docs: avoid errors by using type hints instead of doc types (#34707)

There are a number of places in our docstrings where we write "list of X" as the type, even though napoleon doesn't actually support this. It ends up causing warnings when generating docs.

Now that we require Python 3, we don't have to rely on type hints in docs -- we can just use Python type hints and omit the types of args and return values from docstrings.

We should probably do this for all types in docstrings eventually, but this PR focuses on the ones that generate warnings during doc builds.

Some `mypy` annoyances we should consider in the future:
1. Adding some of these type annotations gets you:
    ```
    note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
    ```
   because they are in unannotated functions (like constructors where we don't really need any annotations).
   You can silence these with `disable_error_code = "annotation-unchecked"` in `pyproject.toml`
2. Right now we support running `mypy` in Python `3.6`.  That means we have to support `mypy` `.971`, which does not support `disable_error_code = "annotation-unchecked"`, so I just filter `[annotation-unchecked]` lines out in `spack style`.
3. I would rather just turn on `check_untyped_defs` and get more `mypy` coverage everywhere, but that will require about 1,000 fixes.  We should probably do that eventually.
4. We could also consider only running `mypy` on newer python versions.  This is not easy to do while supporting `3.6`, because you have to use `if TYPE_CHECKING` for a lot of things to ensure that 3.6 still parses correctly.  If we only supported `3.7` and above we could use [`from __future__ import annotations`](https://mypy.readthedocs.io/en/stable/runtime_troubles.html#future-annotations-import-pep-563), but we have to support 3.6 for now. Sigh.

- [x] Convert a number of docstring types to Python type hints
- [x] Get rid of "list of" wherever it appears

13 files changed, 133 insertions, 83 deletions

diff --git a/lib/spack/spack/bootstrap/core.py b/lib/spack/spack/bootstrap/core.py
index e8cb429fa8..f4b435deba 100644
--- a/lib/spack/spack/bootstrap/core.py
+++ b/lib/spack/spack/bootstrap/core.py
@@ -29,6 +29,7 @@ import os
 import os.path
 import sys
 import uuid
+from typing import Callable, List, Optional
 
 from llnl.util import tty
 from llnl.util.lang import GroupedExceptionHandler
@@ -70,12 +71,12 @@ IS_WINDOWS = sys.platform == "win32"
 _bootstrap_methods = {}
 
 
-def bootstrapper(bootstrapper_type):
+def bootstrapper(bootstrapper_type: str):
     """Decorator to register classes implementing bootstrapping
     methods.
 
     Args:
-        bootstrapper_type (str): string identifying the class
+        bootstrapper_type: string identifying the class
     """
 
     def _register(cls):
@@ -119,26 +120,26 @@ class Bootstrapper:
             self.config_scope_name, {"mirrors:": {self.name: self.mirror_url}}
         )
 
-    def try_import(self, module: str, abstract_spec_str: str):  # pylint: disable=unused-argument
+    def try_import(self, module: str, abstract_spec_str: str) -> bool:
         """Try to import a Python module from a spec satisfying the abstract spec
         passed as argument.
 
         Args:
-            module (str): Python module name to try importing
-            abstract_spec_str (str): abstract spec that can provide the Python module
+            module: Python module name to try importing
+            abstract_spec_str: abstract spec that can provide the Python module
 
         Return:
             True if the Python module could be imported, False otherwise
         """
         return False
 
-    def try_search_path(self, executables, abstract_spec_str):  # pylint: disable=unused-argument
+    def try_search_path(self, executables: List[str], abstract_spec_str: str) -> bool:
         """Try to search some executables in the prefix of specs satisfying the abstract
         spec passed as argument.
 
