Python command, libraries, and headers (#3367)

## Motivation

Python installations are both important and unfortunately inconsistent. Depending on the Python version, OS, and the strength of the Earth's magnetic field when it was installed, the name of the Python executable, directory containing its libraries, library names, and the directory containing its headers can vary drastically. 

I originally got into this mess with #3274, where I discovered that Boost could not be built with Python 3 because the executable is called `python3` and we were telling it to use `python`. I got deeper into this mess when I started hacking on #3140, where I discovered just how difficult it is to find the location and name of the Python libraries and headers.

Currently, half of the packages that depend on Python and need to know this information jump through hoops to determine the correct information. The other half are hard-coded to use `python`, `spec['python'].prefix.lib`, and `spec['python'].prefix.include`. Obviously, none of these packages would work for Python 3, and there's no reason to duplicate the effort. The Python package itself should contain all of the information necessary to use it properly. This is in line with the recent work by @alalazo and @davydden with respect to `spec['blas'].libs` and friends.

## Prefix

For most packages in Spack, we assume that the installation directory is `spec['python'].prefix`. This generally works for anything installed with Spack, but gets complicated when we include external packages. Python is a commonly used external package (it needs to be installed just to run Spack). If it was installed with Homebrew, `which python` would return `/usr/local/bin/python`, and most users would erroneously assume that `/usr/local` is the installation directory. If you peruse through #2173, you'll immediately see why this is not the case. Homebrew actually installs Python in `/usr/local/Cellar/python/2.7.12_2` and symlinks the executable to `/usr/local/bin/python`. `PYTHONHOME` (and presumably most things that need to know where Python is installed) needs to be set to the actual installation directory, not `/usr/local`.

Normally I would say, "sounds like user error, make sure to use the real installation directory in your `packages.yaml`". But I think we can make a special case for Python. That's what we decided in #2173 anyway. If we change our minds, I would be more than happy to simplify things.

To solve this problem, I created a `spec['python'].home` attribute that works the same way as `spec['python'].prefix` but queries Python to figure out where it was actually installed. @tgamblin Is there any way to overwrite `spec['python'].prefix`? I think it's currently immutable.

## Command

In general, Python 2 comes with both `python` and `python2` commands, while Python 3 only comes with a `python3` command. But this is up to the OS developers. For example, `/usr/bin/python` on Gentoo is actually Python 3. Worse yet, if someone is using an externally installed Python, all 3 commands may exist in the same directory! Here's what I'm thinking:

If the spec is for Python 3, try searching for the `python3` command.
If the spec is for Python 2, try searching for the `python2` command.
If neither are found, try searching for the `python` command.

## Libraries

Spack installs Python libraries in `spec['python'].prefix.lib`. Except on openSUSE 13, where it installs to `spec['python'].prefix.lib64` (see #2295 and #2253). On my CentOS 6 machine, the Python libraries are installed in `/usr/lib64`. Both need to work.

The libraries themselves change name depending on OS and Python version. For Python 2.7 on macOS, I'm seeing:
```
lib/libpython2.7.dylib
```
For Python 3.6 on CentOS 6, I'm seeing:
```
lib/libpython3.so
lib/libpython3.6m.so.1.0
lib/libpython3.6m.so -> lib/libpython3.6m.so.1.0
```
Notice the `m` after the version number. Yeah, that's a thing.

## Headers

In Python 2.7, I'm seeing:
```
include/python2.7/pyconfig.h
```
In Python 3.6, I'm seeing:
```
include/python3.6m/pyconfig.h
```
It looks like all Python 3 installations have this `m`. Tested with Python 3.2 and 3.6 on macOS and CentOS 6

Spack has really nice support for libraries (`find_libraries` and `LibraryList`), but nothing for headers. Fixed.

44 files changed, 876 insertions, 371 deletions

diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index 035f9a13ff..25dc747499 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -25,27 +25,32 @@
 import collections
 import errno
 import fileinput
+import fnmatch
 import glob
 import numbers
 import os
 import re
 import shutil
+import six
 import stat
 import subprocess
 import sys
 from contextlib import contextmanager
 
-import llnl.util.tty as tty
+from llnl.util import tty
 from llnl.util.lang import dedupe
 
 __all__ = [
     'FileFilter',
+    'HeaderList',
     'LibraryList',
     'ancestor',
     'can_access',
     'change_sed_delimiter',
     'copy_mode',
     'filter_file',
+    'find',
+    'find_headers',
     'find_libraries',
     'find_system_libraries',
     'fix_darwin_install_name',
@@ -66,25 +71,32 @@ __all__ = [
     'touchp',
     'traverse_tree',
     'unset_executable_mode',
-    'working_dir']
+    'working_dir'
+]
 
 
 def filter_file(regex, repl, *filenames, **kwargs):
-    """Like sed, but uses python regular expressions.
-
-       Filters every line of each file through regex and replaces the file
-       with a filtered version.  Preserves mode of filtered files.
-
-       As with re.sub, ``repl`` can be either a string or a callable.
-       If it is a callable, it is passed the match object and should
-       return a suitable replacement string.  If it is a string, it
-       can contain ``\1``, ``\2``, etc. to represent back-substitution
-       as sed would allow.
-
-       Keyword Options:
-         string[=False]         If True, treat regex as a plain string.
-         backup[=True]          Make backup file(s) suffixed with ~
-         ignore_absent[=False]  Ignore any files that don't exist.
+    r"""Like sed, but uses python regular expressions.
+
+    Filters every line of each file through regex and replaces the file
+    with a filtered version.  Preserves mode of filtered files.
+
+    As with re.sub, ``repl`` can be either a string or a callable.
+    If it is a callable, it is passed the match object and should
+    return a suitable replacement string.  If it is a string, it
+    can contain ``\1``, ``\2``, etc. to represent back-substitution
+    as sed would allow.
+
+    Parameters:
+        regex (str): The regular expression to search for
+        repl (str): The string to replace matches with
+        *filenames: One or more files to search and replace
+
+    Keyword Arguments:
+        string (bool): Treat regex as a plain string. Default it False
+        backup (bool): Make backup file(s) suffixed with ``~``. Default is True
+        ignore_absent (bool): Ignore any files that don't exist.
+            Default is False
     """
     string = kwargs.get('string', False)
     backup = kwargs.get('backup', True)
@@ -128,7 +140,7 @@ def filter_file(regex, repl, *filenames, **kwargs):
 
 
 class FileFilter(object):
-    """Convenience class for calling filter_file a lot."""
+    """Convenience class for calling ``filter_file`` a lot."""
 
     def __init__(self, *filenames):
         self.filenames = filenames
@@ -139,12 +151,18 @@ class FileFilter(object):
 
 def change_sed_delimiter(old_delim, new_delim, *filenames):
     """Find all sed search/replace commands and change the delimiter.
-       e.g., if the file contains seds that look like 's///', you can
-       call change_sed_delimiter('/', '@', file) to change the
-       delimiter to '@'.
 
-       NOTE that this routine will fail if the delimiter is ' or ".
-       Handling those is left for future work.
+    e.g., if the file contains seds that look like ``'s///'``, you can
+    call ``change_sed_delimiter('/', '@', file)`` to change the
+    delimiter to ``'@'``.
+
+    Note that this routine will fail if the delimiter is ``'`` or ``"``.
+    Handling those is left for future work.
+
+    Parameters:
+        old_delim (str): The delimiter to search for
+        new_delim (str): The delimiter to replace with
+        *filenames: One or more files to search and replace
     """
     assert(len(old_delim) == 1)
     assert(len(new_delim) == 1)
@@ -239,7 +257,7 @@ def mkdirp(*paths):
 
 
 def force_remove(*paths):
-    """Remove files without printing errors.  Like rm -f, does NOT
+    """Remove files without printing errors.  Like ``rm -f``, does NOT
        remove directories."""
     for path in paths:
         try:
@@ -278,7 +296,8 @@ def touch(path):
 
 
 def touchp(path):
-    """Like touch, but creates any parent directories needed for the file."""
+    """Like ``touch``, but creates any parent directories needed for the file.
+    """
     mkdirp(os.path.dirname(path))
     touch(path)
 
@@ -335,17 +354,13 @@ def traverse_tree(source_root, dest_root, rel_path='', **kwargs):
         ('root/b',       'dest/b')
         ('root/b/file3', 'dest/b/file3')
 
-    Optional args:
-
-    order=[pre|post] -- Whether to do pre- or post-order traversal.
-
-    ignore=<predicate> -- Predicate indicating which files to ignore.
-
-    follow_nonexisting -- Whether to descend into directories in
-                          src that do not exit in dest. Default True.
-
-    follow_links -- Whether to descend into symlinks in src.
-
+    Keyword Arguments:
+        order (str): Whether to do pre- or post-order traversal. Accepted
+            values are 'pre' and 'post'
+        ignore (str): Predicate indicating which files to ignore
+        follow_nonexisting (bool): Whether to descend into directories in
+            ``src`` that do not exit in ``dest``. Default is True
+        follow_links (bool): Whether to descend into symlinks in ``src``
     """
     follow_nonexisting = kwargs.get('follow_nonexisting', True)
     follow_links = kwargs.get('follow_link', False)
@@ -406,12 +421,10 @@ def set_executable(path):
 
 
 def remove_dead_links(root):
-    """
-    Removes any dead link that is present in root
-
-    Args:
-        root: path where to search for dead links
+    """Removes any dead link that is present in root.
 
+    Parameters:
+        root (str): path where to search for dead links
     """
     for file in os.listdir(root):
         path = join_path(root, file)
@@ -419,11 +432,10 @@ def remove_dead_links(root):
 
 
 def remove_if_dead_link(path):
-    """
-    Removes the argument if it is a dead link, does nothing otherwise
+    """Removes the argument if it is a dead link.
 
-    Args:
-        path: the potential dead link
+    Parameters:
+        path (str): The potential dead link
     """
     if os.path.islink(path):
         real_path = os.path.realpath(path)
@@ -432,14 +444,13 @@ def remove_if_dead_link(path):
 
 
 def remove_linked_tree(path):
-    """
-    Removes a directory and its contents.  If the directory is a
-    symlink, follows the link and removes the real directory before
-    removing the link.
+    """Removes a directory and its contents.
 
-    Args:
-        path: directory to be removed
+    If the directory is a symlink, follows the link and removes the real
+    directory before removing the link.
 
+    Parameters:
+        path (str): Directory to be removed
     """
     if os.path.exists(path):
         if os.path.islink(path):
@@ -450,17 +461,17 @@ def remove_linked_tree(path):
 
 
 def fix_darwin_install_name(path):
-    """
-    Fix install name of dynamic libraries on Darwin to have full path.
+    """Fix install name of dynamic libraries on Darwin to have full path.
+
     There are two parts of this task:
-    (i) use install_name('-id',...) to change install name of a single lib;
-    (ii) use install_name('-change',...) to change the cross linking between
-    libs. The function assumes that all libraries are in one folder and
-    currently won't follow subfolders.
 
-    Args:
-        path: directory in which .dylib files are located
+    1. Use ``install_name('-id', ...)`` to change install name of a single lib
+    2. Use ``install_name('-change', ...)`` to change the cross linking between
+       libs. The function assumes that all libraries are in one folder and
+       currently won't follow subfolders.
 
