x = 42`` or by using
+        `setattr` like ``setattr(i, "x", 42)``). It receives the same arguments
+        as validators: the instance, the attribute that is being modified, and
+        the new value.
+
+        If no exception is raised, the attribute is set to the return value of
+        the callable.
+
+        If a list of callables is passed, they're automatically wrapped in an
+        `attr.setters.pipe`.
+
+    :param Optional[callable] field_transformer:
+        A function that is called with the original class object and all
+        fields right before ``attrs`` finalizes the class.  You can use
+        this, e.g., to automatically add converters or validators to
+        fields based on their types.  See `transform-fields` for more details.
+
+    .. versionadded:: 16.0.0 *slots*
+    .. versionadded:: 16.1.0 *frozen*
+    .. versionadded:: 16.3.0 *str*
+    .. versionadded:: 16.3.0 Support for ``__attrs_post_init__``.
+    .. versionchanged:: 17.1.0
+       *hash* supports ``None`` as value which is also the default now.
+    .. versionadded:: 17.3.0 *auto_attribs*
+    .. versionchanged:: 18.1.0
+       If *these* is passed, no attributes are deleted from the class body.
+    .. versionchanged:: 18.1.0 If *these* is ordered, the order is retained.
+    .. versionadded:: 18.2.0 *weakref_slot*
+    .. deprecated:: 18.2.0
+       ``__lt__``, ``__le__``, ``__gt__``, and ``__ge__`` now raise a
+       `DeprecationWarning` if the classes compared are subclasses of
+       each other. ``__eq`` and ``__ne__`` never tried to compared subclasses
+       to each other.
+    .. versionchanged:: 19.2.0
+       ``__lt__``, ``__le__``, ``__gt__``, and ``__ge__`` now do not consider
+       subclasses comparable anymore.
+    .. versionadded:: 18.2.0 *kw_only*
+    .. versionadded:: 18.2.0 *cache_hash*
+    .. versionadded:: 19.1.0 *auto_exc*
+    .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
+    .. versionadded:: 19.2.0 *eq* and *order*
+    .. versionadded:: 20.1.0 *auto_detect*
+    .. versionadded:: 20.1.0 *collect_by_mro*
+    .. versionadded:: 20.1.0 *getstate_setstate*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionadded:: 20.3.0 *field_transformer*
+    .. versionchanged:: 21.1.0
+       ``init=False`` injects ``__attrs_init__``
+    .. versionchanged:: 21.1.0 Support for ``__attrs_pre_init__``
+    .. versionchanged:: 21.1.0 *cmp* undeprecated
+    """
+    if auto_detect and PY2:
+        raise PythonTooOldError(
+            "auto_detect only works on Python 3 and later."
+        )
+
+    eq_, order_ = _determine_attrs_eq_order(cmp, eq, order, None)
+    hash_ = hash  # work around the lack of nonlocal
+
+    if isinstance(on_setattr, (list, tuple)):
+        on_setattr = setters.pipe(*on_setattr)
+
+    def wrap(cls):
+
+        if getattr(cls, "__class__", None) is None:
+            raise TypeError("attrs only works with new-style classes.")
+
+        is_frozen = frozen or _has_frozen_base_class(cls)
+        is_exc = auto_exc is True and issubclass(cls, BaseException)
+        has_own_setattr = auto_detect and _has_own_attribute(
+            cls, "__setattr__"
+        )
+
+        if has_own_setattr and is_frozen:
+            raise ValueError("Can't freeze a class with a custom __setattr__.")
+
+        builder = _ClassBuilder(
+            cls,
+            these,
+            slots,
+            is_frozen,
+            weakref_slot,
+            _determine_whether_to_implement(
+                cls,
+                getstate_setstate,
+                auto_detect,
+                ("__getstate__", "__setstate__"),
+                default=slots,
+            ),
+            auto_attribs,
+            kw_only,
+            cache_hash,
+            is_exc,
+            collect_by_mro,
+            on_setattr,
+            has_own_setattr,
+            field_transformer,
+        )
+        if _determine_whether_to_implement(
+            cls, repr, auto_detect, ("__repr__",)
+        ):
+            builder.add_repr(repr_ns)
+        if str is True:
+            builder.add_str()
+
+        eq = _determine_whether_to_implement(
+            cls, eq_, auto_detect, ("__eq__", "__ne__")
+        )
+        if not is_exc and eq is True:
+            builder.add_eq()
+        if not is_exc and _determine_whether_to_implement(
+            cls, order_, auto_detect, ("__lt__", "__le__", "__gt__", "__ge__")
+        ):
+            builder.add_order()
+
+        builder.add_setattr()
+
+        if (
+            hash_ is None
+            and auto_detect is True
+            and _has_own_attribute(cls, "__hash__")
+        ):
+            hash = False
+        else:
+            hash = hash_
+        if hash is not True and hash is not False and hash is not None:
+            # Can't use `hash in` because 1 == True for example.
+            raise TypeError(
+                "Invalid value for hash.  Must be True, False, or None."
+            )
+        elif hash is False or (hash is None and eq is False) or is_exc:
+            # Don't do anything. Should fall back to __object__'s __hash__
+            # which is by id.
+            if cache_hash:
+                raise TypeError(
+                    "Invalid value for cache_hash.  To use hash caching,"
+                    " hashing must be either explicitly or implicitly "
+                    "enabled."
+                )
+        elif hash is True or (
+            hash is None and eq is True and is_frozen is True
+        ):
+            # Build a __hash__ if told so, or if it's safe.
+            builder.add_hash()
+        else:
+            # Raise TypeError on attempts to hash.
+            if cache_hash:
+                raise TypeError(
+                    "Invalid value for cache_hash.  To use hash caching,"
+                    " hashing must be either explicitly or implicitly "
+                    "enabled."
+                )
+            builder.make_unhashable()
+
+        if _determine_whether_to_implement(
+            cls, init, auto_detect, ("__init__",)
+        ):
+            builder.add_init()
+        else:
+            builder.add_attrs_init()
+            if cache_hash:
+                raise TypeError(
+                    "Invalid value for cache_hash.  To use hash caching,"
+                    " init must be True."
+                )
+
+        return builder.build_class()
+
+    # maybe_cls's type depends on the usage of the decorator.  It's a class
+    # if it's used as `@attrs` but ``None`` if used as `@attrs()`.
+    if maybe_cls is None:
+        return wrap
+    else:
+        return wrap(maybe_cls)
+
+
+_attrs = attrs
+"""
+Internal alias so we can use it in functions that take an argument called
+*attrs*.
+"""
+
+
+if PY2:
+
+    def _has_frozen_base_class(cls):
+        """
+        Check whether *cls* has a frozen ancestor by looking at its
+        __setattr__.
+        """
+        return (
+            getattr(cls.__setattr__, "__module__", None)
+            == _frozen_setattrs.__module__
+            and cls.__setattr__.__name__ == _frozen_setattrs.__name__
+        )
+
+
+else:
+
+    def _has_frozen_base_class(cls):
+        """
+        Check whether *cls* has a frozen ancestor by looking at its
+        __setattr__.
+        """
+        return cls.__setattr__ == _frozen_setattrs
+
+
+def _generate_unique_filename(cls, func_name):
+    """
+    Create a "filename" suitable for a function being generated.
+    """
+    unique_id = uuid.uuid4()
+    extra = ""
+    count = 1
+
+    while True:
+        unique_filename = "<attrs generated {0} {1}.{2}{3}>".format(
+            func_name,
+            cls.__module__,
+            getattr(cls, "__qualname__", cls.__name__),
+            extra,
+        )
+        # To handle concurrency we essentially "reserve" our spot in
+        # the linecache with a dummy line.  The caller can then
+        # set this value correctly.
+        cache_line = (1, None, (str(unique_id),), unique_filename)
+        if (
+            linecache.cache.setdefault(unique_filename, cache_line)
+            == cache_line
+        ):
+            return unique_filename
+
+        # Looks like this spot is taken. Try again.
+        count += 1
+        extra = "-{0}".format(count)
+
+
+def _make_hash(cls, attrs, frozen, cache_hash):
+    attrs = tuple(
+        a for a in attrs if a.hash is True or (a.hash is None and a.eq is True)
+    )
+
+    tab = "        "
+
+    unique_filename = _generate_unique_filename(cls, "hash")
+    type_hash = hash(unique_filename)
+
+    hash_def = "def __hash__(self"
+    hash_func = "hash(("
+    closing_braces = "))"
+    if not cache_hash:
+        hash_def += "):"
+    else:
+        if not PY2:
+            hash_def += ", *"
+
+        hash_def += (
+            ", _cache_wrapper="
+            + "__import__('attr._make')._make._CacheHashWrapper):"
+        )
+        hash_func = "_cache_wrapper(" + hash_func
+        closing_braces += ")"
+
+    method_lines = [hash_def]
+
+    def append_hash_computation_lines(prefix, indent):
+        """
+        Generate the code for actually computing the hash code.