         Args:
-            executables (list of str): executables to be found
-            abstract_spec_str (str): abstract spec that can provide the Python module
+            executables: executables to be found
+            abstract_spec_str: abstract spec that can provide the Python module
 
         Return:
             True if the executables are found, False otherwise
@@ -347,7 +348,7 @@ def source_is_enabled_or_raise(conf):
         raise ValueError("source is not trusted")
 
 
-def ensure_module_importable_or_raise(module, abstract_spec=None):
+def ensure_module_importable_or_raise(module: str, abstract_spec: Optional[str] = None):
     """Make the requested module available for import, or raise.
 
     This function tries to import a Python module in the current interpreter
@@ -357,8 +358,8 @@ def ensure_module_importable_or_raise(module, abstract_spec=None):
     on first success.
 
     Args:
-        module (str): module to be imported in the current interpreter
-        abstract_spec (str): abstract spec that might provide the module. If not
+        module: module to be imported in the current interpreter
+        abstract_spec: abstract spec that might provide the module. If not
             given it defaults to "module"
 
     Raises:
@@ -395,7 +396,11 @@ def ensure_module_importable_or_raise(module, abstract_spec=None):
     raise ImportError(msg)
 
 
-def ensure_executables_in_path_or_raise(executables, abstract_spec, cmd_check=None):
+def ensure_executables_in_path_or_raise(
+    executables: list,
+    abstract_spec: str,
+    cmd_check: Optional[Callable[[spack.util.executable.Executable], bool]] = None,
+):
     """Ensure that some executables are in path or raise.
 
     Args:
@@ -555,11 +560,11 @@ def all_core_root_specs():
     return [clingo_root_spec(), gnupg_root_spec(), patchelf_root_spec()]
 
 
-def bootstrapping_sources(scope=None):
+def bootstrapping_sources(scope: Optional[str] = None):
     """Return the list of configured sources of software for bootstrapping Spack
 
     Args:
-        scope (str or None): if a valid configuration scope is given, return the
+        scope: if a valid configuration scope is given, return the
             list only from that scope
     """
     source_configs = spack.config.get("bootstrap:sources", default=None, scope=scope)
diff --git a/lib/spack/spack/build_systems/_checks.py b/lib/spack/spack/build_systems/_checks.py
index c041098239..4422552b10 100644
--- a/lib/spack/spack/build_systems/_checks.py
+++ b/lib/spack/spack/build_systems/_checks.py
@@ -3,23 +3,25 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import os
+from typing import List
 
 import llnl.util.lang
 
 import spack.builder
 import spack.installer
 import spack.relocate
+import spack.spec
 import spack.store
 
 
-def sanity_check_prefix(builder):
+def sanity_check_prefix(builder: spack.builder.Builder):
     """Check that specific directories and files are created after installation.
 
     The files to be checked are in the ``sanity_check_is_file`` attribute of the
     package object, while the directories are in the ``sanity_check_is_dir``.
 
     Args:
-        builder (spack.builder.Builder): builder that installed the package
+        builder: builder that installed the package
     """
     pkg = builder.pkg
 
@@ -43,7 +45,7 @@ def sanity_check_prefix(builder):
         raise spack.installer.InstallError(msg.format(pkg.name))
 
 
-def apply_macos_rpath_fixups(builder):
+def apply_macos_rpath_fixups(builder: spack.builder.Builder):
     """On Darwin, make installed libraries more easily relocatable.
 
     Some build systems (handrolled, autotools, makefiles) can set their own
@@ -55,20 +57,22 @@ def apply_macos_rpath_fixups(builder):
     packages) that do not install relocatable libraries by default.
 
     Args:
-        builder (spack.builder.Builder): builder that installed the package
+        builder: builder that installed the package
     """
     spack.relocate.fixup_macos_rpaths(builder.spec)
 
 
-def ensure_build_dependencies_or_raise(spec, dependencies, error_msg):
+def ensure_build_dependencies_or_raise(
+    spec: spack.spec.Spec, dependencies: List[spack.spec.Spec], error_msg: str
+):
     """Ensure that some build dependencies are present in the concrete spec.
 
     If not, raise a RuntimeError with a helpful error message.
 