+    Parameters:
+        path (str): directory in which .dylib files are located
     """
     libs = glob.glob(join_path(path, "*.dylib"))
     for lib in libs:
@@ -486,29 +497,108 @@ def fix_darwin_install_name(path):
                         stdout=subprocess.PIPE).communicate()[0]
                     break
 
-# Utilities for libraries
 
+def find(root, files, recurse=True):
+    """Search for ``files`` starting from the ``root`` directory.
 
-class LibraryList(collections.Sequence):
-    """Sequence of absolute paths to libraries
+    Like GNU/BSD find but written entirely in Python.
 
-    Provides a few convenience methods to manipulate library paths and get
-    commonly used compiler flags or names
+    Examples:
+
+    .. code-block:: console
+
+       $ find /usr -name python
+
+    is equivalent to:
+
+    >>> find('/usr', 'python')
+
+    .. code-block:: console
+
+       $ find /usr/local/bin -maxdepth 1 -name python
+
+    is equivalent to:
+
+    >>> find('/usr/local/bin', 'python', recurse=False)
+
+    Accepts any glob characters accepted by fnmatch:
+
+    =======  ====================================
+    Pattern  Meaning
+    =======  ====================================
+    *        matches everything
+    ?        matches any single character
+    [seq]    matches any character in ``seq``
+    [!seq]   matches any character not in ``seq``
+    =======  ====================================
+
+    Parameters:
+        root (str): The root directory to start searching from
+        files (str or collections.Sequence): Library name(s) to search for
+        recurse (bool, optional): if False search only root folder,
+            if True descends top-down from the root. Defaults to True.
+
+    Returns:
+        :func:`list`: The files that have been found
     """
+    if isinstance(files, six.string_types):
+        files = [files]
+
+    if recurse:
+        return _find_recursive(root, files)
+    else:
+        return _find_non_recursive(root, files)
+
+
+def _find_recursive(root, search_files):
+    found_files = []
+
+    for path, _, list_files in os.walk(root):
+        for search_file in search_files:
+            for list_file in list_files:
+                if fnmatch.fnmatch(list_file, search_file):
+                    found_files.append(join_path(path, list_file))
 
-    def __init__(self, libraries):
-        self.libraries = list(libraries)
+    return found_files
+
+
+def _find_non_recursive(root, search_files):
+    found_files = []
+
+    for list_file in os.listdir(root):
+        for search_file in search_files:
+            if fnmatch.fnmatch(list_file, search_file):
+                found_files.append(join_path(root, list_file))
+
+    return found_files
+
+
+# Utilities for libraries and headers
+
+
+class FileList(collections.Sequence):
+    """Sequence of absolute paths to files.
+
+    Provides a few convenience methods to manipulate file paths.
+    """
+
+    def __init__(self, files):
+        if isinstance(files, six.string_types):
+            files = [files]
+
+        self.files = list(dedupe(files))
 
     @property
     def directories(self):
-        """Stable de-duplication of the directories where the libraries
-        reside
+        """Stable de-duplication of the directories where the files reside.
 
         >>> l = LibraryList(['/dir1/liba.a', '/dir2/libb.a', '/dir1/libc.a'])
         >>> assert l.directories == ['/dir1', '/dir2']
+        >>> h = HeaderList(['/dir1/a.h', '/dir1/b.h', '/dir2/c.h'])
+        >>> assert h.directories == ['/dir1', '/dir2']
         """
         return list(dedupe(
-            os.path.dirname(x) for x in self.libraries if os.path.dirname(x)
+            os.path.dirname(x) for x in self.files if os.path.dirname(x)
         ))
 
     @property
@@ -517,8 +607,150 @@ class LibraryList(collections.Sequence):
 
         >>> l = LibraryList(['/dir1/liba.a', '/dir2/libb.a', '/dir3/liba.a'])
         >>> assert l.basenames == ['liba.a', 'libb.a']
+        >>> h = HeaderList(['/dir1/a.h', '/dir2/b.h', '/dir3/a.h'])
+        >>> assert h.basenames == ['a.h', 'b.h']
         """
-        return list(dedupe(os.path.basename(x) for x in self.libraries))
+        return list(dedupe(os.path.basename(x) for x in self.files))
+
+    @property
+    def names(self):
+        """Stable de-duplication of file names in the list
+
+        >>> h = HeaderList(['/dir1/a.h', '/dir2/b.h', '/dir3/a.h'])
+        >>> assert h.names == ['a', 'b']
+        """
+        return list(dedupe(x.split('.')[0] for x in self.basenames))
+
+    def __getitem__(self, item):
+        cls = type(self)
+        if isinstance(item, numbers.Integral):
+            return self.files[item]
+        return cls(self.files[item])
+
+    def __add__(self, other):
+        return self.__class__(dedupe(self.files + list(other)))
+
+    def __radd__(self, other):
+        return self.__add__(other)
+
+    def __eq__(self, other):
+        return self.files == other.files
+
+    def __len__(self):
+        return len(self.files)
+
+    def joined(self, separator=' '):
+        return separator.join(self.files)
+
+    def __repr__(self):
+        return self.__class__.__name__ + '(' + repr(self.files) + ')'
+
+    def __str__(self):
+        return self.joined()
+
+
+class HeaderList(FileList):
+    """Sequence of absolute paths to headers.
+
+    Provides a few convenience methods to manipulate header paths and get
+    commonly used compiler flags or names.
+    """
+
+    def __init__(self, files):
+        super(HeaderList, self).__init__(files)
+
+        self._macro_definitions = []
+
+    @property
+    def headers(self):
+        return self.files
+
+    @property
+    def include_flags(self):
+        """Include flags
+
+        >>> h = HeaderList(['/dir1/a.h', '/dir1/b.h', '/dir2/c.h'])
+        >>> assert h.cpp_flags == '-I/dir1 -I/dir2'
+        """
+        return ' '.join(['-I' + x for x in self.directories])
+
+    @property
+    def macro_definitions(self):
+        """Macro definitions
+
+        >>> h = HeaderList(['/dir1/a.h', '/dir1/b.h', '/dir2/c.h'])
+        >>> h.add_macro('-DBOOST_LIB_NAME=boost_regex')
+        >>> h.add_macro('-DBOOST_DYN_LINK')
+        >>> assert h.macro_definitions == '-DBOOST_LIB_NAME=boost_regex -DBOOST_DYN_LINK'  # noqa
+        """
+        return ' '.join(self._macro_definitions)
+
+    @property
+    def cpp_flags(self):
+        """Include flags + macro definitions
+
+        >>> h = HeaderList(['/dir1/a.h', '/dir1/b.h', '/dir2/c.h'])
+        >>> h.add_macro('-DBOOST_DYN_LINK')
+        >>> assert h.macro_definitions == '-I/dir1 -I/dir2 -DBOOST_DYN_LINK'
+        """
+        return self.include_flags + ' ' + self.macro_definitions
+
+    def add_macro(self, macro):
+        """Add a macro definition"""
+        self._macro_definitions.append(macro)
+
+
+def find_headers(headers, root, recurse=False):
+    """Returns an iterable object containing a list of full paths to
+    headers if found.
+
+    Accepts any glob characters accepted by fnmatch:
+
+    =======  ====================================
+    Pattern  Meaning
+    =======  ====================================
+    *        matches everything
+    ?        matches any single character
+    [seq]    matches any character in ``seq``
+    [!seq]   matches any character not in ``seq``
+    =======  ====================================
+
+    Parameters:
+        headers (str or list of str): Header name(s) to search for
+        root (str): The root directory to start searching from
+        recurses (bool, optional): if False search only root folder,
+            if True descends top-down from the root. Defaults to False.
+
+    Returns:
+        HeaderList: The headers that have been found
+    """
+    if isinstance(headers, six.string_types):
+        headers = [headers]
+    elif not isinstance(headers, collections.Sequence):
+        message = '{0} expects a string or sequence of strings as the '
+        message += 'first argument [got {1} instead]'
+        message = message.format(find_headers.__name__, type(headers))
+        raise TypeError(message)
+
+    # Construct the right suffix for the headers
+    suffix = 'h'
+
+    # List of headers we are searching with suffixes
+    headers = ['{0}.{1}'.format(header, suffix) for header in headers]
+
+    return HeaderList(find(root, headers, recurse))
+
+
+class LibraryList(FileList):
+    """Sequence of absolute paths to libraries
+
+    Provides a few convenience methods to manipulate library paths and get
+    commonly used compiler flags or names
+    """
+
+    @property
+    def libraries(self):
+        return self.files
 
     @property
     def names(self):
@@ -556,36 +788,9 @@ class LibraryList(collections.Sequence):
         """
         return self.search_flags + ' ' + self.link_flags
 
-    def __getitem__(self, item):
-        cls = type(self)
-        if isinstance(item, numbers.Integral):
-            return self.libraries[item]
-        return cls(self.libraries[item])
-
-    def __add__(self, other):
-        return LibraryList(dedupe(self.libraries + list(other)))
-
-    def __radd__(self, other):
-        return self.__add__(other)
-
-    def __eq__(self, other):
-        return self.libraries == other.libraries
-
-    def __len__(self):
-        return len(self.libraries)
-
-    def joined(self, separator=' '):
-        return separator.join(self.libraries)
-
-    def __repr__(self):
-        return self.__class__.__name__ + '(' + repr(self.libraries) + ')'
-
-    def __str__(self):
-        return self.joined()
-
 
-def find_system_libraries(library_names, shared=True):
-    """Searches the usual system library locations for ``library_names``.
+def find_system_libraries(libraries, shared=True):
+    """Searches the usual system library locations for ``libraries``.
 
     Search order is as follows:
 
@@ -596,20 +801,32 @@ def find_system_libraries(library_names, shared=True):
     5. ``/usr/local/lib64``
     6. ``/usr/local/lib``
 
-    Args:
-        library_names (str or list of str): Library name(s) to search for
-        shared (bool): searches for shared libraries if True
+    Accepts any glob characters accepted by fnmatch:
+
+    =======  ====================================
+    Pattern  Meaning
+    =======  ====================================
+    *        matches everything
+    ?        matches any single character
+    [seq]    matches any character in ``seq``
+    [!seq]   matches any character not in ``seq``
+    =======  ====================================
+
+    Parameters:
+        libraries (str or list of str): Library name(s) to search for
+        shared (bool, optional): if True searches for shared libraries,
+            otherwise for static. Defaults to True.
 
     Returns:
         LibraryList: The libraries that have been found
     """
-    if isinstance(library_names, str):
-        library_names = [library_names]
-    elif not isinstance(library_names, collections.Sequence):
+    if isinstance(libraries, six.string_types):
+        libraries = [libraries]
+    elif not isinstance(libraries, collections.Sequence):
         message = '{0} expects a string or sequence of strings as the '
         message += 'first argument [got {1} instead]'
-        message = message.format(
-            find_system_libraries.__name__, type(library_names))
+        message = message.format(find_system_libraries.__name__,
+                                 type(libraries))
         raise TypeError(message)
 
     libraries_found = []
@@ -622,7 +839,7 @@ def find_system_libraries(library_names, shared=True):
         '/usr/local/lib',
     ]
 
-    for library in library_names:
+    for library in libraries:
         for root in search_locations:
             result = find_libraries(library, root, shared, recurse=True)
             if result:
@@ -632,26 +849,38 @@ def find_system_libraries(library_names, shared=True):
     return libraries_found
 
 
-def find_libraries(library_names, root, shared=True, recurse=False):
+def find_libraries(libraries, root, shared=True, recurse=False):
     """Returns an iterable of full paths to libraries found in a root dir.
 