+        Below this will either be returned directly or used to compute
+        a value which is then cached, depending on the value of cache_hash
+        """
+
+        method_lines.extend(
+            [
+                indent + prefix + hash_func,
+                indent + "        %d," % (type_hash,),
+            ]
+        )
+
+        for a in attrs:
+            method_lines.append(indent + "        self.%s," % a.name)
+
+        method_lines.append(indent + "    " + closing_braces)
+
+    if cache_hash:
+        method_lines.append(tab + "if self.%s is None:" % _hash_cache_field)
+        if frozen:
+            append_hash_computation_lines(
+                "object.__setattr__(self, '%s', " % _hash_cache_field, tab * 2
+            )
+            method_lines.append(tab * 2 + ")")  # close __setattr__
+        else:
+            append_hash_computation_lines(
+                "self.%s = " % _hash_cache_field, tab * 2
+            )
+        method_lines.append(tab + "return self.%s" % _hash_cache_field)
+    else:
+        append_hash_computation_lines("return ", tab)
+
+    script = "\n".join(method_lines)
+    return _make_method("__hash__", script, unique_filename)
+
+
+def _add_hash(cls, attrs):
+    """
+    Add a hash method to *cls*.
+    """
+    cls.__hash__ = _make_hash(cls, attrs, frozen=False, cache_hash=False)
+    return cls
+
+
+def _make_ne():
+    """
+    Create __ne__ method.
+    """
+
+    def __ne__(self, other):
+        """
+        Check equality and either forward a NotImplemented or
+        return the result negated.
+        """
+        result = self.__eq__(other)
+        if result is NotImplemented:
+            return NotImplemented
+
+        return not result
+
+    return __ne__
+
+
+def _make_eq(cls, attrs):
+    """
+    Create __eq__ method for *cls* with *attrs*.
+    """
+    attrs = [a for a in attrs if a.eq]
+
+    unique_filename = _generate_unique_filename(cls, "eq")
+    lines = [
+        "def __eq__(self, other):",
+        "    if other.__class__ is not self.__class__:",
+        "        return NotImplemented",
+    ]
+
+    # We can't just do a big self.x = other.x and... clause due to
+    # irregularities like nan == nan is false but (nan,) == (nan,) is true.
+    globs = {}
+    if attrs:
+        lines.append("    return  (")
+        others = ["    ) == ("]
+        for a in attrs:
+            if a.eq_key:
+                cmp_name = "_%s_key" % (a.name,)
+                # Add the key function to the global namespace
+                # of the evaluated function.
+                globs[cmp_name] = a.eq_key
+                lines.append(
+                    "        %s(self.%s),"
+                    % (
+                        cmp_name,
+                        a.name,
+                    )
+                )
+                others.append(
+                    "        %s(other.%s),"
+                    % (
+                        cmp_name,
+                        a.name,
+                    )
+                )
+            else:
+                lines.append("        self.%s," % (a.name,))
+                others.append("        other.%s," % (a.name,))
+
+        lines += others + ["    )"]
+    else:
+        lines.append("    return True")
+
+    script = "\n".join(lines)
+
+    return _make_method("__eq__", script, unique_filename, globs)
+
+
+def _make_order(cls, attrs):
+    """
+    Create ordering methods for *cls* with *attrs*.
+    """
+    attrs = [a for a in attrs if a.order]
+
+    def attrs_to_tuple(obj):
+        """
+        Save us some typing.
+        """
+        return tuple(
+            key(value) if key else value
+            for value, key in (
+                (getattr(obj, a.name), a.order_key) for a in attrs
+            )
+        )
+
+    def __lt__(self, other):
+        """
+        Automatically created by attrs.
+        """
+        if other.__class__ is self.__class__:
+            return attrs_to_tuple(self) < attrs_to_tuple(other)
+
+        return NotImplemented
+
+    def __le__(self, other):
+        """
+        Automatically created by attrs.
+        """
+        if other.__class__ is self.__class__:
+            return attrs_to_tuple(self) <= attrs_to_tuple(other)
+
+        return NotImplemented
+
+    def __gt__(self, other):
+        """
+        Automatically created by attrs.
+        """
+        if other.__class__ is self.__class__:
+            return attrs_to_tuple(self) > attrs_to_tuple(other)
+
+        return NotImplemented
+
+    def __ge__(self, other):
+        """
+        Automatically created by attrs.
+        """
+        if other.__class__ is self.__class__:
+            return attrs_to_tuple(self) >= attrs_to_tuple(other)
+
+        return NotImplemented
+
+    return __lt__, __le__, __gt__, __ge__
+
+
+def _add_eq(cls, attrs=None):
+    """
+    Add equality methods to *cls* with *attrs*.
+    """
+    if attrs is None:
+        attrs = cls.__attrs_attrs__
+
+    cls.__eq__ = _make_eq(cls, attrs)
+    cls.__ne__ = _make_ne()
+
+    return cls
+
+
+_already_repring = threading.local()
+
+
+def _make_repr(attrs, ns):
+    """
+    Make a repr method that includes relevant *attrs*, adding *ns* to the full
+    name.
+    """
+
+    # Figure out which attributes to include, and which function to use to
+    # format them. The a.repr value can be either bool or a custom callable.
+    attr_names_with_reprs = tuple(
+        (a.name, repr if a.repr is True else a.repr)
+        for a in attrs
+        if a.repr is not False
+    )
+
+    def __repr__(self):
+        """
+        Automatically created by attrs.
+        """
+        try:
+            working_set = _already_repring.working_set
+        except AttributeError:
+            working_set = set()
+            _already_repring.working_set = working_set
+
+        if id(self) in working_set:
+            return "..."
+        real_cls = self.__class__
+        if ns is None:
+            qualname = getattr(real_cls, "__qualname__", None)
+            if qualname is not None:
+                class_name = qualname.rsplit(">.", 1)[-1]
+            else:
+                class_name = real_cls.__name__
+        else:
+            class_name = ns + "." + real_cls.__name__
+
+        # Since 'self' remains on the stack (i.e.: strongly referenced) for the
+        # duration of this call, it's safe to depend on id(...) stability, and
+        # not need to track the instance and therefore worry about properties
+        # like weakref- or hash-ability.
+        working_set.add(id(self))
+        try:
+            result = [class_name, "("]
+            first = True
+            for name, attr_repr in attr_names_with_reprs:
+                if first:
+                    first = False
+                else:
+                    result.append(", ")
+                result.extend(
+                    (name, "=", attr_repr(getattr(self, name, NOTHING)))
+                )
+            return "".join(result) + ")"
+        finally:
+            working_set.remove(id(self))
+
+    return __repr__
+
+
+def _add_repr(cls, ns=None, attrs=None):
+    """
+    Add a repr method to *cls*.
+    """
+    if attrs is None:
+        attrs = cls.__attrs_attrs__
+
+    cls.__repr__ = _make_repr(attrs, ns)
+    return cls
+
+
+def fields(cls):
+    """
+    Return the tuple of ``attrs`` attributes for a class.
+
+    The tuple also allows accessing the fields by their names (see below for
+    examples).
+
+    :param type cls: Class to introspect.
+
+    :raise TypeError: If *cls* is not a class.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    :rtype: tuple (with name accessors) of `attr.Attribute`
+
+    ..  versionchanged:: 16.2.0 Returned tuple allows accessing the fields
+        by name.
+    """
+    if not isclass(cls):
+        raise TypeError("Passed object must be a class.")
+    attrs = getattr(cls, "__attrs_attrs__", None)
+    if attrs is None:
+        raise NotAnAttrsClassError(
+            "{cls!r} is not an attrs-decorated class.".format(cls=cls)
+        )
+    return attrs
+
+
+def fields_dict(cls):
+    """
+    Return an ordered dictionary of ``attrs`` attributes for a class, whose
+    keys are the attribute names.
+
+    :param type cls: Class to introspect.
+
+    :raise TypeError: If *cls* is not a class.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+
+    :rtype: an ordered dict where keys are attribute names and values are
+        `attr.Attribute`\\ s. This will be a `dict` if it's
+        naturally ordered like on Python 3.6+ or an
+        :class:`~collections.OrderedDict` otherwise.
+
+    .. versionadded:: 18.1.0
+    """
+    if not isclass(cls):
+        raise TypeError("Passed object must be a class.")
+    attrs = getattr(cls, "__attrs_attrs__", None)
+    if attrs is None:
+        raise NotAnAttrsClassError(
+            "{cls!r} is not an attrs-decorated class.".format(cls=cls)
+        )
+    return ordered_dict(((a.name, a) for a in attrs))
+
+
+def validate(inst):
+    """
+    Validate all attributes on *inst* that have a validator.
+
+    Leaves all exceptions through.
+
+    :param inst: Instance of a class with ``attrs`` attributes.
+    """
+    if _config._run_validators is False:
+        return
+
+    for a in fields(inst.__class__):
+        v = a.validator
+        if v is not None:
+            v(inst, a, getattr(inst, a.name))
+
+
+def _is_slot_cls(cls):
+    return "__slots__" in cls.__dict__
+
+
+def _is_slot_attr(a_name, base_attr_map):
+    """
+    Check if the attribute name comes from a slot class.