     Args:
-        spec (spack.spec.Spec): concrete spec to be checked.
-        dependencies (list of spack.spec.Spec): list of abstract specs to be satisfied
-        error_msg (str): brief error message to be prepended to a longer description
+        spec: concrete spec to be checked.
+        dependencies: list of abstract specs to be satisfied
+        error_msg: brief error message to be prepended to a longer description
 
     Raises:
           RuntimeError: when the required build dependencies are not found
@@ -83,7 +87,9 @@ def ensure_build_dependencies_or_raise(spec, dependencies, error_msg):
     # Raise an exception on missing deps.
     msg = (
         "{0}: missing dependencies: {1}.\n\nPlease add "
-        "the following lines to the package:\n\n".format(error_msg, ", ".join(missing_deps))
+        "the following lines to the package:\n\n".format(
+            error_msg, ", ".join(str(d) for d in missing_deps)
+        )
     )
 
     for dep in missing_deps:
@@ -95,21 +101,21 @@ def ensure_build_dependencies_or_raise(spec, dependencies, error_msg):
     raise RuntimeError(msg)
 
 
-def execute_build_time_tests(builder):
+def execute_build_time_tests(builder: spack.builder.Builder):
     """Execute the build-time tests prescribed by builder.
 
     Args:
-        builder (Builder): builder prescribing the test callbacks. The name of the callbacks is
+        builder: builder prescribing the test callbacks. The name of the callbacks is
             stored as a list of strings in the ``build_time_test_callbacks`` attribute.
     """
     builder.pkg.run_test_callbacks(builder, builder.build_time_test_callbacks, "build")
 
 
-def execute_install_time_tests(builder):
+def execute_install_time_tests(builder: spack.builder.Builder):
     """Execute the install-time tests prescribed by builder.
 
     Args:
-        builder (Builder): builder prescribing the test callbacks. The name of the callbacks is
+        builder: builder prescribing the test callbacks. The name of the callbacks is
             stored as a list of strings in the ``install_time_test_callbacks`` attribute.
     """
     builder.pkg.run_test_callbacks(builder, builder.install_time_test_callbacks, "install")
diff --git a/lib/spack/spack/builder.py b/lib/spack/spack/builder.py
index ae4f4f2fc2..211d7e218e 100644
--- a/lib/spack/spack/builder.py
+++ b/lib/spack/spack/builder.py
@@ -478,6 +478,10 @@ class Builder(collections.abc.Sequence, metaclass=BuilderMeta):
     legacy_methods: Tuple[str, ...] = ()
     legacy_attributes: Tuple[str, ...] = ()
 
+    # type hints for some of the legacy methods
+    build_time_test_callbacks: List[str]
+    install_time_test_callbacks: List[str]
+
     #: List of glob expressions. Each expression must either be
     #: absolute or relative to the package source path.
     #: Matching artifacts found at the end of the build process will be
diff --git a/lib/spack/spack/cmd/style.py b/lib/spack/spack/cmd/style.py
index f090819879..922e81ef8b 100644
--- a/lib/spack/spack/cmd/style.py
+++ b/lib/spack/spack/cmd/style.py
@@ -48,6 +48,13 @@ tool_names = [
 #: tools we run in spack style
 tools = {}
 
+#: warnings to ignore in mypy
+mypy_ignores = [
+    # same as `disable_error_code = "annotation-unchecked"` in pyproject.toml, which
+    # doesn't exist in mypy 0.971 for Python 3.6
+    "[annotation-unchecked]",
+]
+
 
 def is_package(f):
     """Whether flake8 should consider a file as a core file or a package.
@@ -211,6 +218,10 @@ def rewrite_and_print_output(
     for line in output.split("\n"):
         if not line:
             continue
+        if any(ignore in line for ignore in mypy_ignores):
+            # some mypy annotations can't be disabled in older mypys (e.g. .971, which
+            # is the only mypy that supports python 3.6), so we filter them here.
+            continue
         if not args.root_relative and re_obj:
             line = re_obj.sub(translate, line)
         print(line)
diff --git a/lib/spack/spack/compiler.py b/lib/spack/spack/compiler.py
index 52c2db8c79..d985f21434 100644
--- a/lib/spack/spack/compiler.py
+++ b/lib/spack/spack/compiler.py
@@ -156,14 +156,14 @@ def _parse_link_paths(string):
 