-    Args:
-        library_names (str or list of str): Library names to search for
+    Accepts any glob characters accepted by fnmatch:
+
+    =======  ====================================
+    Pattern  Meaning
+    =======  ====================================
+    *        matches everything
+    ?        matches any single character
+    [seq]    matches any character in ``seq``
+    [!seq]   matches any character not in ``seq``
+    =======  ====================================
+
+    Parameters:
+        libraries (str or list of str): Library name(s) to search for
         root (str): The root directory to start searching from
-        shared (bool): if True searches for shared libraries, otherwise static.
-        recurse (bool): if False search only root folder,
-            if True descends top-down from the root
+        shared (bool, optional): if True searches for shared libraries,
+            otherwise for static. Defaults to True.
+        recurse (bool, optional): if False search only root folder,
+            if True descends top-down from the root. Defaults to False.
 
     Returns:
         LibraryList: The libraries that have been found
     """
-    if isinstance(library_names, str):
-        library_names = [library_names]
-    elif not isinstance(library_names, collections.Sequence):
+    if isinstance(libraries, six.string_types):
+        libraries = [libraries]
+    elif not isinstance(libraries, collections.Sequence):
         message = '{0} expects a string or sequence of strings as the '
         message += 'first argument [got {1} instead]'
-        raise TypeError(message.format(
-            find_libraries.__name__, type(library_names)))
+        message = message.format(find_libraries.__name__, type(libraries))
+        raise TypeError(message)
 
     # Construct the right suffix for the library
     if shared is True:
@@ -659,38 +888,6 @@ def find_libraries(library_names, root, shared=True, recurse=False):
     else:
         suffix = 'a'
     # List of libraries we are searching with suffixes
-    libraries = ['{0}.{1}'.format(lib, suffix) for lib in library_names]
-    # Search method
-    if recurse is False:
-        search_method = _find_libraries_non_recursive
-    else:
-        search_method = _find_libraries_recursive
-
-    return search_method(libraries, root)
-
-
-def _find_libraries_recursive(libraries, root):
-    library_dict = collections.defaultdict(list)
-    for path, _, files in os.walk(root):
-        for lib in libraries:
-            if lib in files:
-                library_dict[lib].append(
-                    join_path(path, lib)
-                )
-    answer = []
-    for lib in libraries:
-        answer.extend(library_dict[lib])
-    return LibraryList(answer)
-
-
-def _find_libraries_non_recursive(libraries, root):
-
-    def lib_or_none(lib):
-        library = join_path(root, lib)
-        if not os.path.exists(library):
-            return None
-        return library
+    libraries = ['{0}.{1}'.format(lib, suffix) for lib in libraries]
 
-    return LibraryList(
-        [lib_or_none(lib) for lib in libraries if lib_or_none(lib) is not None]
-    )
+    return LibraryList(find(root, libraries, recurse))
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 1e2210f928..12b91bcdb0 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -64,6 +64,7 @@ from llnl.util.lang import *
 from llnl.util.link_tree import LinkTree
 from llnl.util.tty.log import log_output
 from spack import directory_layout
+from spack.util.executable import which
 from spack.stage import Stage, ResourceStage, StageComposite
 from spack.util.environment import dump_environment
 from spack.version import *
@@ -1025,17 +1026,27 @@ class PackageBase(with_metaclass(PackageMeta, object)):
         return namespace
 
     def do_fake_install(self):
-        """Make a fake install directory containing a 'fake' file in bin."""
-        # FIXME : Make this part of the 'install' behavior ?
+        """Make a fake install directory containing fake executables,
+        headers, and libraries."""
+
+        name = self.name
+        library_name = 'lib' + self.name
+        dso_suffix = '.dylib' if sys.platform == 'darwin' else '.so'
+        chmod = which('chmod')
+
         mkdirp(self.prefix.bin)
-        touch(join_path(self.prefix.bin, 'fake'))
+        touch(join_path(self.prefix.bin, name))
+        chmod('+x', join_path(self.prefix.bin, name))
+
         mkdirp(self.prefix.include)
+        touch(join_path(self.prefix.include, name + '.h'))
+
         mkdirp(self.prefix.lib)
-        library_name = 'lib' + self.name
-        dso_suffix = 'dylib' if sys.platform == 'darwin' else 'so'
         touch(join_path(self.prefix.lib, library_name + dso_suffix))
         touch(join_path(self.prefix.lib, library_name + '.a'))
+
         mkdirp(self.prefix.man1)
+
         packages_dir = spack.store.layout.build_packages_path(self.spec)
         dump_packages(self.spec, packages_dir)
 
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 4cd89f59bf..0d8fb2893b 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -101,6 +101,8 @@ import collections
 import ctypes
 import hashlib
 import itertools
+import os
+
 from operator import attrgetter
 from six import StringIO
 from six import string_types
@@ -115,12 +117,13 @@ import spack.store
 import spack.util.spack_json as sjson
 import spack.util.spack_yaml as syaml
 
-from llnl.util.filesystem import find_libraries
+from llnl.util.filesystem import find_headers, find_libraries, is_exe
 from llnl.util.lang import *
 from llnl.util.tty.color import *
 from spack.build_environment import get_path_from_module, load_module
 from spack.provider_index import ProviderIndex
 from spack.util.crypto import prefix_bits
+from spack.util.executable import Executable
 from spack.util.prefix import Prefix
 from spack.util.spack_yaml import syaml_dict
 from spack.util.string import *
@@ -745,9 +748,9 @@ class FlagMap(HashableMap):
 
 
 class DependencyMap(HashableMap):
-
     """Each spec has a DependencyMap containing specs for its dependencies.
        The DependencyMap is keyed by name. """
+
     @property
     def concrete(self):
         return all((d.spec.concrete and d.deptypes)
@@ -757,38 +760,104 @@ class DependencyMap(HashableMap):
         return "{deps: %s}" % ', '.join(str(d) for d in sorted(self.values()))
 
 
-def _libs_default_handler(descriptor, spec, cls):
-    """Default handler when for ``libs`` attribute in Spec interface.
+def _command_default_handler(descriptor, spec, cls):
+    """Default handler when looking for the 'command' attribute.
 
-    Tries to search for ``lib{spec.name}`` recursively starting from
-    `spec.prefix`.
+    Tries to search for ``spec.name`` in the ``spec.prefix.bin`` directory.
 
-    Args:
-        descriptor (ForwardQueryToPackage): descriptor that triggered
-            the call
+    Parameters:
+        descriptor (ForwardQueryToPackage): descriptor that triggered the call
         spec (Spec): spec that is being queried
         cls (type(spec)): type of spec, to match the signature of the
-            descriptor `__get__` method
+            descriptor ``__get__`` method
+
+    Returns:
+        Executable: An executable of the command
+
+    Raises:
+        RuntimeError: If the command is not found
     """
-    name = 'lib' + spec.name
-    shared = '+shared' in spec
-    return find_libraries(name, root=spec.prefix, shared=shared, recurse=True)
+    path = os.path.join(spec.prefix.bin, spec.name)
+
+    if is_exe(path):
+        return Executable(path)
+    else:
+        msg = 'Unable to locate {0} command in {1}'
+        raise RuntimeError(msg.format(spec.name, spec.prefix.bin))
 
 
-def _cppflags_default_handler(descriptor, spec, cls):
-    """Default handler for the ``cppflags`` attribute in the Spec interface.
+def _headers_default_handler(descriptor, spec, cls):
+    """Default handler when looking for the 'headers' attribute.
 
-    The default just returns ``-I{spec.prefix.include}``.
+    Tries to search for ``*.h`` files recursively starting from
+    ``spec.prefix.include``.
 
-    Args:
-        descriptor (ForwardQueryToPackage): descriptor that triggered
-            the call
+    Parameters:
+        descriptor (ForwardQueryToPackage): descriptor that triggered the call
         spec (Spec): spec that is being queried
+        cls (type(spec)): type of spec, to match the signature of the
+            descriptor ``__get__`` method
+
+    Returns:
+        HeaderList: The headers in ``prefix.include``
+
+    Raises:
+        RuntimeError: If no headers are found
+    """
+    headers = find_headers('*', root=spec.prefix.include, recurse=True)
+
+    if headers:
+        return headers
+    else:
+        msg = 'Unable to locate {0} headers in {1}'
+        raise RuntimeError(msg.format(spec.name, spec.prefix.include))
+
+
+def _libs_default_handler(descriptor, spec, cls):
+    """Default handler when looking for the 'libs' attribute.
 
+    Tries to search for ``lib{spec.name}`` recursively starting from
+    ``spec.prefix``.
+
+    Parameters:
+        descriptor (ForwardQueryToPackage): descriptor that triggered the call
+        spec (Spec): spec that is being queried
         cls (type(spec)): type of spec, to match the signature of the
             descriptor ``__get__`` method
+
+    Returns:
+        LibraryList: The libraries found
+
+    Raises:
+        RuntimeError: If no libraries are found
     """
-    return '-I' + spec.prefix.include
+    name = 'lib' + spec.name
+
+    if '+shared' in spec:
+        libs = find_libraries(
+            name, root=spec.prefix, shared=True, recurse=True
+        )
+    elif '~shared' in spec:
+        libs = find_libraries(
+            name, root=spec.prefix, shared=False, recurse=True
+        )
+    else:
+        # Prefer shared
+        libs = find_libraries(
+            name, root=spec.prefix, shared=True, recurse=True
+        )
+        if libs:
+            return libs
+
+        libs = find_libraries(
+            name, root=spec.prefix, shared=False, recurse=True
+        )
+
+    if libs:
+        return libs
+    else:
+        msg = 'Unable to recursively locate {0} libraries in {1}'
+        raise RuntimeError(msg.format(spec.name, spec.prefix))
 
 
 class ForwardQueryToPackage(object):
@@ -797,7 +866,7 @@ class ForwardQueryToPackage(object):
     def __init__(self, attribute_name, default_handler=None):
         """Create a new descriptor.
 
-        Args:
+        Parameters:
             attribute_name (str): name of the attribute to be
                 searched for in the Package instance
             default_handler (callable, optional): default function to be
@@ -815,7 +884,7 @@ class ForwardQueryToPackage(object):
         """Retrieves the property from Package using a well defined chain
         of responsibility.
 