+    """
+    return a_name in base_attr_map and _is_slot_cls(base_attr_map[a_name])
+
+
+def _make_init(
+    cls,
+    attrs,
+    pre_init,
+    post_init,
+    frozen,
+    slots,
+    cache_hash,
+    base_attr_map,
+    is_exc,
+    has_global_on_setattr,
+    attrs_init,
+):
+    if frozen and has_global_on_setattr:
+        raise ValueError("Frozen classes can't use on_setattr.")
+
+    needs_cached_setattr = cache_hash or frozen
+    filtered_attrs = []
+    attr_dict = {}
+    for a in attrs:
+        if not a.init and a.default is NOTHING:
+            continue
+
+        filtered_attrs.append(a)
+        attr_dict[a.name] = a
+
+        if a.on_setattr is not None:
+            if frozen is True:
+                raise ValueError("Frozen classes can't use on_setattr.")
+
+            needs_cached_setattr = True
+        elif (
+            has_global_on_setattr and a.on_setattr is not setters.NO_OP
+        ) or _is_slot_attr(a.name, base_attr_map):
+            needs_cached_setattr = True
+
+    unique_filename = _generate_unique_filename(cls, "init")
+
+    script, globs, annotations = _attrs_to_init_script(
+        filtered_attrs,
+        frozen,
+        slots,
+        pre_init,
+        post_init,
+        cache_hash,
+        base_attr_map,
+        is_exc,
+        needs_cached_setattr,
+        has_global_on_setattr,
+        attrs_init,
+    )
+    if cls.__module__ in sys.modules:
+        # This makes typing.get_type_hints(CLS.__init__) resolve string types.
+        globs.update(sys.modules[cls.__module__].__dict__)
+
+    globs.update({"NOTHING": NOTHING, "attr_dict": attr_dict})
+
+    if needs_cached_setattr:
+        # Save the lookup overhead in __init__ if we need to circumvent
+        # setattr hooks.
+        globs["_cached_setattr"] = _obj_setattr
+
+    init = _make_method(
+        "__attrs_init__" if attrs_init else "__init__",
+        script,
+        unique_filename,
+        globs,
+    )
+    init.__annotations__ = annotations
+
+    return init
+
+
+def _setattr(attr_name, value_var, has_on_setattr):
+    """
+    Use the cached object.setattr to set *attr_name* to *value_var*.
+    """
+    return "_setattr('%s', %s)" % (attr_name, value_var)
+
+
+def _setattr_with_converter(attr_name, value_var, has_on_setattr):
+    """
+    Use the cached object.setattr to set *attr_name* to *value_var*, but run
+    its converter first.
+    """
+    return "_setattr('%s', %s(%s))" % (
+        attr_name,
+        _init_converter_pat % (attr_name,),
+        value_var,
+    )
+
+
+def _assign(attr_name, value, has_on_setattr):
+    """
+    Unless *attr_name* has an on_setattr hook, use normal assignment. Otherwise
+    relegate to _setattr.
+    """
+    if has_on_setattr:
+        return _setattr(attr_name, value, True)
+
+    return "self.%s = %s" % (attr_name, value)
+
+
+def _assign_with_converter(attr_name, value_var, has_on_setattr):
+    """
+    Unless *attr_name* has an on_setattr hook, use normal assignment after
+    conversion. Otherwise relegate to _setattr_with_converter.
+    """
+    if has_on_setattr:
+        return _setattr_with_converter(attr_name, value_var, True)
+
+    return "self.%s = %s(%s)" % (
+        attr_name,
+        _init_converter_pat % (attr_name,),
+        value_var,
+    )
+
+
+if PY2:
+
+    def _unpack_kw_only_py2(attr_name, default=None):
+        """
+        Unpack *attr_name* from _kw_only dict.
+        """
+        if default is not None:
+            arg_default = ", %s" % default
+        else:
+            arg_default = ""
+        return "%s = _kw_only.pop('%s'%s)" % (
+            attr_name,
+            attr_name,
+            arg_default,
+        )
+
+    def _unpack_kw_only_lines_py2(kw_only_args):
+        """
+        Unpack all *kw_only_args* from _kw_only dict and handle errors.
+
+        Given a list of strings "{attr_name}" and "{attr_name}={default}"
+        generates list of lines of code that pop attrs from _kw_only dict and
+        raise TypeError similar to builtin if required attr is missing or
+        extra key is passed.
+
+        >>> print("\n".join(_unpack_kw_only_lines_py2(["a", "b=42"])))
+        try:
+            a = _kw_only.pop('a')
+            b = _kw_only.pop('b', 42)
+        except KeyError as _key_error:
+            raise TypeError(
+                ...
+        if _kw_only:
+            raise TypeError(
+                ...
+        """
+        lines = ["try:"]
+        lines.extend(
+            "    " + _unpack_kw_only_py2(*arg.split("="))
+            for arg in kw_only_args
+        )
+        lines += """\
+except KeyError as _key_error:
+    raise TypeError(
+        '__init__() missing required keyword-only argument: %s' % _key_error
+    )
+if _kw_only:
+    raise TypeError(
+        '__init__() got an unexpected keyword argument %r'
+        % next(iter(_kw_only))
+    )
+""".split(
+            "\n"
+        )
+        return lines
+
+
+def _attrs_to_init_script(
+    attrs,
+    frozen,
+    slots,
+    pre_init,
+    post_init,
+    cache_hash,
+    base_attr_map,
+    is_exc,
+    needs_cached_setattr,
+    has_global_on_setattr,
+    attrs_init,
+):
+    """
+    Return a script of an initializer for *attrs* and a dict of globals.
+
+    The globals are expected by the generated script.
+
+    If *frozen* is True, we cannot set the attributes directly so we use
+    a cached ``object.__setattr__``.
+    """
+    lines = []
+    if pre_init:
+        lines.append("self.__attrs_pre_init__()")
+
+    if needs_cached_setattr:
+        lines.append(
+            # Circumvent the __setattr__ descriptor to save one lookup per
+            # assignment.
+            # Note _setattr will be used again below if cache_hash is True
+            "_setattr = _cached_setattr.__get__(self, self.__class__)"
+        )
+
+    if frozen is True:
+        if slots is True:
+            fmt_setter = _setattr
+            fmt_setter_with_converter = _setattr_with_converter
+        else:
+            # Dict frozen classes assign directly to __dict__.
+            # But only if the attribute doesn't come from an ancestor slot
+            # class.
+            # Note _inst_dict will be used again below if cache_hash is True
+            lines.append("_inst_dict = self.__dict__")
+
+            def fmt_setter(attr_name, value_var, has_on_setattr):
+                if _is_slot_attr(attr_name, base_attr_map):
+                    return _setattr(attr_name, value_var, has_on_setattr)
+
+                return "_inst_dict['%s'] = %s" % (attr_name, value_var)
+
+            def fmt_setter_with_converter(
+                attr_name, value_var, has_on_setattr
+            ):
+                if has_on_setattr or _is_slot_attr(attr_name, base_attr_map):
+                    return _setattr_with_converter(
+                        attr_name, value_var, has_on_setattr
+                    )
+
+                return "_inst_dict['%s'] = %s(%s)" % (
+                    attr_name,
+                    _init_converter_pat % (attr_name,),
+                    value_var,
+                )
+
+    else:
+        # Not frozen.
+        fmt_setter = _assign
+        fmt_setter_with_converter = _assign_with_converter
+
+    args = []
+    kw_only_args = []
+    attrs_to_validate = []
+
+    # This is a dictionary of names to validator and converter callables.
+    # Injecting this into __init__ globals lets us avoid lookups.