 
 @system_path_filter
-def _parse_non_system_link_dirs(string):
+def _parse_non_system_link_dirs(string: str) -> List[str]:
     """Parses link paths out of compiler debug output.
 
     Args:
-        string (str): compiler debug output as a string
+        string: compiler debug output as a string
 
     Returns:
-        (list of str): implicit link paths parsed from the compiler output
+        Implicit link paths parsed from the compiler output
     """
     link_dirs = _parse_link_paths(string)
 
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 3bfd0b18ed..46cf0232d0 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -36,7 +36,7 @@ import os
 import re
 import sys
 from contextlib import contextmanager
-from typing import List
+from typing import Dict, List, Optional
 
 import ruamel.yaml as yaml
 from ruamel.yaml.error import MarkedYAMLError
@@ -391,41 +391,44 @@ class Configuration(object):
     This class makes it easy to add a new scope on top of an existing one.
     """
 
-    def __init__(self, *scopes):
+    # convert to typing.OrderedDict when we drop 3.6, or OrderedDict when we reach 3.9
+    scopes: Dict[str, ConfigScope]
+
+    def __init__(self, *scopes: ConfigScope):
         """Initialize a configuration with an initial list of scopes.
 
         Args:
-            scopes (list of ConfigScope): list of scopes to add to this
+            scopes: list of scopes to add to this
                 Configuration, ordered from lowest to highest precedence
 
         """
         self.scopes = collections.OrderedDict()
         for scope in scopes:
             self.push_scope(scope)
-        self.format_updates = collections.defaultdict(list)
+        self.format_updates: Dict[str, List[str]] = collections.defaultdict(list)
 
     @_config_mutator
-    def push_scope(self, scope):
+    def push_scope(self, scope: ConfigScope):
         """Add a higher precedence scope to the Configuration."""
         tty.debug("[CONFIGURATION: PUSH SCOPE]: {}".format(str(scope)), level=2)
         self.scopes[scope.name] = scope
 
     @_config_mutator
-    def pop_scope(self):
+    def pop_scope(self) -> ConfigScope:
         """Remove the highest precedence scope and return it."""
-        name, scope = self.scopes.popitem(last=True)
+        name, scope = self.scopes.popitem(last=True)  # type: ignore[call-arg]
         tty.debug("[CONFIGURATION: POP SCOPE]: {}".format(str(scope)), level=2)
         return scope
 
     @_config_mutator
-    def remove_scope(self, scope_name):
+    def remove_scope(self, scope_name: str) -> Optional[ConfigScope]:
         """Remove scope by name; has no effect when ``scope_name`` does not exist"""
         scope = self.scopes.pop(scope_name, None)
         tty.debug("[CONFIGURATION: POP SCOPE]: {}".format(str(scope)), level=2)
         return scope
 
     @property
-    def file_scopes(self):
+    def file_scopes(self) -> List[ConfigScope]:
         """List of writable scopes with an associated file."""
         return [
             s
@@ -433,21 +436,21 @@ class Configuration(object):
             if (type(s) == ConfigScope or type(s) == SingleFileScope)
         ]
 
-    def highest_precedence_scope(self):
+    def highest_precedence_scope(self) -> ConfigScope:
         """Non-internal scope with highest precedence."""
-        return next(reversed(self.file_scopes), None)
+        return next(reversed(self.file_scopes))
 
-    def highest_precedence_non_platform_scope(self):
+    def highest_precedence_non_platform_scope(self) -> ConfigScope:
         """Non-internal non-platform scope with highest precedence
 
         Platform-specific scopes are of the form scope/platform"""
         generator = reversed(self.file_scopes)
-        highest = next(generator, None)
+        highest = next(generator)
         while highest and highest.is_platform_dependent:
-            highest = next(generator, None)
+            highest = next(generator)
         return highest
 
-    def matching_scopes(self, reg_expr):
+    def matching_scopes(self, reg_expr) -> List[ConfigScope]:
         """
         List of all scopes whose names match the provided regular expression.
 