-        The order of call is :
+        The order of call is:
 
         1. if the query was through the name of a virtual package try to
             search for the attribute `{virtual_name}_{attribute_name}`
@@ -885,17 +954,21 @@ class ForwardQueryToPackage(object):
 
 
 class SpecBuildInterface(ObjectWrapper):
+    command = ForwardQueryToPackage(
+        'command',
+        default_handler=_command_default_handler
+    )
+
+    headers = ForwardQueryToPackage(
+        'headers',
+        default_handler=_headers_default_handler
+    )
 
     libs = ForwardQueryToPackage(
         'libs',
         default_handler=_libs_default_handler
     )
 
-    cppflags = ForwardQueryToPackage(
-        'cppflags',
-        default_handler=_cppflags_default_handler
-    )
-
     def __init__(self, spec, name, query_parameters):
         super(SpecBuildInterface, self).__init__(spec)
 
diff --git a/lib/spack/spack/test/database.py b/lib/spack/spack/test/database.py
index a4b35e1df7..6f8a2ef5d2 100644
--- a/lib/spack/spack/test/database.py
+++ b/lib/spack/spack/test/database.py
@@ -32,6 +32,7 @@ import os.path
 import pytest
 import spack
 import spack.store
+from spack.util.executable import Executable
 from llnl.util.tty.colify import colify
 
 
@@ -114,11 +115,17 @@ def test_default_queries(database):
     rec = install_db.get_record('zmpi')
 
     spec = rec.spec
+
     libraries = spec['zmpi'].libs
     assert len(libraries) == 1
 
-    cppflags_expected = '-I' + spec.prefix.include
-    assert spec['zmpi'].cppflags == cppflags_expected
+    headers = spec['zmpi'].headers
+    assert len(headers) == 1
+
+    command = spec['zmpi'].command
+    assert isinstance(command, Executable)
+    assert command.name == 'zmpi'
+    assert os.path.exists(command.path)
 
 
 def test_005_db_exists(database):
diff --git a/lib/spack/spack/test/library_list.py b/lib/spack/spack/test/file_list.py
index 7fc2fd222f..09517b4dab 100644
--- a/lib/spack/spack/test/library_list.py
+++ b/lib/spack/spack/test/file_list.py
@@ -25,7 +25,7 @@
 
 import unittest
 
-from llnl.util.filesystem import LibraryList
+from llnl.util.filesystem import LibraryList, HeaderList
 
 
 class LibraryListTest(unittest.TestCase):
@@ -63,15 +63,29 @@ class LibraryListTest(unittest.TestCase):
         self.assertTrue('-L/dir2' in search_flags)
         self.assertTrue('-L/dir3' in search_flags)
         self.assertTrue(isinstance(search_flags, str))
+        self.assertEqual(
+            search_flags,
+            '-L/dir1 -L/dir2 -L/dir3'
+        )
 
         link_flags = self.liblist.link_flags
+        self.assertTrue('-llapack' in link_flags)
+        self.assertTrue('-lfoo'    in link_flags)
+        self.assertTrue('-lblas'   in link_flags)
+        self.assertTrue('-lbar'    in link_flags)
+        self.assertTrue('-lbaz'    in link_flags)
+        self.assertTrue(isinstance(link_flags, str))
         self.assertEqual(
             link_flags,
             '-llapack -lfoo -lblas -lbar -lbaz'
         )
 
         ld_flags = self.liblist.ld_flags
-        self.assertEqual(ld_flags, search_flags + ' ' + link_flags)
+        self.assertTrue(isinstance(ld_flags, str))
+        self.assertEqual(
+            ld_flags,
+            search_flags + ' ' + link_flags
+        )
 
     def test_paths_manipulation(self):
         names = self.liblist.names
@@ -109,3 +123,103 @@ class LibraryListTest(unittest.TestCase):
             type(pylist + self.liblist),
             type(self.liblist)
         )
+
+
+class HeaderListTest(unittest.TestCase):
+    def setUp(self):
+        h = [
+            '/dir1/Python.h',
+            '/dir2/datetime.h',
+            '/dir1/pyconfig.h',
+            '/dir3/core.h',
+            'pymem.h'
+        ]
+        headlist = HeaderList(h)
+        headlist.add_macro('-DBOOST_LIB_NAME=boost_regex')
+        headlist.add_macro('-DBOOST_DYN_LINK')
+        self.headlist = headlist
+
+    def test_repr(self):
+        x = eval(repr(self.headlist))
+        self.assertEqual(self.headlist, x)
+
+    def test_joined_and_str(self):
+        s1 = self.headlist.joined()
+        self.assertEqual(
+            s1,
+            '/dir1/Python.h /dir2/datetime.h /dir1/pyconfig.h /dir3/core.h pymem.h'  # NOQA: ignore=E501
+        )
+        s2 = str(self.headlist)
+        self.assertEqual(s1, s2)
+        s3 = self.headlist.joined(';')
+        self.assertEqual(
+            s3,
+            '/dir1/Python.h;/dir2/datetime.h;/dir1/pyconfig.h;/dir3/core.h;pymem.h'  # NOQA: ignore=E501
+        )
+
+    def test_flags(self):
+        include_flags = self.headlist.include_flags
+        self.assertTrue('-I/dir1' in include_flags)
+        self.assertTrue('-I/dir2' in include_flags)
+        self.assertTrue('-I/dir3' in include_flags)
+        self.assertTrue(isinstance(include_flags, str))
+        self.assertEqual(
+            include_flags,
+            '-I/dir1 -I/dir2 -I/dir3'
+        )
+
+        macros = self.headlist.macro_definitions
+        self.assertTrue('-DBOOST_LIB_NAME=boost_regex' in macros)
+        self.assertTrue('-DBOOST_DYN_LINK' in macros)
+        self.assertTrue(isinstance(macros, str))
+        self.assertEqual(
+            macros,
+            '-DBOOST_LIB_NAME=boost_regex -DBOOST_DYN_LINK'
+        )
+
+        cpp_flags = self.headlist.cpp_flags
+        self.assertTrue(isinstance(cpp_flags, str))
+        self.assertEqual(
+            cpp_flags,
+            include_flags + ' ' + macros
+        )
+
+    def test_paths_manipulation(self):
+        names = self.headlist.names
+        self.assertEqual(
+            names,
+            ['Python', 'datetime', 'pyconfig', 'core', 'pymem']
+        )
+
+        directories = self.headlist.directories
+        self.assertEqual(directories, ['/dir1', '/dir2', '/dir3'])
+
+    def test_get_item(self):
+        a = self.headlist[0]
+        self.assertEqual(a, '/dir1/Python.h')
+
+        b = self.headlist[:]
+        self.assertEqual(type(b), type(self.headlist))
+        self.assertEqual(self.headlist, b)
+        self.assertTrue(self.headlist is not b)
+
+    def test_add(self):
+        pylist = [
+            '/dir1/Python.h',  # removed from the final list
+            '/dir2/pyconfig.h',
+            '/dir4/datetime.h'
+        ]
+        another = HeaderList(pylist)
+        h = self.headlist + another
+        self.assertEqual(len(h), 7)
+        # Invariant : l == l + l
+        self.assertEqual(h, h + h)
+        # Always produce an instance of HeaderList
+        self.assertEqual(
+            type(self.headlist),
+            type(self.headlist + pylist)
+        )
+        self.assertEqual(
+            type(pylist + self.headlist),
+            type(self.headlist)
+        )
diff --git a/lib/spack/spack/util/executable.py b/lib/spack/spack/util/executable.py
index 1d7f019fdf..44865e7bdb 100644
--- a/lib/spack/spack/util/executable.py
+++ b/lib/spack/spack/util/executable.py
@@ -53,8 +53,31 @@ class Executable(object):
 
     @property
     def command(self):
+        """The command-line string.
+
+        Returns:
+            str: The executable and default arguments
+        """
         return ' '.join(self.exe)
 
+    @property
+    def name(self):
+        """The executable name.
+
+        Returns:
+            str: The basename of the executable
+        """
+        return os.path.basename(self.path)
+
+    @property
+    def path(self):
+        """The path to the executable.
+
+        Returns:
+            str: The path to the executable
+        """
+        return self.exe[0]
+
     def __call__(self, *args, **kwargs):
         """Run this executable in a subprocess.
 
diff --git a/var/spack/repos/builtin/packages/abinit/package.py b/var/spack/repos/builtin/packages/abinit/package.py
index fe1c7ec778..d73b89ea69 100644
--- a/var/spack/repos/builtin/packages/abinit/package.py
+++ b/var/spack/repos/builtin/packages/abinit/package.py
@@ -33,9 +33,9 @@ class Abinit(AutotoolsPackage):
     energy, charge density and electronic structure of systems made of
     electrons and nuclei (molecules and periodic solids) within
     Density Functional Theory (DFT), using pseudopotentials and a planewave
-    or wavelet basis. 
-    
-    ABINIT also includes options to optimize the geometry according to the 
+    or wavelet basis.
+
+    ABINIT also includes options to optimize the geometry according to the
     DFT forces and stresses, or to perform molecular dynamics
     simulations using these forces, or to generate dynamical matrices,
     Born effective charges, and dielectric tensors, based on Density-Functional
@@ -149,7 +149,7 @@ class Abinit(AutotoolsPackage):
         # LibXC library
         libxc = spec['libxc:fortran']
         options.extend([
-            'with_libxc_incs={0}'.format(libxc.cppflags),
+            'with_libxc_incs={0}'.format(libxc.headers.cpp_flags),
             'with_libxc_libs={0}'.format(libxc.libs.ld_flags + ' -lm')
         ])
 
@@ -161,7 +161,7 @@ class Abinit(AutotoolsPackage):
             hdf5 = spec['hdf5:hl']
             netcdff = spec['netcdf-fortran:shared']
             options.extend([
-                '--with-netcdf-incs={0}'.format(netcdff.cppflags),
+                '--with-netcdf-incs={0}'.format(netcdff.headers.cpp_flags),
                 '--with-netcdf-libs={0}'.format(
                     netcdff.libs.ld_flags + ' ' + hdf5.libs.ld_flags
                 ),
@@ -175,7 +175,7 @@ class Abinit(AutotoolsPackage):
 
     def check(self):
         """This method is called after the build phase if tests have been
-        explicitly activated by user. 
+        explicitly activated by user.
         """
         make('check')
         make('tests_in')
diff --git a/var/spack/repos/builtin/packages/ack/package.py b/var/spack/repos/builtin/packages/ack/package.py
index dd6685a829..b377c30fe9 100644
--- a/var/spack/repos/builtin/packages/ack/package.py
+++ b/var/spack/repos/builtin/packages/ack/package.py
@@ -45,7 +45,7 @@ class Ack(Package):
         ack = 'ack-{0}-single-file'.format(self.version)
 