+    names_for_globals = {}
+    annotations = {"return": None}
+
+    for a in attrs:
+        if a.validator:
+            attrs_to_validate.append(a)
+
+        attr_name = a.name
+        has_on_setattr = a.on_setattr is not None or (
+            a.on_setattr is not setters.NO_OP and has_global_on_setattr
+        )
+        arg_name = a.name.lstrip("_")
+
+        has_factory = isinstance(a.default, Factory)
+        if has_factory and a.default.takes_self:
+            maybe_self = "self"
+        else:
+            maybe_self = ""
+
+        if a.init is False:
+            if has_factory:
+                init_factory_name = _init_factory_pat.format(a.name)
+                if a.converter is not None:
+                    lines.append(
+                        fmt_setter_with_converter(
+                            attr_name,
+                            init_factory_name + "(%s)" % (maybe_self,),
+                            has_on_setattr,
+                        )
+                    )
+                    conv_name = _init_converter_pat % (a.name,)
+                    names_for_globals[conv_name] = a.converter
+                else:
+                    lines.append(
+                        fmt_setter(
+                            attr_name,
+                            init_factory_name + "(%s)" % (maybe_self,),
+                            has_on_setattr,
+                        )
+                    )
+                names_for_globals[init_factory_name] = a.default.factory
+            else:
+                if a.converter is not None:
+                    lines.append(
+                        fmt_setter_with_converter(
+                            attr_name,
+                            "attr_dict['%s'].default" % (attr_name,),
+                            has_on_setattr,
+                        )
+                    )
+                    conv_name = _init_converter_pat % (a.name,)
+                    names_for_globals[conv_name] = a.converter
+                else:
+                    lines.append(
+                        fmt_setter(
+                            attr_name,
+                            "attr_dict['%s'].default" % (attr_name,),
+                            has_on_setattr,
+                        )
+                    )
+        elif a.default is not NOTHING and not has_factory:
+            arg = "%s=attr_dict['%s'].default" % (arg_name, attr_name)
+            if a.kw_only:
+                kw_only_args.append(arg)
+            else:
+                args.append(arg)
+
+            if a.converter is not None:
+                lines.append(
+                    fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
+                )
+                names_for_globals[
+                    _init_converter_pat % (a.name,)
+                ] = a.converter
+            else:
+                lines.append(fmt_setter(attr_name, arg_name, has_on_setattr))
+
+        elif has_factory:
+            arg = "%s=NOTHING" % (arg_name,)
+            if a.kw_only:
+                kw_only_args.append(arg)
+            else:
+                args.append(arg)
+            lines.append("if %s is not NOTHING:" % (arg_name,))
+
+            init_factory_name = _init_factory_pat.format(a.name)
+            if a.converter is not None:
+                lines.append(
+                    "    "
+                    + fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
+                )
+                lines.append("else:")
+                lines.append(
+                    "    "
+                    + fmt_setter_with_converter(
+                        attr_name,
+                        init_factory_name + "(" + maybe_self + ")",
+                        has_on_setattr,
+                    )
+                )
+                names_for_globals[
+                    _init_converter_pat % (a.name,)
+                ] = a.converter
+            else:
+                lines.append(
+                    "    " + fmt_setter(attr_name, arg_name, has_on_setattr)
+                )
+                lines.append("else:")
+                lines.append(
+                    "    "
+                    + fmt_setter(
+                        attr_name,
+                        init_factory_name + "(" + maybe_self + ")",
+                        has_on_setattr,
+                    )
+                )
+            names_for_globals[init_factory_name] = a.default.factory
+        else:
+            if a.kw_only:
+                kw_only_args.append(arg_name)
+            else:
+                args.append(arg_name)
+
+            if a.converter is not None:
+                lines.append(
+                    fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
+                )
+                names_for_globals[
+                    _init_converter_pat % (a.name,)
+                ] = a.converter
+            else:
+                lines.append(fmt_setter(attr_name, arg_name, has_on_setattr))
+
+        if a.init is True:
+            if a.type is not None and a.converter is None:
+                annotations[arg_name] = a.type
+            elif a.converter is not None and not PY2:
+                # Try to get the type from the converter.
+                sig = None
+                try:
+                    sig = inspect.signature(a.converter)
+                except (ValueError, TypeError):  # inspect failed
+                    pass
+                if sig:
+                    sig_params = list(sig.parameters.values())
+                    if (
+                        sig_params
+                        and sig_params[0].annotation
+                        is not inspect.Parameter.empty
+                    ):
+                        annotations[arg_name] = sig_params[0].annotation
+
+    if attrs_to_validate:  # we can skip this if there are no validators.
+        names_for_globals["_config"] = _config
+        lines.append("if _config._run_validators is True:")
+        for a in attrs_to_validate:
+            val_name = "__attr_validator_" + a.name
+            attr_name = "__attr_" + a.name
+            lines.append(
+                "    %s(self, %s, self.%s)" % (val_name, attr_name, a.name)
+            )
+            names_for_globals[val_name] = a.validator
+            names_for_globals[attr_name] = a
+
+    if post_init:
+        lines.append("self.__attrs_post_init__()")
+
+    # because this is set only after __attrs_post_init is called, a crash
+    # will result if post-init tries to access the hash code.  This seemed
+    # preferable to setting this beforehand, in which case alteration to
+    # field values during post-init combined with post-init accessing the
+    # hash code would result in silent bugs.
+    if cache_hash:
+        if frozen:
+            if slots:
+                # if frozen and slots, then _setattr defined above
+                init_hash_cache = "_setattr('%s', %s)"
+            else:
+                # if frozen and not slots, then _inst_dict defined above
+                init_hash_cache = "_inst_dict['%s'] = %s"
+        else:
+            init_hash_cache = "self.%s = %s"
+        lines.append(init_hash_cache % (_hash_cache_field, "None"))
+
+    # For exceptions we rely on BaseException.__init__ for proper
+    # initialization.
+    if is_exc:
+        vals = ",".join("self." + a.name for a in attrs if a.init)
+
+        lines.append("BaseException.__init__(self, %s)" % (vals,))
+
+    args = ", ".join(args)
+    if kw_only_args:
+        if PY2:
+            lines = _unpack_kw_only_lines_py2(kw_only_args) + lines
+
+            args += "%s**_kw_only" % (", " if args else "",)  # leading comma
+        else:
+            args += "%s*, %s" % (
+                ", " if args else "",  # leading comma
+                ", ".join(kw_only_args),  # kw_only args
+            )
+    return (
+        """\
+def {init_name}(self, {args}):
+    {lines}
+""".format(
+            init_name=("__attrs_init__" if attrs_init else "__init__"),
+            args=args,
+            lines="\n    ".join(lines) if lines else "pass",
+        ),
+        names_for_globals,
+        annotations,
+    )
+
+
+class Attribute(object):
+    """
+    *Read-only* representation of an attribute.
+
+    Instances of this class are frequently used for introspection purposes
+    like:
+
+    - `fields` returns a tuple of them.
+    - Validators get them passed as the first argument.
+    - The *field transformer* hook receives a list of them.
+
+    :attribute name: The name of the attribute.
+    :attribute inherited: Whether or not that attribute has been inherited from
+        a base class.
+
+    Plus *all* arguments of `attr.ib` (except for ``factory``
+    which is only syntactic sugar for ``default=Factory(...)``.
+
+    .. versionadded:: 20.1.0 *inherited*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.2.0 *inherited* is not taken into account for
+        equality checks and hashing anymore.
+    .. versionadded:: 21.1.0 *eq_key* and *order_key*
+
+    For the full version history of the fields, see `attr.ib`.
+    """
+
+    __slots__ = (
+        "name",
+        "default",
+        "validator",
+        "repr",
+        "eq",
+        "eq_key",
+        "order",
+        "order_key",
+        "hash",
+        "init",
+        "metadata",
+        "type",
+        "converter",
+        "kw_only",
+        "inherited",
+        "on_setattr",
+    )
+
+    def __init__(
+        self,
+        name,
+        default,
+        validator,
+        repr,
+        cmp,  # XXX: unused, remove along with other cmp code.
+        hash,
+        init,
+        inherited,
+        metadata=None,
+        type=None,
+        converter=None,
+        kw_only=False,
+        eq=None,
+        eq_key=None,
+        order=None,
+        order_key=None,
+        on_setattr=None,
+    ):
+        eq, eq_key, order, order_key = _determine_attrib_eq_order(
+            cmp, eq_key or eq, order_key or order, True
+        )
+
+        # Cache this descriptor here to speed things up later.
+        bound_setattr = _obj_setattr.__get__(self, Attribute)
+
+        # Despite the big red warning, people *do* instantiate `Attribute`
+        # themselves.
+        bound_setattr("name", name)
+        bound_setattr("default", default)
+        bound_setattr("validator", validator)
+        bound_setattr("repr", repr)
+        bound_setattr("eq", eq)
+        bound_setattr("eq_key", eq_key)
+        bound_setattr("order", order)
+        bound_setattr("order_key", order_key)
+        bound_setattr("hash", hash)
+        bound_setattr("init", init)
+        bound_setattr("converter", converter)
+        bound_setattr(
+            "metadata",
+            (
+                metadata_proxy(metadata)
+                if metadata
+                else _empty_metadata_singleton
+            ),
+        )
+        bound_setattr("type", type)
+        bound_setattr("kw_only", kw_only)
+        bound_setattr("inherited", inherited)
+        bound_setattr("on_setattr", on_setattr)
+
+    def __setattr__(self, name, value):
+        raise FrozenInstanceError()
+
+    @classmethod
+    def from_counting_attr(cls, name, ca, type=None):
+        # type holds the annotated value. deal with conflicts:
+        if type is None:
+            type = ca.type
+        elif ca.type is not None:
+            raise ValueError(
+                "Type annotation and type argument cannot both be present"
+            )
+        inst_dict = {
+            k: getattr(ca, k)
+            for k in Attribute.__slots__
+            if k
+            not in (
+                "name",
+                "validator",
+                "default",
+                "type",
+                "inherited",
+            )  # exclude methods and deprecated alias
+        }
+        return cls(
+            name=name,
+            validator=ca._validator,
+            default=ca._default,
+            type=type,
+            cmp=None,
+            inherited=False,
+            **inst_dict
+        )
+
+    @property
+    def cmp(self):
+        """
+        Simulate the presence of a cmp attribute and warn.
+        """
+        warnings.warn(_CMP_DEPRECATION, DeprecationWarning, stacklevel=2)
+
+        return self.eq and self.order
+
+    # Don't use attr.evolve since fields(Attribute) doesn't work
+    def evolve(self, **changes):
+        """
+        Copy *self* and apply *changes*.
+
+        This works similarly to `attr.evolve` but that function does not work
+        with ``Attribute``.
+
+        It is mainly meant to be used for `transform-fields`.