@@ -456,7 +459,7 @@ class Configuration(object):
         """
         return [s for s in self.scopes.values() if re.search(reg_expr, s.name)]
 
-    def _validate_scope(self, scope):
+    def _validate_scope(self, scope: Optional[str]) -> ConfigScope:
         """Ensure that scope is valid in this configuration.
 
         This should be used by routines in ``config.py`` to validate
@@ -481,7 +484,7 @@ class Configuration(object):
                 "Invalid config scope: '%s'.  Must be one of %s" % (scope, self.scopes.keys())
             )
 
-    def get_config_filename(self, scope, section):
+    def get_config_filename(self, scope, section) -> str:
         """For some scope and section, get the name of the configuration file."""
         scope = self._validate_scope(scope)
         return scope.get_section_filename(section)
@@ -495,7 +498,9 @@ class Configuration(object):
             scope.clear()
 
     @_config_mutator
-    def update_config(self, section, update_data, scope=None, force=False):
+    def update_config(
+        self, section: str, update_data: Dict, scope: Optional[str] = None, force: bool = False
+    ):
         """Update the configuration file for a particular scope.
 
         Overwrites contents of a section in a scope with update_data,
@@ -1315,14 +1320,15 @@ def raw_github_gitlab_url(url):
     return url
 
 
-def collect_urls(base_url):
+def collect_urls(base_url: str) -> list:
     """Return a list of configuration URLs.
 
     Arguments:
-        base_url (str): URL for a configuration (yaml) file or a directory
+        base_url: URL for a configuration (yaml) file or a directory
             containing yaml file(s)
 
-    Returns: (list) list of configuration file(s) or empty list if none
+    Returns:
+        List of configuration file(s) or empty list if none
     """
     if not base_url:
         return []
@@ -1337,20 +1343,21 @@ def collect_urls(base_url):
     return [link for link in links if link.endswith(extension)]
 
 
-def fetch_remote_configs(url, dest_dir, skip_existing=True):
+def fetch_remote_configs(url: str, dest_dir: str, skip_existing: bool = True) -> str:
     """Retrieve configuration file(s) at the specified URL.
 
     Arguments:
-        url (str): URL for a configuration (yaml) file or a directory containing
+        url: URL for a configuration (yaml) file or a directory containing
             yaml file(s)
-        dest_dir (str): destination directory
-        skip_existing (bool): Skip files that already exist in dest_dir if
+        dest_dir: destination directory
+        skip_existing: Skip files that already exist in dest_dir if
             ``True``; otherwise, replace those files
 
-    Returns: (str) path to the corresponding file if URL is or contains a
-       single file and it is the only file in the destination directory or
-       the root (dest_dir) directory if multiple configuration files exist
-       or are retrieved.
+    Returns:
+        Path to the corresponding file if URL is or contains a
+        single file and it is the only file in the destination directory or
+        the root (dest_dir) directory if multiple configuration files exist
+        or are retrieved.
     """
 
     def _fetch_file(url):
diff --git a/lib/spack/spack/graph.py b/lib/spack/spack/graph.py
index 481b699390..3187334b90 100644
--- a/lib/spack/spack/graph.py
+++ b/lib/spack/spack/graph.py
@@ -555,9 +555,9 @@ def static_graph_dot(
     """Static DOT graph with edges to all possible dependencies.
 