         # rewrite the script's #! line to call the perl dependency
-        shbang = '#!' + join_path(spec['perl'].prefix.bin, 'perl')
+        shbang = '#!' + spec['perl'].command.path
         filter_file(r'^#!/usr/bin/env perl', shbang, ack)
 
         install(ack, join_path(prefix.bin, "ack"))
diff --git a/var/spack/repos/builtin/packages/blast-plus/package.py b/var/spack/repos/builtin/packages/blast-plus/package.py
index 7cab51b0be..3faf852739 100644
--- a/var/spack/repos/builtin/packages/blast-plus/package.py
+++ b/var/spack/repos/builtin/packages/blast-plus/package.py
@@ -206,7 +206,7 @@ class BlastPlus(AutotoolsPackage):
 
         if '+python' in spec:
             config_args.append(
-                '--with-python={0}'.format(self.spec['python'].prefix)
+                '--with-python={0}'.format(self.spec['python'].home)
             )
         else:
             config_args.append('--without-python')
diff --git a/var/spack/repos/builtin/packages/boost/package.py b/var/spack/repos/builtin/packages/boost/package.py
index 487445ab10..d4c772ed03 100644
--- a/var/spack/repos/builtin/packages/boost/package.py
+++ b/var/spack/repos/builtin/packages/boost/package.py
@@ -25,7 +25,6 @@
 from spack import *
 import sys
 import os
-from glob import glob
 
 
 class Boost(Package):
@@ -173,23 +172,11 @@ class Boost(Package):
         return 'gcc'
 
     def bjam_python_line(self, spec):
-        from os.path import dirname, splitext
-        pydir = 'python%s.%s*' % spec['python'].version.version[:2]
-        incs = join_path(spec['python'].prefix.include, pydir, "pyconfig.h")
-        incs = glob(incs)
-        incs = " ".join([dirname(u) for u in incs])
-
-        pylib = 'libpython%s.%s*' % spec['python'].version.version[:2]
-        all_libs = join_path(spec['python'].prefix.lib, pylib)
-        libs = [u for u in all_libs if splitext(u)[1] == dso_suffix]
-        if len(libs) == 0:
-            libs = [u for u in all_libs if splitext(u)[1] == '.a']
-
-        libs = " ".join(libs)
-        return 'using python : %s : %s : %s : %s ;\n' % (
+        return 'using python : {0} : {1} : {2} : {3} ;\n'.format(
             spec['python'].version.up_to(2),
-            join_path(spec['python'].prefix.bin, 'python'),
-            incs, libs
+            spec['python'].command.path,
+            spec['python'].headers.directories[0],
+            spec['python'].libs[0]
         )
 
     def determine_bootstrap_options(self, spec, withLibs, options):
@@ -198,7 +185,7 @@ class Boost(Package):
         options.append("--with-libraries=%s" % ','.join(withLibs))
 
         if '+python' in spec:
-            options.append('--with-python=%s' % python_exe)
+            options.append('--with-python=%s' % spec['python'].command.path)
 
         with open('user-config.jam', 'w') as f:
             # Boost may end up using gcc even though clang+gfortran is set in
diff --git a/var/spack/repos/builtin/packages/cantera/package.py b/var/spack/repos/builtin/packages/cantera/package.py
index 0685772f82..aa7c7dc98c 100644
--- a/var/spack/repos/builtin/packages/cantera/package.py
+++ b/var/spack/repos/builtin/packages/cantera/package.py
@@ -117,15 +117,13 @@ class Cantera(Package):
         if '+python' in spec:
             options.extend([
                 'python_package=full',
-                'python_cmd={0}'.format(
-                    join_path(spec['python'].prefix.bin, 'python')),
+                'python_cmd={0}'.format(spec['python'].command.path),
                 'python_array_home={0}'.format(spec['py-numpy'].prefix)
             ])
             if spec['python'].satisfies('@3'):
                 options.extend([
                     'python3_package=y',
-                    'python3_cmd={0}'.format(
-                        join_path(spec['python'].prefix.bin, 'python')),
+                    'python3_cmd={0}'.format(spec['python'].command.path),
                     'python3_array_home={0}'.format(spec['py-numpy'].prefix)
                 ])
             else:
diff --git a/var/spack/repos/builtin/packages/conduit/package.py b/var/spack/repos/builtin/packages/conduit/package.py
index ddefde4fb7..d97c6534e4 100644
--- a/var/spack/repos/builtin/packages/conduit/package.py
+++ b/var/spack/repos/builtin/packages/conduit/package.py
@@ -174,13 +174,13 @@ class Conduit(Package):
         ##############################################
 
         if "+cmake" in spec:
-            cmake_exe = join_path(spec['cmake'].prefix.bin, "cmake")
+            cmake_exe = spec['cmake'].command.path
         else:
             cmake_exe = which("cmake")
             if cmake_exe is None:
                 msg = 'failed to find CMake (and cmake variant is off)'
                 raise RuntimeError(msg)
-            cmake_exe = cmake_exe.command
+            cmake_exe = cmake_exe.path
 
         host_cfg_fname = "%s-%s-%s.cmake" % (socket.gethostname(),
                                              sys_type,
@@ -224,21 +224,15 @@ class Conduit(Package):
         cfg.write("# Python Support\n")
 
         if "+python" in spec:
-            python_exe = join_path(spec['python'].prefix.bin, "python")
             cfg.write("# Enable python module builds\n")
             cfg.write(cmake_cache_entry("ENABLE_PYTHON", "ON"))
             cfg.write("# python from spack \n")
-            cfg.write(cmake_cache_entry("PYTHON_EXECUTABLE", python_exe))
+            cfg.write(cmake_cache_entry("PYTHON_EXECUTABLE",
+                      spec['python'].command.path))
             # install module to standard style site packages dir
             # so we can support spack activate
-            py_ver_short = "python{0}".format(spec["python"].version.up_to(2))
-            pym_prefix = join_path("${CMAKE_INSTALL_PREFIX}",
-                                   "lib",
-                                   py_ver_short,
-                                   "site-packages")
-            # use pym_prefix as the install path
             cfg.write(cmake_cache_entry("PYTHON_MODULE_INSTALL_PREFIX",
-                                        pym_prefix))
+                                        site_packages_dir))
         else:
             cfg.write(cmake_cache_entry("ENABLE_PYTHON", "OFF"))
 
@@ -251,7 +245,7 @@ class Conduit(Package):
             cfg.write(cmake_cache_entry("SPHINX_EXECUTABLE", sphinx_build_exe))
 
             cfg.write("# doxygen from uberenv\n")
-            doxygen_exe = join_path(spec['doxygen'].prefix.bin, "doxygen")
+            doxygen_exe = spec['doxygen'].command.path
             cfg.write(cmake_cache_entry("DOXYGEN_EXECUTABLE", doxygen_exe))
         else:
             cfg.write(cmake_cache_entry("ENABLE_DOCS", "OFF"))
diff --git a/var/spack/repos/builtin/packages/cosmomc/package.py b/var/spack/repos/builtin/packages/cosmomc/package.py
index 1e83e02d65..6c41002079 100644
--- a/var/spack/repos/builtin/packages/cosmomc/package.py
+++ b/var/spack/repos/builtin/packages/cosmomc/package.py
@@ -178,7 +178,7 @@ class Cosmomc(Package):
         os.environ.pop('CLIKPATH', '')
         os.environ.pop('PLANCKLIKE', '')
 
-        exe = join_path(prefix.bin, 'cosmomc')
+        exe = spec['cosmomc'].command.path
         args = []
         if '+mpi' in spec:
             # Add mpirun prefix
diff --git a/var/spack/repos/builtin/packages/cp2k/package.py b/var/spack/repos/builtin/packages/cp2k/package.py
index 9bc1026ba9..06e4697904 100644
--- a/var/spack/repos/builtin/packages/cp2k/package.py
+++ b/var/spack/repos/builtin/packages/cp2k/package.py
@@ -93,10 +93,10 @@ class Cp2k(Package):
             cppflags = [
                 '-D__FFTW3',
                 '-D__LIBINT',
-                spec['fftw'].cppflags
+                spec['fftw'].headers.cpp_flags
             ]
             fcflags = copy.deepcopy(optflags[self.spec.compiler.name])
-            fcflags.append(spec['fftw'].cppflags)
+            fcflags.append(spec['fftw'].headers.cpp_flags)
             fftw = find_libraries('libfftw3', root=spec['fftw'].prefix.lib)
             ldflags = [fftw.search_flags]
             if 'superlu-dist@4.3' in spec:
@@ -161,14 +161,14 @@ class Cp2k(Package):
                     cppflags.append('-D__WANNIER90')
 
                 fcflags.extend([
-                    # spec['elpa:fortran'].cppflags
+                    # spec['elpa:fortran'].headers.cpp_flags
                     '-I' + join_path(
                         spec['elpa'].prefix,
                         'include',
                         'elpa-{0}'.format(str(spec['elpa'].version)),
                         'modules'
                     ),
-                    # spec[pexsi:fortran].cppflags
+                    # spec[pexsi:fortran].headers.cpp_flags
                     '-I' + join_path(spec['pexsi'].prefix, 'fortran')
                 ])
                 scalapack = spec['scalapack'].libs
diff --git a/var/spack/repos/builtin/packages/foam-extend/package.py b/var/spack/repos/builtin/packages/foam-extend/package.py
index 559cc45d7a..c68e570c0f 100644
--- a/var/spack/repos/builtin/packages/foam-extend/package.py
+++ b/var/spack/repos/builtin/packages/foam-extend/package.py
@@ -356,8 +356,8 @@ echo WM_PROJECT_DIR = $WM_PROJECT_DIR
                 'CMAKE_BIN_DIR': spec['cmake'].prefix.bin,
             },
             'python': {
-                'PYTHON_DIR':     spec['python'].prefix,
-                'PYTHON_BIN_DIR': spec['python'].prefix.bin,
+                'PYTHON_DIR':     spec['python'].home,
+                'PYTHON_BIN_DIR': spec['python'].home.bin,
             },
             'flex': {
                 'FLEX_SYSTEM': 1,
diff --git a/var/spack/repos/builtin/packages/gcc/package.py b/var/spack/repos/builtin/packages/gcc/package.py
index 00518c8cf4..b362060e61 100644
--- a/var/spack/repos/builtin/packages/gcc/package.py
+++ b/var/spack/repos/builtin/packages/gcc/package.py
@@ -176,7 +176,7 @@ class Gcc(AutotoolsPackage):
                      self.spec.format('$_$@'))
             return
 