+
+        .. versionadded:: 20.3.0
+        """
+        new = copy.copy(self)
+
+        new._setattrs(changes.items())
+
+        return new
+
+    # Don't use _add_pickle since fields(Attribute) doesn't work
+    def __getstate__(self):
+        """
+        Play nice with pickle.
+        """
+        return tuple(
+            getattr(self, name) if name != "metadata" else dict(self.metadata)
+            for name in self.__slots__
+        )
+
+    def __setstate__(self, state):
+        """
+        Play nice with pickle.
+        """
+        self._setattrs(zip(self.__slots__, state))
+
+    def _setattrs(self, name_values_pairs):
+        bound_setattr = _obj_setattr.__get__(self, Attribute)
+        for name, value in name_values_pairs:
+            if name != "metadata":
+                bound_setattr(name, value)
+            else:
+                bound_setattr(
+                    name,
+                    metadata_proxy(value)
+                    if value
+                    else _empty_metadata_singleton,
+                )
+
+
+_a = [
+    Attribute(
+        name=name,
+        default=NOTHING,
+        validator=None,
+        repr=True,
+        cmp=None,
+        eq=True,
+        order=False,
+        hash=(name != "metadata"),
+        init=True,
+        inherited=False,
+    )
+    for name in Attribute.__slots__
+]
+
+Attribute = _add_hash(
+    _add_eq(
+        _add_repr(Attribute, attrs=_a),
+        attrs=[a for a in _a if a.name != "inherited"],
+    ),
+    attrs=[a for a in _a if a.hash and a.name != "inherited"],
+)
+
+
+class _CountingAttr(object):
+    """
+    Intermediate representation of attributes that uses a counter to preserve
+    the order in which the attributes have been defined.
+
+    *Internal* data structure of the attrs library.  Running into is most
+    likely the result of a bug like a forgotten `@attr.s` decorator.
+    """
+
+    __slots__ = (
+        "counter",
+        "_default",
+        "repr",
+        "eq",
+        "eq_key",
+        "order",
+        "order_key",
+        "hash",
+        "init",
+        "metadata",
+        "_validator",
+        "converter",
+        "type",
+        "kw_only",
+        "on_setattr",
+    )
+    __attrs_attrs__ = tuple(
+        Attribute(
+            name=name,
+            default=NOTHING,
+            validator=None,
+            repr=True,
+            cmp=None,
+            hash=True,
+            init=True,
+            kw_only=False,
+            eq=True,
+            eq_key=None,
+            order=False,
+            order_key=None,
+            inherited=False,
+            on_setattr=None,
+        )
+        for name in (
+            "counter",
+            "_default",
+            "repr",
+            "eq",
+            "order",
+            "hash",
+            "init",
+            "on_setattr",
+        )
+    ) + (
+        Attribute(
+            name="metadata",
+            default=None,
+            validator=None,
+            repr=True,
+            cmp=None,
+            hash=False,
+            init=True,
+            kw_only=False,
+            eq=True,
+            eq_key=None,
+            order=False,
+            order_key=None,
+            inherited=False,
+            on_setattr=None,
+        ),
+    )
+    cls_counter = 0
+
+    def __init__(
+        self,
+        default,
+        validator,
+        repr,
+        cmp,
+        hash,
+        init,
+        converter,
+        metadata,
+        type,
+        kw_only,
+        eq,
+        eq_key,
+        order,
+        order_key,
+        on_setattr,
+    ):
+        _CountingAttr.cls_counter += 1
+        self.counter = _CountingAttr.cls_counter
+        self._default = default
+        self._validator = validator
+        self.converter = converter
+        self.repr = repr
+        self.eq = eq
+        self.eq_key = eq_key
+        self.order = order
+        self.order_key = order_key
+        self.hash = hash
+        self.init = init
+        self.metadata = metadata
+        self.type = type
+        self.kw_only = kw_only
+        self.on_setattr = on_setattr
+
+    def validator(self, meth):
+        """
+        Decorator that adds *meth* to the list of validators.
+
+        Returns *meth* unchanged.
+
+        .. versionadded:: 17.1.0
+        """
+        if self._validator is None:
+            self._validator = meth
+        else:
+            self._validator = and_(self._validator, meth)
+        return meth
+
+    def default(self, meth):
+        """
+        Decorator that allows to set the default for an attribute.
+
+        Returns *meth* unchanged.
+
+        :raises DefaultAlreadySetError: If default has been set before.
+
+        .. versionadded:: 17.1.0
+        """
+        if self._default is not NOTHING:
+            raise DefaultAlreadySetError()
+
+        self._default = Factory(meth, takes_self=True)
+
+        return meth
+
+
+_CountingAttr = _add_eq(_add_repr(_CountingAttr))
+
+
+class Factory(object):
+    """
+    Stores a factory callable.
+
+    If passed as the default value to `attr.ib`, the factory is used to
+    generate a new value.
+
+    :param callable factory: A callable that takes either none or exactly one
+        mandatory positional argument depending on *takes_self*.
+    :param bool takes_self: Pass the partially initialized instance that is
+        being initialized as a positional argument.
+
+    .. versionadded:: 17.1.0  *takes_self*
+    """
+
+    __slots__ = ("factory", "takes_self")
+
+    def __init__(self, factory, takes_self=False):
+        """
+        `Factory` is part of the default machinery so if we want a default
+        value here, we have to implement it ourselves.
+        """
+        self.factory = factory
+        self.takes_self = takes_self
+
+    def __getstate__(self):
+        """
+        Play nice with pickle.
+        """
+        return tuple(getattr(self, name) for name in self.__slots__)
+
+    def __setstate__(self, state):
+        """
+        Play nice with pickle.
+        """
+        for name, value in zip(self.__slots__, state):
+            setattr(self, name, value)
+
+
+_f = [
+    Attribute(
+        name=name,
+        default=NOTHING,
+        validator=None,
+        repr=True,
+        cmp=None,
+        eq=True,
+        order=False,
+        hash=True,
+        init=True,
+        inherited=False,
+    )
+    for name in Factory.__slots__
+]
+
+Factory = _add_hash(_add_eq(_add_repr(Factory, attrs=_f), attrs=_f), attrs=_f)
+
+
+def make_class(name, attrs, bases=(object,), **attributes_arguments):
+    """
+    A quick way to create a new class called *name* with *attrs*.
+
+    :param str name: The name for the new class.
+
+    :param attrs: A list of names or a dictionary of mappings of names to
+        attributes.
+
+        If *attrs* is a list or an ordered dict (`dict` on Python 3.6+,
+        `collections.OrderedDict` otherwise), the order is deduced from
+        the order of the names or attributes inside *attrs*.  Otherwise the
+        order of the definition of the attributes is used.
+    :type attrs: `list` or `dict`
+
+    :param tuple bases: Classes that the new class will subclass.
+
+    :param attributes_arguments: Passed unmodified to `attr.s`.
+
+    :return: A new class with *attrs*.
+    :rtype: type
+
+    .. versionadded:: 17.1.0 *bases*
+    .. versionchanged:: 18.1.0 If *attrs* is ordered, the order is retained.
+    """
+    if isinstance(attrs, dict):
+        cls_dict = attrs
+    elif isinstance(attrs, (list, tuple)):
+        cls_dict = dict((a, attrib()) for a in attrs)
+    else:
+        raise TypeError("attrs argument must be a dict or a list.")
+
+    pre_init = cls_dict.pop("__attrs_pre_init__", None)
+    post_init = cls_dict.pop("__attrs_post_init__", None)
+    user_init = cls_dict.pop("__init__", None)
+
+    body = {}
+    if pre_init is not None:
+        body["__attrs_pre_init__"] = pre_init
+    if post_init is not None:
+        body["__attrs_post_init__"] = post_init
+    if user_init is not None:
+        body["__init__"] = user_init
+
+    type_ = new_class(name, bases, {}, lambda ns: ns.update(body))
+
+    # For pickling to work, the __module__ variable needs to be set to the
+    # frame where the class is created.  Bypass this step in environments where
+    # sys._getframe is not defined (Jython for example) or sys._getframe is not
+    # defined for arguments greater than 0 (IronPython).
+    try:
+        type_.__module__ = sys._getframe(1).f_globals.get(
+            "__name__", "__main__"
+        )
+    except (AttributeError, ValueError):
+        pass
+
+    # We do it here for proper warnings with meaningful stacklevel.
+    cmp = attributes_arguments.pop("cmp", None)
+    (
+        attributes_arguments["eq"],
+        attributes_arguments["order"],
+    ) = _determine_attrs_eq_order(
+        cmp,
+        attributes_arguments.get("eq"),
+        attributes_arguments.get("order"),
+        True,
+    )
+
+    return _attrs(these=cls_dict, **attributes_arguments)(type_)
+
+
+# These are required by within this module so we define them here and merely
+# import into .validators / .converters.
+
+
+@attrs(slots=True, hash=True)
+class _AndValidator(object):
+    """
+    Compose many validators to a single one.
+    """
+
+    _validators = attrib()
+
+    def __call__(self, inst, attr, value):
+        for v in self._validators:
+            v(inst, attr, value)
+
+
+def and_(*validators):
+    """
+    A validator that composes multiple validators into one.