     Args:
-        specs (list of spack.spec.Spec): abstract specs to be represented
-        deptype (str or tuple): dependency types to consider
-        out (TextIO or None): optional output stream. If None sys.stdout is used
+        specs: abstract specs to be represented
+        deptype: dependency types to consider
+        out: optional output stream. If None sys.stdout is used
     """
     out = out or sys.stdout
     builder = StaticDag()
@@ -575,10 +575,10 @@ def graph_dot(
     """DOT graph of the concrete specs passed as input.
 
     Args:
-        specs (list of spack.spec.Spec): specs to be represented
-        builder (DotGraphBuilder): builder to use to render the graph
-        deptype (str or tuple): dependency types to consider
-        out (TextIO or None): optional output stream. If None sys.stdout is used
+        specs: specs to be represented
+        builder: builder to use to render the graph
+        deptype: dependency types to consider
+        out: optional output stream. If None sys.stdout is used
     """
     if not specs:
         raise ValueError("Must provide specs to graph_dot")
diff --git a/lib/spack/spack/provider_index.py b/lib/spack/spack/provider_index.py
index 1ffd19236b..7b2d99d6c4 100644
--- a/lib/spack/spack/provider_index.py
+++ b/lib/spack/spack/provider_index.py
@@ -4,8 +4,10 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 """Classes and functions to manage providers of virtual dependencies"""
 import itertools
+from typing import Dict, List, Optional, Set
 
 import spack.error
+import spack.spec
 import spack.util.spack_json as sjson
 
 
@@ -53,7 +55,7 @@ class _IndexBase(object):
     #: Calling providers_for(spec) will find specs that provide a
     #: matching implementation of MPI. Derived class need to construct
     #: this attribute according to the semantics above.
-    providers = None
+    providers: Dict[str, Dict[str, Set[str]]]
 
     def providers_for(self, virtual_spec):
         """Return a list of specs of all packages that provide virtual
@@ -127,11 +129,16 @@ class _IndexBase(object):
 
 
 class ProviderIndex(_IndexBase):
-    def __init__(self, repository, specs=None, restrict=False):
+    def __init__(
+        self,
+        repository: "spack.repo.RepoType",
+        specs: Optional[List["spack.spec.Spec"]] = None,
+        restrict: bool = False,
+    ):
         """Provider index based on a single mapping of providers.
 
         Args:
-            specs (list of specs): if provided, will call update on each
+            specs: if provided, will call update on each
                 single spec to initialize this provider index.
 
             restrict: "restricts" values to the verbatim input specs; do not
diff --git a/lib/spack/spack/repo.py b/lib/spack/spack/repo.py
index 9386e424c9..ac8a598ded 100644
--- a/lib/spack/spack/repo.py
+++ b/lib/spack/spack/repo.py
@@ -24,7 +24,7 @@ import sys
 import traceback
 import types
 import uuid
-from typing import Dict
+from typing import Dict, Union
 
 import ruamel.yaml as yaml
 
@@ -1286,6 +1286,9 @@ class Repo(object):
         return self.exists(pkg_name)
 
 
+RepoType = Union[Repo, RepoPath]
+
+
 def create_repo(root, namespace=None):
     """Create a new repository in root with the specified namespace.
 
diff --git a/lib/spack/spack/report.py b/lib/spack/spack/report.py
index 8d4fb2b81d..bc7c4f3ac8 100644
--- a/lib/spack/spack/report.py
+++ b/lib/spack/spack/report.py
@@ -3,12 +3,14 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 """Tools to produce reports of spec installations"""
+import argparse
 import codecs
 import collections
 import functools
 import os
 import time
 import traceback
+from typing import Any, Callable, Dict, List, Type
 
 import llnl.util.lang
 
@@ -51,12 +53,16 @@ class InfoCollector(object):
     attribute once exited, and it's organized as a list where
     each item represents the installation of one of the spec.
 