-        gcc = Executable(join_path(self.prefix.bin, 'gcc'))
+        gcc = self.spec['gcc'].command
         lines = gcc('-dumpspecs', output=str).strip().split("\n")
         specs_file = join_path(self.spec_dir, 'specs')
         with closing(open(specs_file, 'w')) as out:
diff --git a/var/spack/repos/builtin/packages/gdal/package.py b/var/spack/repos/builtin/packages/gdal/package.py
index dc58cc8993..1b9a4f0818 100644
--- a/var/spack/repos/builtin/packages/gdal/package.py
+++ b/var/spack/repos/builtin/packages/gdal/package.py
@@ -72,7 +72,7 @@ class Gdal(Package):
         args.append("--prefix=%s" % prefix)
         args.append("--with-liblzma=yes")
         args.append("--with-zlib=%s" % spec['zlib'].prefix)
-        args.append("--with-python=%s" % spec['python'].prefix.bin + "/python")
+        args.append("--with-python=%s" % spec['python'].command.path)
         args.append("--without-libtool")
 
         if '+geos' in spec:
diff --git a/var/spack/repos/builtin/packages/geos/package.py b/var/spack/repos/builtin/packages/geos/package.py
index 324186cfbc..2419eed038 100644
--- a/var/spack/repos/builtin/packages/geos/package.py
+++ b/var/spack/repos/builtin/packages/geos/package.py
@@ -60,9 +60,8 @@ class Geos(Package):
     def install(self, spec, prefix):
         args = ["--prefix=%s" % prefix]
 #        if '+python' in spec:
-#            os.environ['PYTHON'] = join_path(spec['python'].prefix, 'bin',
-#                'python' if spec['python'].version[:1][0] <= 2 else 'python3')
-#            os.environ['SWIG'] = join_path(spec['swig'].prefix, 'bin', 'swig')
+#            os.environ['PYTHON'] = spec['python'].command.path
+#            os.environ['SWIG'] = spec['swig'].command.path
 #
 #            args.append("--enable-python")
 
diff --git a/var/spack/repos/builtin/packages/git/package.py b/var/spack/repos/builtin/packages/git/package.py
index dfa4e08643..d71f49bff6 100644
--- a/var/spack/repos/builtin/packages/git/package.py
+++ b/var/spack/repos/builtin/packages/git/package.py
@@ -162,8 +162,7 @@ class Git(AutotoolsPackage):
             '--with-iconv={0}'.format(spec['libiconv'].prefix),
             '--with-libpcre={0}'.format(spec['pcre'].prefix),
             '--with-openssl={0}'.format(spec['openssl'].prefix),
-            '--with-perl={0}'.format(
-                join_path(spec['perl'].prefix.bin, 'perl')),
+            '--with-perl={0}'.format(spec['perl'].command.path),
             '--with-zlib={0}'.format(spec['zlib'].prefix),
         ]
 
diff --git a/var/spack/repos/builtin/packages/go/package.py b/var/spack/repos/builtin/packages/go/package.py
index 7f1523ca6d..05a903ea74 100644
--- a/var/spack/repos/builtin/packages/go/package.py
+++ b/var/spack/repos/builtin/packages/go/package.py
@@ -117,7 +117,7 @@ class Go(Package):
         shutil.copytree('bin', os.path.join(prefix, '/bin'))
         """
         #  Add a go command/compiler for extensions
-        module.go = Executable(join_path(self.spec.prefix.bin, 'go'))
+        module.go = self.spec['go'].command
 
     def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         if os.environ.get('GOROOT', False):
diff --git a/var/spack/repos/builtin/packages/hdf5-blosc/package.py b/var/spack/repos/builtin/packages/hdf5-blosc/package.py
index 79f23d7d92..053a4d4852 100644
--- a/var/spack/repos/builtin/packages/hdf5-blosc/package.py
+++ b/var/spack/repos/builtin/packages/hdf5-blosc/package.py
@@ -69,7 +69,7 @@ class Hdf5Blosc(Package):
         # if sys.platform == "darwin":
         #     fix_darwin_install_name(prefix.lib)
 
-        libtool = Executable(join_path(spec["libtool"].prefix.bin, "libtool"))
+        libtool = spec["libtool"].command
 
         # TODO: these vars are not used.
         # if "+mpi" in spec["hdf5"]:
diff --git a/var/spack/repos/builtin/packages/hdf5/package.py b/var/spack/repos/builtin/packages/hdf5/package.py
index 4da24dd7f1..5a3f9e5657 100644
--- a/var/spack/repos/builtin/packages/hdf5/package.py
+++ b/var/spack/repos/builtin/packages/hdf5/package.py
@@ -257,7 +257,7 @@ HDF5 version {version} {version}
                 cc = Executable(spec['mpi'].mpicc)
             else:
                 cc = Executable(self.compiler.cc)
-            cc(*(['-c', "check.c"] + spec['hdf5'].cppflags.split()))
+            cc(*(['-c', "check.c"] + spec['hdf5'].headers.cpp_flags.split()))
             cc(*(['-o', "check", "check.o"] +
                  spec['hdf5'].libs.ld_flags.split()))
             try:
diff --git a/var/spack/repos/builtin/packages/hoomd-blue/package.py b/var/spack/repos/builtin/packages/hoomd-blue/package.py
index 3d56f08dc0..0aa9574a6d 100644
--- a/var/spack/repos/builtin/packages/hoomd-blue/package.py
+++ b/var/spack/repos/builtin/packages/hoomd-blue/package.py
@@ -81,7 +81,7 @@ class HoomdBlue(CMakePackage):
         spec = self.spec
 
         cmake_args = [
-            '-DPYTHON_EXECUTABLE={0}/python'.format(spec['python'].prefix.bin),
+            '-DPYTHON_EXECUTABLE={0}'.format(spec['python'].command.path),
         ]
 
         # MPI support
diff --git a/var/spack/repos/builtin/packages/julia/package.py b/var/spack/repos/builtin/packages/julia/package.py
index 4e7c65f53a..007a384f10 100644
--- a/var/spack/repos/builtin/packages/julia/package.py
+++ b/var/spack/repos/builtin/packages/julia/package.py
@@ -185,7 +185,7 @@ class Julia(Package):
             juliarc.write('\n')
 
         # Install some commonly used packages
-        julia = Executable(join_path(prefix.bin, "julia"))
+        julia = spec['julia'].command
         julia("-e", 'Pkg.init(); Pkg.update()')
 
         # Install HDF5
@@ -216,7 +216,7 @@ class Julia(Package):
             with open(join_path(prefix, "etc", "julia", "juliarc.jl"),
                       "a") as juliarc:
                 juliarc.write('# Python\n')
-                juliarc.write('ENV["PYTHON"] = "%s"\n' % spec["python"].prefix)
+                juliarc.write('ENV["PYTHON"] = "%s"\n' % spec["python"].home)
                 juliarc.write('\n')
             # Python's OpenSSL package installer complains:
             # Error: PREFIX too long: 166 characters, but only 128 allowed
diff --git a/var/spack/repos/builtin/packages/libxml2/package.py b/var/spack/repos/builtin/packages/libxml2/package.py
index 470998d882..4324b1dcd3 100644
--- a/var/spack/repos/builtin/packages/libxml2/package.py
+++ b/var/spack/repos/builtin/packages/libxml2/package.py
@@ -49,7 +49,7 @@ class Libxml2(AutotoolsPackage):
         spec = self.spec
         if '+python' in spec:
             python_args = [
-                '--with-python={0}'.format(spec['python'].prefix),
+                '--with-python={0}'.format(spec['python'].home),
                 '--with-python-install-dir={0}'.format(site_packages_dir)
             ]
         else:
diff --git a/var/spack/repos/builtin/packages/llvm/package.py b/var/spack/repos/builtin/packages/llvm/package.py
index d2bf06e358..868e336778 100644
--- a/var/spack/repos/builtin/packages/llvm/package.py
+++ b/var/spack/repos/builtin/packages/llvm/package.py
@@ -339,7 +339,7 @@ class Llvm(CMakePackage):
         cmake_args = [
             '-DLLVM_REQUIRES_RTTI:BOOL=ON',
             '-DCLANG_DEFAULT_OPENMP_RUNTIME:STRING=libomp',
-            '-DPYTHON_EXECUTABLE:PATH=%s/bin/python' % spec['python'].prefix
+            '-DPYTHON_EXECUTABLE:PATH={0}'.format(spec['python'].command.path),
         ]
 
         if '+gold' in spec:
diff --git a/var/spack/repos/builtin/packages/nwchem/package.py b/var/spack/repos/builtin/packages/nwchem/package.py
index 3a8be3f56e..10c5ec5701 100644
--- a/var/spack/repos/builtin/packages/nwchem/package.py
+++ b/var/spack/repos/builtin/packages/nwchem/package.py
@@ -88,7 +88,7 @@ class Nwchem(Package):
             'MPI_LOC=%s' % spec['mpi'].prefix,
             'USE_PYTHONCONFIG=y',
             'PYTHONVERSION=%s' % spec['python'].version.up_to(2),
-            'PYTHONHOME=%s' % spec['python'].prefix,
+            'PYTHONHOME=%s' % spec['python'].home,
             'BLASOPT=%s' % ((lapack + blas).ld_flags),
             'BLAS_LIB=%s' % blas.ld_flags,
             'LAPACK_LIB=%s' % lapack.ld_flags,
diff --git a/var/spack/repos/builtin/packages/openblas/package.py b/var/spack/repos/builtin/packages/openblas/package.py
index 81f4841931..4ebb38aa7a 100644
--- a/var/spack/repos/builtin/packages/openblas/package.py
+++ b/var/spack/repos/builtin/packages/openblas/package.py
@@ -150,7 +150,7 @@ class Openblas(MakefilePackage):
         blessed_file = join_path(os.path.dirname(self.module.__file__),
                                  'test_cblas_dgemm.output')
 
-        include_flags = spec['openblas'].cppflags
+        include_flags = spec['openblas'].headers.cpp_flags
         link_flags = spec['openblas'].libs.ld_flags
         if self.compiler.name == 'intel':
             link_flags += ' -lifcore'
diff --git a/var/spack/repos/builtin/packages/opencv/package.py b/var/spack/repos/builtin/packages/opencv/package.py
index 8a721032a6..b052cfa12a 100644
--- a/var/spack/repos/builtin/packages/opencv/package.py
+++ b/var/spack/repos/builtin/packages/opencv/package.py
@@ -23,7 +23,6 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 from spack import *
-from glob import glob
 
 
 class Opencv(Package):
@@ -166,22 +165,11 @@ class Opencv(Package):
 
         # Python
         if '+python' in spec:
-            python = spec['python']
-
-            try:
-                python_lib = glob(join_path(
-                    python.prefix.lib, 'libpython*.{0}'.format(dso_suffix)))[0]
-            except KeyError:
-                raise InstallError('Cannot find libpython')
-
-            try:
-                python_include_dir = glob(join_path(python.prefix.include,
-                                                    'python*'))[0]
-            except KeyError:
-                raise InstallError('Cannot find python include directory')
+            python_exe = spec['python'].command.path
+            python_lib = spec['python'].libs[0]
+            python_include_dir = spec['python'].headers.directories[0]
 
             if '^python@3:' in spec:
-                python_exe = join_path(python.prefix.bin, 'python3')
                 cmake_options.extend([
                     '-DBUILD_opencv_python3=ON',
                     '-DPYTHON3_EXECUTABLE={0}'.format(python_exe),
@@ -190,7 +178,6 @@ class Opencv(Package):
                     '-DBUILD_opencv_python2=OFF',
                 ])
             elif '^python@2:3' in spec:
-                python_exe = join_path(python.prefix.bin, 'python2')
                 cmake_options.extend([
                     '-DBUILD_opencv_python2=ON',
                     '-DPYTHON2_EXECUTABLE={0}'.format(python_exe),
diff --git a/var/spack/repos/builtin/packages/openspeedshop/package.py b/var/spack/repos/builtin/packages/openspeedshop/package.py
index c4b150f56c..6ad28ce3b7 100644
--- a/var/spack/repos/builtin/packages/openspeedshop/package.py
+++ b/var/spack/repos/builtin/packages/openspeedshop/package.py
@@ -167,24 +167,18 @@ class Openspeedshop(Package):
         # Appends to cmakeOptions the options that will enable
         # the appropriate base level options to the openspeedshop
         # cmake build.
-        python_vers = format(spec['python'].version.up_to(2))
-        python_pv = '/python' + python_vers
-        python_pvs = '/libpython' + python_vers + '.' + format(dso_suffix)
+        python_exe = spec['python'].command.path
+        python_library = spec['python'].libs[0]
+        python_include = spec['python'].headers.directories[0]
 