+
+    When called on a value, it runs all wrapped validators.
+
+    :param callables validators: Arbitrary number of validators.
+
+    .. versionadded:: 17.1.0
+    """
+    vals = []
+    for validator in validators:
+        vals.extend(
+            validator._validators
+            if isinstance(validator, _AndValidator)
+            else [validator]
+        )
+
+    return _AndValidator(tuple(vals))
+
+
+def pipe(*converters):
+    """
+    A converter that composes multiple converters into one.
+
+    When called on a value, it runs all wrapped converters, returning the
+    *last* value.
+
+    Type annotations will be inferred from the wrapped converters', if
+    they have any.
+
+    :param callables converters: Arbitrary number of converters.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def pipe_converter(val):
+        for converter in converters:
+            val = converter(val)
+
+        return val
+
+    if not PY2:
+        if not converters:
+            # If the converter list is empty, pipe_converter is the identity.
+            A = typing.TypeVar("A")
+            pipe_converter.__annotations__ = {"val": A, "return": A}
+        else:
+            # Get parameter type.
+            sig = None
+            try:
+                sig = inspect.signature(converters[0])
+            except (ValueError, TypeError):  # inspect failed
+                pass
+            if sig:
+                params = list(sig.parameters.values())
+                if (
+                    params
+                    and params[0].annotation is not inspect.Parameter.empty
+                ):
+                    pipe_converter.__annotations__["val"] = params[
+                        0
+                    ].annotation
+            # Get return type.
+            sig = None
+            try:
+                sig = inspect.signature(converters[-1])
+            except (ValueError, TypeError):  # inspect failed
+                pass
+            if sig and sig.return_annotation is not inspect.Signature().empty:
+                pipe_converter.__annotations__[
+                    "return"
+                ] = sig.return_annotation
+
+    return pipe_converter
diff --git a/lib/spack/external/attr/_next_gen.py b/lib/spack/external/attr/_next_gen.py
new file mode 100644
index 0000000000..fab0af966a
--- /dev/null
+++ b/lib/spack/external/attr/_next_gen.py
@@ -0,0 +1,158 @@
+"""
+These are Python 3.6+-only and keyword-only APIs that call `attr.s` and
+`attr.ib` with different default values.
+"""
+
+from functools import partial
+
+from attr.exceptions import UnannotatedAttributeError
+
+from . import setters
+from ._make import NOTHING, _frozen_setattrs, attrib, attrs
+
+
+def define(
+    maybe_cls=None,
+    *,
+    these=None,
+    repr=None,
+    hash=None,
+    init=None,
+    slots=True,
+    frozen=False,
+    weakref_slot=True,
+    str=False,
+    auto_attribs=None,
+    kw_only=False,
+    cache_hash=False,
+    auto_exc=True,
+    eq=None,
+    order=False,
+    auto_detect=True,
+    getstate_setstate=None,
+    on_setattr=None,
+    field_transformer=None,
+):
+    r"""
+    The only behavioral differences are the handling of the *auto_attribs*
+    option:
+
+    :param Optional[bool] auto_attribs: If set to `True` or `False`, it behaves
+       exactly like `attr.s`. If left `None`, `attr.s` will try to guess:
+
+       1. If any attributes are annotated and no unannotated `attr.ib`\ s
+          are found, it assumes *auto_attribs=True*.
+       2. Otherwise it assumes *auto_attribs=False* and tries to collect
+          `attr.ib`\ s.
+
+    and that mutable classes (``frozen=False``) validate on ``__setattr__``.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def do_it(cls, auto_attribs):
+        return attrs(
+            maybe_cls=cls,
+            these=these,
+            repr=repr,
+            hash=hash,
+            init=init,
+            slots=slots,
+            frozen=frozen,
+            weakref_slot=weakref_slot,
+            str=str,
+            auto_attribs=auto_attribs,
+            kw_only=kw_only,
+            cache_hash=cache_hash,
+            auto_exc=auto_exc,
+            eq=eq,
+            order=order,
+            auto_detect=auto_detect,
+            collect_by_mro=True,
+            getstate_setstate=getstate_setstate,
+            on_setattr=on_setattr,
+            field_transformer=field_transformer,
+        )
+
+    def wrap(cls):
+        """
+        Making this a wrapper ensures this code runs during class creation.
+
+        We also ensure that frozen-ness of classes is inherited.
+        """
+        nonlocal frozen, on_setattr
+
+        had_on_setattr = on_setattr not in (None, setters.NO_OP)
+
+        # By default, mutable classes validate on setattr.
+        if frozen is False and on_setattr is None:
+            on_setattr = setters.validate
+
+        # However, if we subclass a frozen class, we inherit the immutability
+        # and disable on_setattr.
+        for base_cls in cls.__bases__:
+            if base_cls.__setattr__ is _frozen_setattrs:
+                if had_on_setattr:
+                    raise ValueError(
+                        "Frozen classes can't use on_setattr "
+                        "(frozen-ness was inherited)."
+                    )
+
+                on_setattr = setters.NO_OP
+                break
+
+        if auto_attribs is not None:
+            return do_it(cls, auto_attribs)
+
+        try:
+            return do_it(cls, True)
+        except UnannotatedAttributeError:
+            return do_it(cls, False)
+
+    # maybe_cls's type depends on the usage of the decorator.  It's a class
+    # if it's used as `@attrs` but ``None`` if used as `@attrs()`.
+    if maybe_cls is None:
+        return wrap
+    else:
+        return wrap(maybe_cls)
+
+
+mutable = define
+frozen = partial(define, frozen=True, on_setattr=None)
+
+
+def field(
+    *,
+    default=NOTHING,
+    validator=None,
+    repr=True,
+    hash=None,
+    init=True,
+    metadata=None,
+    converter=None,
+    factory=None,
+    kw_only=False,
+    eq=None,
+    order=None,
+    on_setattr=None,
+):
+    """
+    Identical to `attr.ib`, except keyword-only and with some arguments
+    removed.
+
+    .. versionadded:: 20.1.0
+    """
+    return attrib(
+        default=default,
+        validator=validator,
+        repr=repr,
+        hash=hash,
+        init=init,
+        metadata=metadata,
+        converter=converter,
+        factory=factory,
+        kw_only=kw_only,
+        eq=eq,
+        order=order,
+        on_setattr=on_setattr,
+    )
diff --git a/lib/spack/external/attr/_version_info.py b/lib/spack/external/attr/_version_info.py
new file mode 100644
index 0000000000..014e78a1b4
--- /dev/null
+++ b/lib/spack/external/attr/_version_info.py
@@ -0,0 +1,85 @@
+from __future__ import absolute_import, division, print_function
+
+from functools import total_ordering
+
+from ._funcs import astuple
+from ._make import attrib, attrs
+
+
+@total_ordering
+@attrs(eq=False, order=False, slots=True, frozen=True)
+class VersionInfo(object):
+    """
+    A version object that can be compared to tuple of length 1--4:
+
+    >>> attr.VersionInfo(19, 1, 0, "final")  <= (19, 2)
+    True
+    >>> attr.VersionInfo(19, 1, 0, "final") < (19, 1, 1)
+    True
+    >>> vi = attr.VersionInfo(19, 2, 0, "final")
+    >>> vi < (19, 1, 1)
+    False
+    >>> vi < (19,)
+    False
+    >>> vi == (19, 2,)
+    True
+    >>> vi == (19, 2, 1)
+    False
+
+    .. versionadded:: 19.2
+    """
+
+    year = attrib(type=int)
+    minor = attrib(type=int)
+    micro = attrib(type=int)
+    releaselevel = attrib(type=str)
+
+    @classmethod
+    def _from_version_string(cls, s):
+        """
+        Parse *s* and return a _VersionInfo.
+        """
+        v = s.split(".")
+        if len(v) == 3:
+            v.append("final")
+
+        return cls(
+            year=int(v[0]), minor=int(v[1]), micro=int(v[2]), releaselevel=v[3]
+        )
+
+    def _ensure_tuple(self, other):
+        """
+        Ensure *other* is a tuple of a valid length.
+
+        Returns a possibly transformed *other* and ourselves as a tuple of
+        the same length as *other*.
+        """
+
+        if self.__class__ is other.__class__:
+            other = astuple(other)
+
+        if not isinstance(other, tuple):
+            raise NotImplementedError
+
+        if not (1 <= len(other) <= 4):
+            raise NotImplementedError
+
+        return astuple(self)[: len(other)], other
+
+    def __eq__(self, other):
+        try:
+            us, them = self._ensure_tuple(other)
+        except NotImplementedError:
+            return NotImplemented
+
+        return us == them
+
+    def __lt__(self, other):
+        try:
+            us, them = self._ensure_tuple(other)
+        except NotImplementedError:
+            return NotImplemented
+
+        # Since alphabetically "dev0" < "final" < "post1" < "post2", we don't
+        # have to do anything special with releaselevel for now.