-    Args:
-        specs (list of Spec): specs whose install information will
-           be recorded
     """
 
-    def __init__(self, wrap_class, do_fn, specs, dir):
+    wrap_class: Type
+    do_fn: str
+    _backup_do_fn: Callable
+    input_specs: List["spack.spec.Spec"]
+    specs: List[Dict[str, Any]]
+    dir: str
+
+    def __init__(self, wrap_class: Type, do_fn: str, specs: List["spack.spec.Spec"], dir: str):
         #: Class for which to wrap a function
         self.wrap_class = wrap_class
         #: Action to be reported on
@@ -234,14 +240,14 @@ class collect_info(object):
     Args:
         class: class on which to wrap a function
         function: function to wrap
-        format_name (str or None): one of the supported formats
-        args (dict): args passed to function
+        format_name: one of the supported formats
+        args: args passed to function
 
     Raises:
         ValueError: when ``format_name`` is not in ``valid_formats``
     """
 
-    def __init__(self, cls, function, format_name, args):
+    def __init__(self, cls: Type, function: str, format_name: str, args: argparse.Namespace):
         self.cls = cls
         self.function = function
         self.filename = None
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 012a75c89c..85cfa70ca3 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -4868,7 +4868,7 @@ class Spec(object):
         return Spec.from_dict, (self.to_dict(hash=ht.process_hash),)
 
 
-def merge_abstract_anonymous_specs(*abstract_specs):
+def merge_abstract_anonymous_specs(*abstract_specs: Spec):
     """Merge the abstracts specs passed as input and return the result.
 
     The root specs must be anonymous, and it's duty of the caller to ensure that.
@@ -4877,7 +4877,7 @@ def merge_abstract_anonymous_specs(*abstract_specs):
     it doesn't try to resolve virtual dependencies.
 
     Args:
-        *abstract_specs (list of Specs): abstract specs to be merged
+        *abstract_specs: abstract specs to be merged
     """
     merged_spec = spack.spec.Spec()
     for current_spec_constraint in abstract_specs:
diff --git a/lib/spack/spack/test/cmd/style.py b/lib/spack/spack/test/cmd/style.py
index 1a925f5722..a643307a3b 100644
--- a/lib/spack/spack/test/cmd/style.py
+++ b/lib/spack/spack/test/cmd/style.py
@@ -213,7 +213,7 @@ def test_fix_style(external_style_root):
 @pytest.mark.skipif(not which("isort"), reason="isort is not installed.")
 @pytest.mark.skipif(not which("mypy"), reason="mypy is not installed.")
 @pytest.mark.skipif(not which("black"), reason="black is not installed.")
-def test_external_root(external_style_root):
+def test_external_root(external_style_root, capfd):
     """Ensure we can run in a separate root directory w/o configuration files."""
     tmpdir, py_file = external_style_root
 
diff --git a/lib/spack/spack/util/timer.py b/lib/spack/spack/util/timer.py
index 840bfb3c0d..d0b6927b65 100644
--- a/lib/spack/spack/util/timer.py
+++ b/lib/spack/spack/util/timer.py
@@ -9,16 +9,17 @@
 a stack trace and drops the user into an interpreter.
 
 """
+import collections
 import sys
 import time
-from collections import OrderedDict, namedtuple
 from contextlib import contextmanager
+from typing import Dict
 
 from llnl.util.lang import pretty_seconds_formatter
 
 import spack.util.spack_json as sjson
 
-Interval = namedtuple("Interval", ("begin", "end"))
+Interval = collections.namedtuple("Interval", ("begin", "end"))
 
 #: name for the global timer (used in start(), stop(), duration() without arguments)
 global_timer_name = "_global"
@@ -65,7 +66,7 @@ class Timer(object):
             now: function that gives the seconds since e.g. epoch
         """
         self._now = now
-        self._timers: OrderedDict[str, Interval] = OrderedDict()
+        self._timers: Dict[str, Interval] = collections.OrderedDict()
 
         # _global is the overal timer since the instance was created
         self._timers[global_timer_name] = Interval(self._now(), end=None)