         BaseOptions = []
 
         BaseOptions.append('-DBINUTILS_DIR=%s' % spec['binutils'].prefix)
         BaseOptions.append('-DLIBELF_DIR=%s' % spec['libelf'].prefix)
         BaseOptions.append('-DLIBDWARF_DIR=%s' % spec['libdwarf'].prefix)
-        BaseOptions.append(
-            '-DPYTHON_EXECUTABLE=%s'
-            % join_path(spec['python'].prefix + '/bin/python'))
-        BaseOptions.append(
-            '-DPYTHON_INCLUDE_DIR=%s'
-            % join_path(spec['python'].prefix.include) + python_pv)
-        BaseOptions.append(
-            '-DPYTHON_LIBRARY=%s'
-            % join_path(spec['python'].prefix.lib) + python_pvs)
+        BaseOptions.append('-DPYTHON_EXECUTABLE=%s' % python_exe)
+        BaseOptions.append('-DPYTHON_INCLUDE_DIR=%s' % python_include)
+        BaseOptions.append('-DPYTHON_LIBRARY=%s' % python_library)
         BaseOptions.append('-DBoost_NO_SYSTEM_PATHS=TRUE')
         BaseOptions.append('-DBoost_NO_BOOST_CMAKE=TRUE')
         BaseOptions.append('-DBOOST_ROOT=%s' % spec['boost'].prefix)
diff --git a/var/spack/repos/builtin/packages/pagmo/package.py b/var/spack/repos/builtin/packages/pagmo/package.py
index 5a06b70e38..eeeb437db7 100644
--- a/var/spack/repos/builtin/packages/pagmo/package.py
+++ b/var/spack/repos/builtin/packages/pagmo/package.py
@@ -110,7 +110,7 @@ class Pagmo(CMakePackage):
         if '+python' in spec:
             args.extend([
                 # By default picks up the system python not the Spack build
-                '-DPYTHON_EXECUTABLE={0}'.format(python_exe),
+                '-DPYTHON_EXECUTABLE={0}'.format(spec['python'].command.path),
                 # By default installs to the python prefix not the pagmo prefix
                 '-DPYTHON_MODULES_DIR={0}'.format(site_packages_dir),
             ])
diff --git a/var/spack/repos/builtin/packages/paraview/package.py b/var/spack/repos/builtin/packages/paraview/package.py
index 9b2b33625a..ea0722dab8 100644
--- a/var/spack/repos/builtin/packages/paraview/package.py
+++ b/var/spack/repos/builtin/packages/paraview/package.py
@@ -120,8 +120,7 @@ class Paraview(CMakePackage):
         if '+python' in spec:
             cmake_args.extend([
                 '-DPARAVIEW_ENABLE_PYTHON:BOOL=ON',
-                '-DPYTHON_EXECUTABLE:FILEPATH=%s/bin/python'
-                % spec['python'].prefix
+                '-DPYTHON_EXECUTABLE:FILEPATH=%s' % spec['python'].command.path
             ])
 
         if '+mpi' in spec:
diff --git a/var/spack/repos/builtin/packages/perl/package.py b/var/spack/repos/builtin/packages/perl/package.py
index ad67cae48c..dc1f7be93c 100644
--- a/var/spack/repos/builtin/packages/perl/package.py
+++ b/var/spack/repos/builtin/packages/perl/package.py
@@ -113,11 +113,10 @@ class Perl(Package):  # Perl doesn't use Autotools, it should subclass Package
     @run_after('install')
     def install_cpanm(self):
         spec = self.spec
-        prefix = self.prefix
 
         if '+cpanm' in spec:
             with working_dir(join_path('cpanm', 'cpanm')):
-                perl = Executable(join_path(prefix.bin, 'perl'))
+                perl = spec['perl'].command
                 perl('Makefile.PL')
                 make()
                 make('install')
diff --git a/var/spack/repos/builtin/packages/plumed/package.py b/var/spack/repos/builtin/packages/plumed/package.py
index 60443cbcc6..3022cd7959 100644
--- a/var/spack/repos/builtin/packages/plumed/package.py
+++ b/var/spack/repos/builtin/packages/plumed/package.py
@@ -122,7 +122,7 @@ class Plumed(AutotoolsPackage):
 
     def setup_dependent_package(self, module, dependent_spec):
         # Make plumed visible from dependent packages
-        module.plumed = Executable(join_path(self.spec.prefix.bin, 'plumed'))
+        module.plumed = self.spec['plumed'].command
 
     @run_before('autoreconf')
     def filter_gslcblas(self):
diff --git a/var/spack/repos/builtin/packages/pocl/package.py b/var/spack/repos/builtin/packages/pocl/package.py
index b6baee7e07..8becd9e9f7 100644
--- a/var/spack/repos/builtin/packages/pocl/package.py
+++ b/var/spack/repos/builtin/packages/pocl/package.py
@@ -104,7 +104,7 @@ class Pocl(CMakePackage):
         with working_dir(checkdir, create=True):
             source = join_path(os.path.dirname(self.module.__file__),
                                "example1.c")
-            cflags = spec["pocl"].cppflags.split()
+            cflags = spec["pocl"].headers.cpp_flags.split()
             # ldflags = spec["pocl"].libs.ld_flags.split()
             ldflags = ["-L%s" % spec["pocl"].prefix.lib,
                        "-lOpenCL", "-lpoclu"]
diff --git a/var/spack/repos/builtin/packages/psi4/package.py b/var/spack/repos/builtin/packages/psi4/package.py
index d14aff363b..976b5d3c4e 100644
--- a/var/spack/repos/builtin/packages/psi4/package.py
+++ b/var/spack/repos/builtin/packages/psi4/package.py
@@ -115,8 +115,7 @@ class Psi4(Package):
             ' -I'.join([
                 os.path.join(spec['psi4'].prefix.include, 'psi4'),
                 os.path.join(spec['boost'].prefix.include, 'boost'),
-                os.path.join(spec['python'].prefix.include, 'python{0}'.format(
-                    spec['python'].version.up_to(2))),
+                os.path.join(spec['python'].headers.directories[0]),
                 spec['lapack'].prefix.include,
                 spec['blas'].prefix.include,
                 '/usr/include'
diff --git a/var/spack/repos/builtin/packages/python/package.py b/var/spack/repos/builtin/packages/python/package.py
index bc3de0479b..12b77813d1 100644
--- a/var/spack/repos/builtin/packages/python/package.py
+++ b/var/spack/repos/builtin/packages/python/package.py
@@ -24,7 +24,9 @@
 ##############################################################################
 import ast
 import os
+import platform
 import re
+import sys
 from contextlib import closing
 
 import spack
@@ -33,10 +35,11 @@ from llnl.util.lang import match_predicate
 from llnl.util.filesystem import force_remove
 from spack import *
 from spack.util.environment import *
+from spack.util.prefix import Prefix
 import spack.util.spack_json as sjson
 
 
-class Python(Package):
+class Python(AutotoolsPackage):
     """The Python programming language."""
 
     homepage = "http://www.python.org"
@@ -62,6 +65,10 @@ class Python(Package):
 
     extendable = True
 
+    # --enable-shared is known to cause problems for some users on macOS
+    # See http://bugs.python.org/issue29846
+    variant('shared', default=sys.platform != 'darwin',
+            description='Enable shared libraries')
     variant('tk', default=False, description='Provide support for Tkinter')
     variant('ucs4', default=False,
             description='Enable UCS4 (wide) unicode strings')
@@ -82,13 +89,14 @@ class Python(Package):
     depends_on("tk", when="+tk")
     depends_on("tcl", when="+tk")
 
-    patch('ncurses.patch')
+    # Patch does not work for Python 3.1
+    patch('ncurses.patch', when='@:2.8,3.2:')
 
     _DISTUTIL_VARS_TO_SAVE = ['LDSHARED']
     _DISTUTIL_CACHE_FILENAME = 'sysconfig.json'
     _distutil_vars = None
 
-    @when('@2.7,3.4:')
+    @when('@2.7:2.8,3.4:')
     def patch(self):
         # NOTE: Python's default installation procedure makes it possible for a
         # user's local configurations to change the Spack installation.  In
@@ -100,7 +108,10 @@ class Python(Package):
             r'\1setup.py\2 --no-user-cfg \3\6'
         )
 
-    def install(self, spec, prefix):
+    def setup_environment(self, spack_env, run_env):
+        spec = self.spec
+        prefix = self.prefix
+
         # TODO: The '--no-user-cfg' option for Python installation is only in
         # Python v2.7 and v3.4+ (see https://bugs.python.org/issue1180) and
         # adding support for ignoring user configuration will require
@@ -110,23 +121,27 @@ class Python(Package):
                       'user configurations are present.').format(self.version))
 
         # Need this to allow python build to find the Python installation.
-        env['PYTHONHOME'], env['PYTHONPATH'] = prefix, prefix
-        env['MACOSX_DEPLOYMENT_TARGET'] = '10.6'
+        spack_env.set('PYTHONHOME', prefix)
+        spack_env.set('PYTHONPATH', prefix)
+        spack_env.set('MACOSX_DEPLOYMENT_TARGET', platform.mac_ver()[0])
+
+    def configure_args(self):
+        spec = self.spec
 
-        # Rest of install is pretty standard except setup.py needs to
-        # be able to read the CPPFLAGS and LDFLAGS as it scans for the
-        # library and headers to build
+        # setup.py needs to be able to read the CPPFLAGS and LDFLAGS
+        # as it scans for the library and headers to build
         dep_pfxs = [dspec.prefix for dspec in spec.dependencies('link')]
         config_args = [
-            '--prefix={0}'.format(prefix),
             '--with-threads',
-            '--enable-shared',
             'CPPFLAGS=-I{0}'.format(' -I'.join(dp.include for dp in dep_pfxs)),
             'LDFLAGS=-L{0}'.format(' -L'.join(dp.lib for dp in dep_pfxs)),
         ]
-        if spec.satisfies("platform=darwin") and ('%gcc' in spec):
+        if spec.satisfies('%gcc platform=darwin'):
             config_args.append('--disable-toolbox-glue')
 
+        if '+shared' in spec:
+            config_args.append('--enable-shared')
+
         if '+ucs4' in spec:
             if spec.satisfies('@:2.7'):
                 config_args.append('--enable-unicode=ucs4')
@@ -140,18 +155,18 @@ class Python(Package):
         if spec.satisfies('@3:'):
             config_args.append('--without-ensurepip')
 