+        return us < them
diff --git a/lib/spack/external/attr/converters.py b/lib/spack/external/attr/converters.py
new file mode 100644
index 0000000000..2777db6d0a
--- /dev/null
+++ b/lib/spack/external/attr/converters.py
@@ -0,0 +1,111 @@
+"""
+Commonly useful converters.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from ._compat import PY2
+from ._make import NOTHING, Factory, pipe
+
+
+if not PY2:
+    import inspect
+    import typing
+
+
+__all__ = [
+    "pipe",
+    "optional",
+    "default_if_none",
+]
+
+
+def optional(converter):
+    """
+    A converter that allows an attribute to be optional. An optional attribute
+    is one which can be set to ``None``.
+
+    Type annotations will be inferred from the wrapped converter's, if it
+    has any.
+
+    :param callable converter: the converter that is used for non-``None``
+        values.
+
+    .. versionadded:: 17.1.0
+    """
+
+    def optional_converter(val):
+        if val is None:
+            return None
+        return converter(val)
+
+    if not PY2:
+        sig = None
+        try:
+            sig = inspect.signature(converter)
+        except (ValueError, TypeError):  # inspect failed
+            pass
+        if sig:
+            params = list(sig.parameters.values())
+            if params and params[0].annotation is not inspect.Parameter.empty:
+                optional_converter.__annotations__["val"] = typing.Optional[
+                    params[0].annotation
+                ]
+            if sig.return_annotation is not inspect.Signature.empty:
+                optional_converter.__annotations__["return"] = typing.Optional[
+                    sig.return_annotation
+                ]
+
+    return optional_converter
+
+
+def default_if_none(default=NOTHING, factory=None):
+    """
+    A converter that allows to replace ``None`` values by *default* or the
+    result of *factory*.
+
+    :param default: Value to be used if ``None`` is passed. Passing an instance
+       of `attr.Factory` is supported, however the ``takes_self`` option
+       is *not*.
+    :param callable factory: A callable that takes no parameters whose result
+       is used if ``None`` is passed.
+
+    :raises TypeError: If **neither** *default* or *factory* is passed.
+    :raises TypeError: If **both** *default* and *factory* are passed.
+    :raises ValueError: If an instance of `attr.Factory` is passed with
+       ``takes_self=True``.
+
+    .. versionadded:: 18.2.0
+    """
+    if default is NOTHING and factory is None:
+        raise TypeError("Must pass either `default` or `factory`.")
+
+    if default is not NOTHING and factory is not None:
+        raise TypeError(
+            "Must pass either `default` or `factory` but not both."
+        )
+
+    if factory is not None:
+        default = Factory(factory)
+
+    if isinstance(default, Factory):
+        if default.takes_self:
+            raise ValueError(
+                "`takes_self` is not supported by default_if_none."
+            )
+
+        def default_if_none_converter(val):
+            if val is not None:
+                return val
+
+            return default.factory()
+
+    else:
+
+        def default_if_none_converter(val):
+            if val is not None:
+                return val
+
+            return default
+
+    return default_if_none_converter
diff --git a/lib/spack/external/attr/exceptions.py b/lib/spack/external/attr/exceptions.py
new file mode 100644
index 0000000000..f6f9861bea
--- /dev/null
+++ b/lib/spack/external/attr/exceptions.py
@@ -0,0 +1,92 @@
+from __future__ import absolute_import, division, print_function
+
+
+class FrozenError(AttributeError):
+    """
+    A frozen/immutable instance or attribute have been attempted to be
+    modified.
+
+    It mirrors the behavior of ``namedtuples`` by using the same error message
+    and subclassing `AttributeError`.
+
+    .. versionadded:: 20.1.0
+    """
+
+    msg = "can't set attribute"
+    args = [msg]
+
+
+class FrozenInstanceError(FrozenError):
+    """
+    A frozen instance has been attempted to be modified.
+
+    .. versionadded:: 16.1.0
+    """
+
+
+class FrozenAttributeError(FrozenError):
+    """
+    A frozen attribute has been attempted to be modified.
+
+    .. versionadded:: 20.1.0
+    """
+
+
+class AttrsAttributeNotFoundError(ValueError):
+    """
+    An ``attrs`` function couldn't find an attribute that the user asked for.
+
+    .. versionadded:: 16.2.0
+    """
+
+
+class NotAnAttrsClassError(ValueError):
+    """
+    A non-``attrs`` class has been passed into an ``attrs`` function.
+
+    .. versionadded:: 16.2.0
+    """
+
+
+class DefaultAlreadySetError(RuntimeError):
+    """
+    A default has been set using ``attr.ib()`` and is attempted to be reset
+    using the decorator.
+
+    .. versionadded:: 17.1.0
+    """
+
+
+class UnannotatedAttributeError(RuntimeError):
+    """
+    A class with ``auto_attribs=True`` has an ``attr.ib()`` without a type
+    annotation.
+
+    .. versionadded:: 17.3.0
+    """
+
+
+class PythonTooOldError(RuntimeError):
+    """
+    It was attempted to use an ``attrs`` feature that requires a newer Python
+    version.
+
+    .. versionadded:: 18.2.0
+    """
+
+
+class NotCallableError(TypeError):
+    """
+    A ``attr.ib()`` requiring a callable has been set with a value
+    that is not callable.
+
+    .. versionadded:: 19.2.0
+    """
+
+    def __init__(self, msg, value):
+        super(TypeError, self).__init__(msg, value)
+        self.msg = msg
+        self.value = value
+
+    def __str__(self):
+        return str(self.msg)
diff --git a/lib/spack/external/attr/filters.py b/lib/spack/external/attr/filters.py
new file mode 100644
index 0000000000..dc47e8fa38
--- /dev/null
+++ b/lib/spack/external/attr/filters.py
@@ -0,0 +1,52 @@
+"""
+Commonly useful filters for `attr.asdict`.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from ._compat import isclass
+from ._make import Attribute
+
+
+def _split_what(what):
+    """
+    Returns a tuple of `frozenset`s of classes and attributes.
+    """
+    return (
+        frozenset(cls for cls in what if isclass(cls)),
+        frozenset(cls for cls in what if isinstance(cls, Attribute)),
+    )
+
+
+def include(*what):
+    """
+    Whitelist *what*.
+
+    :param what: What to whitelist.
+    :type what: `list` of `type` or `attr.Attribute`\\ s
+
+    :rtype: `callable`
+    """
+    cls, attrs = _split_what(what)
+
+    def include_(attribute, value):
+        return value.__class__ in cls or attribute in attrs
+
+    return include_
+
+
+def exclude(*what):
+    """
+    Blacklist *what*.
+
+    :param what: What to blacklist.
+    :type what: `list` of classes or `attr.Attribute`\\ s.
+
+    :rtype: `callable`
+    """
+    cls, attrs = _split_what(what)
+
+    def exclude_(attribute, value):
+        return value.__class__ not in cls and attribute not in attrs
+
+    return exclude_
diff --git a/lib/spack/external/attr/setters.py b/lib/spack/external/attr/setters.py
new file mode 100644
index 0000000000..240014b3c1
--- /dev/null
+++ b/lib/spack/external/attr/setters.py
@@ -0,0 +1,77 @@
+"""
+Commonly used hooks for on_setattr.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from . import _config
+from .exceptions import FrozenAttributeError
+
+
+def pipe(*setters):
+    """
+    Run all *setters* and return the return value of the last one.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def wrapped_pipe(instance, attrib, new_value):
+        rv = new_value
+
+        for setter in setters:
+            rv = setter(instance, attrib, rv)
+
+        return rv
+
+    return wrapped_pipe
+
+
+def frozen(_, __, ___):
+    """
+    Prevent an attribute to be modified.
+
+    .. versionadded:: 20.1.0
+    """
+    raise FrozenAttributeError()
+
+
+def validate(instance, attrib, new_value):
+    """
+    Run *attrib*'s validator on *new_value* if it has one.
+
+    .. versionadded:: 20.1.0
+    """
+    if _config._run_validators is False:
+        return new_value
+
+    v = attrib.validator
+    if not v:
+        return new_value
+
+    v(instance, attrib, new_value)
+
+    return new_value
+
+
+def convert(instance, attrib, new_value):
+    """
+    Run *attrib*'s converter -- if it has one --  on *new_value* and return the
+    result.
+
+    .. versionadded:: 20.1.0
+    """
+    c = attrib.converter
+    if c:
+        return c(new_value)
+
+    return new_value
+
+
+NO_OP = object()
+"""
+Sentinel for disabling class-wide *on_setattr* hooks for certain attributes.
+
+Does not work in `pipe` or within lists.
+
+.. versionadded:: 20.1.0
+"""
diff --git a/lib/spack/external/attr/validators.py b/lib/spack/external/attr/validators.py
new file mode 100644
index 0000000000..b9a73054e9
--- /dev/null
+++ b/lib/spack/external/attr/validators.py
@@ -0,0 +1,379 @@
+"""
+Commonly useful validators.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+import re
+
+from ._make import _AndValidator, and_, attrib, attrs
+from .exceptions import NotCallableError
+
+
+__all__ = [
+    "and_",
+    "deep_iterable",
+    "deep_mapping",
+    "in_",
+    "instance_of",
+    "is_callable",
+    "matches_re",
+    "optional",
+    "provides",
+]
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _InstanceOfValidator(object):
+    type = attrib()
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if not isinstance(value, self.type):
+            raise TypeError(
+                "'{name}' must be {type!r} (got {value!r} that is a "
+                "{actual!r}).".format(
+                    name=attr.name,
+                    type=self.type,
+                    actual=value.__class__,
+                    value=value,
+                ),
+                attr,
+                self.type,
+                value,
+            )
+
+    def __repr__(self):
+        return "<instance_of validator for type {type!r}>".format(
+            type=self.type
+        )
+
+
+def instance_of(type):
+    """
+    A validator that raises a `TypeError` if the initializer is called
+    with a wrong type for this particular attribute (checks are performed using
+    `isinstance` therefore it's also valid to pass a tuple of types).
+
+    :param type: The type to check for.
+    :type type: type or tuple of types
+
+    :raises TypeError: With a human readable error message, the attribute
+        (of type `attr.Attribute`), the expected type, and the value it
+        got.
+    """
+    return _InstanceOfValidator(type)
+
+
+@attrs(repr=False, frozen=True, slots=True)
+class _MatchesReValidator(object):
+    regex = attrib()
+    flags = attrib()
+    match_func = attrib()
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if not self.match_func(value):
+            raise ValueError(
+                "'{name}' must match regex {regex!r}"
+                " ({value!r} doesn't)".format(
+                    name=attr.name, regex=self.regex.pattern, value=value
+                ),
+                attr,
+                self.regex,
+                value,
+            )
+
+    def __repr__(self):
+        return "<matches_re validator for pattern {regex!r}>".format(
+            regex=self.regex
+        )
+
+
+def matches_re(regex, flags=0, func=None):
+    r"""
+    A validator that raises `ValueError` if the initializer is called
+    with a string that doesn't match *regex*.
+
+    :param str regex: a regex string to match against
+    :param int flags: flags that will be passed to the underlying re function
+        (default 0)
+    :param callable func: which underlying `re` function to call (options
+        are `re.fullmatch`, `re.search`, `re.match`, default
+        is ``None`` which means either `re.fullmatch` or an emulation of
+        it on Python 2). For performance reasons, they won't be used directly
+        but on a pre-`re.compile`\ ed pattern.
+
+    .. versionadded:: 19.2.0
+    """
+    fullmatch = getattr(re, "fullmatch", None)
+    valid_funcs = (fullmatch, None, re.search, re.match)
+    if func not in valid_funcs:
+        raise ValueError(
+            "'func' must be one of %s."
+            % (
+                ", ".join(
+                    sorted(
+                        e and e.__name__ or "None" for e in set(valid_funcs)
+                    )
+                ),
+            )
+        )
+
+    pattern = re.compile(regex, flags)
+    if func is re.match:
+        match_func = pattern.match
+    elif func is re.search:
+        match_func = pattern.search
+    else:
+        if fullmatch:
+            match_func = pattern.fullmatch
+        else:
+            pattern = re.compile(r"(?:{})\Z".format(regex), flags)
+            match_func = pattern.match
+
+    return _MatchesReValidator(pattern, flags, match_func)
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _ProvidesValidator(object):
+    interface = attrib()
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if not self.interface.providedBy(value):
+            raise TypeError(
+                "'{name}' must provide {interface!r} which {value!r} "
+                "doesn't.".format(
+                    name=attr.name, interface=self.interface, value=value
+                ),
+                attr,
+                self.interface,
+                value,
+            )
+
+    def __repr__(self):
+        return "<provides validator for interface {interface!r}>".format(
+            interface=self.interface
+        )
+
+
+def provides(interface):
+    """
+    A validator that raises a `TypeError` if the initializer is called
+    with an object that does not provide the requested *interface* (checks are
+    performed using ``interface.providedBy(value)`` (see `zope.interface
+    <https://zopeinterface.readthedocs.io/en/latest/>`_).
+
+    :param interface: The interface to check for.
+    :type interface: ``zope.interface.Interface``
+
+    :raises TypeError: With a human readable error message, the attribute
+        (of type `attr.Attribute`), the expected interface, and the
+        value it got.
+    """
+    return _ProvidesValidator(interface)
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _OptionalValidator(object):
+    validator = attrib()
+
+    def __call__(self, inst, attr, value):
+        if value is None:
+            return
+
+        self.validator(inst, attr, value)
+
+    def __repr__(self):
+        return "<optional validator for {what} or None>".format(
+            what=repr(self.validator)
+        )
+
+
+def optional(validator):
+    """
+    A validator that makes an attribute optional.  An optional attribute is one
+    which can be set to ``None`` in addition to satisfying the requirements of
+    the sub-validator.
+
+    :param validator: A validator (or a list of validators) that is used for
+        non-``None`` values.
+    :type validator: callable or `list` of callables.
+
+    .. versionadded:: 15.1.0
+    .. versionchanged:: 17.1.0 *validator* can be a list of validators.
+    """
+    if isinstance(validator, list):
+        return _OptionalValidator(_AndValidator(validator))
+    return _OptionalValidator(validator)
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _InValidator(object):
+    options = attrib()
+
+    def __call__(self, inst, attr, value):
+        try:
+            in_options = value in self.options
+        except TypeError:  # e.g. `1 in "abc"`
+            in_options = False
+
+        if not in_options:
+            raise ValueError(
+                "'{name}' must be in {options!r} (got {value!r})".format(
+                    name=attr.name, options=self.options, value=value
+                )
+            )
+
+    def __repr__(self):
+        return "<in_ validator with options {options!r}>".format(
+            options=self.options
+        )
+
+
+def in_(options):
+    """
+    A validator that raises a `ValueError` if the initializer is called
+    with a value that does not belong in the options provided.  The check is
+    performed using ``value in options``.
+
+    :param options: Allowed options.
+    :type options: list, tuple, `enum.Enum`, ...
+
+    :raises ValueError: With a human readable error message, the attribute (of
+       type `attr.Attribute`), the expected options, and the value it
+       got.
+
+    .. versionadded:: 17.1.0
+    """
+    return _InValidator(options)
+
+
+@attrs(repr=False, slots=False, hash=True)
+class _IsCallableValidator(object):
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if not callable(value):
+            message = (
+                "'{name}' must be callable "
+                "(got {value!r} that is a {actual!r})."
+            )
+            raise NotCallableError(
+                msg=message.format(
+                    name=attr.name, value=value, actual=value.__class__
+                ),
+                value=value,
+            )
+
+    def __repr__(self):
+        return "<is_callable validator>"
+
+
+def is_callable():
+    """
+    A validator that raises a `attr.exceptions.NotCallableError` if the
+    initializer is called with a value for this particular attribute
+    that is not callable.
+
+    .. versionadded:: 19.1.0
+
+    :raises `attr.exceptions.NotCallableError`: With a human readable error
+        message containing the attribute (`attr.Attribute`) name,
+        and the value it got.
+    """
+    return _IsCallableValidator()
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _DeepIterable(object):
+    member_validator = attrib(validator=is_callable())
+    iterable_validator = attrib(
+        default=None, validator=optional(is_callable())
+    )
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if self.iterable_validator is not None:
+            self.iterable_validator(inst, attr, value)
+
+        for member in value:
+            self.member_validator(inst, attr, member)
+
+    def __repr__(self):
+        iterable_identifier = (
+            ""
+            if self.iterable_validator is None
+            else " {iterable!r}".format(iterable=self.iterable_validator)
+        )
+        return (
+            "<deep_iterable validator for{iterable_identifier}"
+            " iterables of {member!r}>"
+        ).format(
+            iterable_identifier=iterable_identifier,
+            member=self.member_validator,
+        )
+
+
+def deep_iterable(member_validator, iterable_validator=None):
+    """
+    A validator that performs deep validation of an iterable.
+
+    :param member_validator: Validator to apply to iterable members
+    :param iterable_validator: Validator to apply to iterable itself
+        (optional)
+
+    .. versionadded:: 19.1.0
+
+    :raises TypeError: if any sub-validators fail
+    """
+    return _DeepIterable(member_validator, iterable_validator)
+
+
+@attrs(repr=False, slots=True, hash=True)
+class _DeepMapping(object):
+    key_validator = attrib(validator=is_callable())
+    value_validator = attrib(validator=is_callable())
+    mapping_validator = attrib(default=None, validator=optional(is_callable()))
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if self.mapping_validator is not None:
+            self.mapping_validator(inst, attr, value)
+
+        for key in value:
+            self.key_validator(inst, attr, key)
+            self.value_validator(inst, attr, value[key])
+
+    def __repr__(self):
+        return (
+            "<deep_mapping validator for objects mapping {key!r} to {value!r}>"
+        ).format(key=self.key_validator, value=self.value_validator)
+
+
+def deep_mapping(key_validator, value_validator, mapping_validator=None):
+    """
+    A validator that performs deep validation of a dictionary.
+
+    :param key_validator: Validator to apply to dictionary keys
+    :param value_validator: Validator to apply to dictionary values
+    :param mapping_validator: Validator to apply to top-level mapping
+        attribute (optional)
+
+    .. versionadded:: 19.1.0
+
+    :raises TypeError: if any sub-validators fail
+    """
+    return _DeepMapping(key_validator, value_validator, mapping_validator)