-        configure(*config_args)
-        make()
-        make('install')
+        return config_args
+
+    @run_after('install')
+    def post_install(self):
+        spec = self.spec
+        prefix = self.prefix
 
         self.sysconfigfilename = '_sysconfigdata.py'
         if spec.satisfies('@3.6:'):
             # Python 3.6.0 renamed the sys config file
-            python3 = os.path.join(prefix.bin,
-                                   'python{0}'.format(self.version.up_to(1)))
-            python = Executable(python3)
             sc = 'import sysconfig; print(sysconfig._get_sysconfigdata_name())'
-            cf = python('-c', sc, output=str).strip('\n')
+            cf = self.command('-c', sc, output=str).strip()
             self.sysconfigfilename = '{0}.py'.format(cf)
 
         self._save_distutil_vars(prefix)
@@ -205,7 +220,7 @@ class Python(Package):
         Spack partially covers this by setting environment variables that
         are also accounted for by distutils. Currently there is one more known
         variable that must be set, which is LDSHARED, so the method saves its
-        autogenerated value to pass it to the dependant package's setup script.
+        autogenerated value to pass it to the dependent package's setup script.
         """
 
         self._distutil_vars = {}
@@ -234,20 +249,20 @@ class Python(Package):
             pass
 
         if not input_dict:
-            tty.warn('Failed to find \'build_time_vars\' dictionary in file '
-                     '\'%s\'. This might cause the extensions that are '
-                     'installed with distutils to call compilers directly '
-                     'avoiding Spack\'s wrappers.' % input_filename)
+            tty.warn("Failed to find 'build_time_vars' dictionary in file "
+                     "'%s'. This might cause the extensions that are "
+                     "installed with distutils to call compilers directly "
+                     "avoiding Spack's wrappers." % input_filename)
             return
 
         for var_name in Python._DISTUTIL_VARS_TO_SAVE:
             if var_name in input_dict:
                 self._distutil_vars[var_name] = input_dict[var_name]
             else:
-                tty.warn('Failed to find key \'%s\' in \'build_time_vars\' '
-                         'dictionary in file \'%s\'. This might cause the '
-                         'extensions that are installed with distutils to '
-                         'call compilers directly avoiding Spack\'s wrappers.'
+                tty.warn("Failed to find key '%s' in 'build_time_vars' "
+                         "dictionary in file '%s'. This might cause the "
+                         "extensions that are installed with distutils to "
+                         "call compilers directly avoiding Spack's wrappers."
                          % (var_name, input_filename))
 
         if len(self._distutil_vars) > 0:
@@ -259,10 +274,10 @@ class Python(Package):
                 with open(output_filename, 'w') as output_file:
                     sjson.dump(self._distutil_vars, output_file)
             except:
-                tty.warn('Failed to save metadata for distutils. This might '
-                         'cause the extensions that are installed with '
-                         'distutils to call compilers directly avoiding '
-                         'Spack\'s wrappers.')
+                tty.warn("Failed to save metadata for distutils. This might "
+                         "cause the extensions that are installed with "
+                         "distutils to call compilers directly avoiding "
+                         "Spack's wrappers.")
                 # We make the cache empty if we failed to save it to file
                 # to provide the same behaviour as in the case when the cache
                 # is initialized by the method load_distutils_data().
@@ -320,6 +335,144 @@ class Python(Package):
     # ========================================================================
 
     @property
+    def command(self):
+        """Returns the Python command, which may vary depending
+        on the version of Python and how it was installed.
+
+        In general, Python 2 comes with ``python`` and ``python2`` commands,
+        while Python 3 only comes with a ``python3`` command.
+
+        :returns: The Python command
+        :rtype: Executable
+        """
+        # We need to be careful here. If the user is using an externally
+        # installed python, all 3 commands could be in the same directory.
+
+        # Search for `python2` iff using Python 2
+        if (self.spec.satisfies('@:2') and
+                os.path.exists(os.path.join(self.prefix.bin, 'python2'))):
+            command = 'python2'
+        # Search for `python3` iff using Python 3
+        elif (self.spec.satisfies('@3:') and
+                os.path.exists(os.path.join(self.prefix.bin, 'python3'))):
+            command = 'python3'
+        # If neither were found, try `python`
+        elif os.path.exists(os.path.join(self.prefix.bin, 'python')):
+            command = 'python'
+        else:
+            msg = 'Unable to locate {0} command in {1}'
+            raise RuntimeError(msg.format(self.name, self.prefix.bin))
+
+        # The python command may be a symlink if it was installed
+        # with Homebrew. Since some packages try to determine the
+        # location of libraries and headers based on the path,
+        # return the realpath
+        path = os.path.realpath(os.path.join(self.prefix.bin, command))
+
+        return Executable(path)
+
+    def print_string(self, string):
+        """Returns the appropriate print string depending on the
+        version of Python.
+
+        Examples:
+
+        * Python 2
+
+          .. code-block:: python
+
+             >>> self.print_string('sys.prefix')
+             'print sys.prefix'
+
+        * Python 3
+
+          .. code-block:: python
+
+             >>> self.print_string('sys.prefix')
+             'print(sys.prefix)'
+        """
+        if self.spec.satisfies('@:2'):
+            return 'print {0}'.format(string)
+        else:
+            return 'print({0})'.format(string)
+
+    def get_config_var(self, key):
+        """Returns the value of a single variable. Wrapper around
+        ``distutils.sysconfig.get_config_var()``."""
+
+        cmd = 'from distutils.sysconfig import get_config_var; '
+        cmd += self.print_string("get_config_var('{0}')".format(key))
+
+        return self.command('-c', cmd, output=str).strip()
+
+    def get_config_h_filename(self):
+        """Returns the full path name of the configuration header.
+        Wrapper around ``distutils.sysconfig.get_config_h_filename()``."""
+
+        cmd = 'from distutils.sysconfig import get_config_h_filename; '
+        cmd += self.print_string('get_config_h_filename()')
+
+        return self.command('-c', cmd, output=str).strip()
+
+    @property
+    def home(self):
+        """Most of the time, ``PYTHONHOME`` is simply
+        ``spec['python'].prefix``. However, if the user is using an
+        externally installed python, it may be symlinked. For example,
+        Homebrew installs python in ``/usr/local/Cellar/python/2.7.12_2``
+        and symlinks it to ``/usr/local``. Users may not know the actual
+        installation directory and add ``/usr/local`` to their
+        ``packages.yaml`` unknowingly. Query the python executable to
+        determine exactly where it is installed."""
+
+        prefix = self.get_config_var('prefix')
+        return Prefix(prefix)
+
+    @property
+    def libs(self):
+        # Spack installs libraries into lib, except on openSUSE where it
+        # installs them into lib64. If the user is using an externally
+        # installed package, it may be in either lib or lib64, so we need
+        # to ask Python where its LIBDIR is.
+        libdir = self.get_config_var('LIBDIR')
+
+        # The system Python installation on macOS and Homebrew installations
+        # install libraries into a Frameworks directory
+        frameworkprefix = self.get_config_var('PYTHONFRAMEWORKPREFIX')
+
+        if '+shared' in self.spec:
+            ldlibrary = self.get_config_var('LDLIBRARY')
+
+            if os.path.exists(os.path.join(libdir, ldlibrary)):
+                return LibraryList(os.path.join(libdir, ldlibrary))
+            elif os.path.exists(os.path.join(frameworkprefix, ldlibrary)):
+                return LibraryList(os.path.join(frameworkprefix, ldlibrary))
+            else:
+                msg = 'Unable to locate {0} libraries in {1}'
+                raise RuntimeError(msg.format(self.name, libdir))
+        else:
+            library = self.get_config_var('LIBRARY')
+
+            if os.path.exists(os.path.join(libdir, library)):
+                return LibraryList(os.path.join(libdir, library))
+            elif os.path.exists(os.path.join(frameworkprefix, library)):
+                return LibraryList(os.path.join(frameworkprefix, library))
+            else:
+                msg = 'Unable to locate {0} libraries in {1}'
+                raise RuntimeError(msg.format(self.name, libdir))
+
+    @property
+    def headers(self):
+        config_h = self.get_config_h_filename()
+
+        if os.path.exists(config_h):
+            return HeaderList(config_h)
+        else:
+            includepy = self.get_config_var('INCLUDEPY')
+            msg = 'Unable to locate {0} headers in {1}'
+            raise RuntimeError(msg.format(self.name, includepy))
+
+    @property
     def python_lib_dir(self):
         return join_path('lib', 'python{0}'.format(self.version.up_to(2)))
 
@@ -332,29 +485,10 @@ class Python(Package):
         return join_path(self.python_lib_dir, 'site-packages')
 
     def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
-        """Set PYTHONPATH to include site-packages dir for the
+        """Set PYTHONPATH to include the site-packages directory for the
         extension and any other python extensions it depends on."""
-        # The python executable for version 3 may be python3 or python
-        # See https://github.com/LLNL/spack/pull/2173#issuecomment-257170199
-        pythonex = 'python{0}'.format('3' if self.spec.satisfies('@3') else '')
-        if os.path.isdir(self.prefix.bin):
-            base = self.prefix.bin
-        else:
-            base = self.prefix
-        if not os.path.isfile(os.path.join(base, pythonex)):
-            if self.spec.satisfies('@3'):
-                python = Executable(os.path.join(base, 'python'))
-                version = python('-c', 'import sys; print(sys.version)',
-                                 output=str)
-                if version.startswith('3'):
-                    pythonex = 'python'
-                else:
-                    raise RuntimeError('Cannot locate python executable')
-            else:
-                raise RuntimeError('Cannot locate python executable')
-        python = Executable(os.path.join(base, pythonex))
-        prefix = python('-c', 'import sys; print(sys.prefix)', output=str)
-        spack_env.set('PYTHONHOME', prefix.strip('\n'))
+
+        spack_env.set('PYTHONHOME', self.home)
 
         python_paths = []
         for d in dependent_spec.traverse(
@@ -378,19 +512,15 @@ class Python(Package):
         In most cases, extensions will only need to have one line::
 
         setup_py('install', '--prefix={0}'.format(prefix))"""
-        python_path = join_path(
-            self.spec.prefix.bin,
-            'python{0}'.format('3' if self.spec.satisfies('@3') else '')
-        )
 
-        module.python_exe = python_path
-        module.python = Executable(python_path)
-        module.setup_py = Executable(python_path + ' setup.py --no-user-cfg')
+        module.python = self.command
+        module.setup_py = Executable(
+            self.command.path + ' setup.py --no-user-cfg')
 
         distutil_vars = self._load_distutil_vars()
 
         if distutil_vars:
-            for key, value in distutil_vars.iteritems():
+            for key, value in distutil_vars.items():
                 module.setup_py.add_default_env(key, value)
 
         # Add variables for lib/pythonX.Y and lib/pythonX.Y/site-packages dirs.
@@ -429,7 +559,7 @@ class Python(Package):
         if ext_pkg.name != 'py-pygments':
             patterns.append(r'bin/pygmentize$')
         if ext_pkg.name != 'py-numpy':
-            patterns.append(r'bin/f2py3?$')
+            patterns.append(r'bin/f2py[0-9.]*$')
 
         return match_predicate(ignore_arg, patterns)
 
@@ -457,7 +587,7 @@ class Python(Package):
 
                     paths.append(line)
 
-        site_packages = join_path(self.prefix, self.site_packages_dir)
+        site_packages = join_path(self.home, self.site_packages_dir)
         main_pth = join_path(site_packages, "easy-install.pth")
 
         if not paths:
diff --git a/var/spack/repos/builtin/packages/scotch/package.py b/var/spack/repos/builtin/packages/scotch/package.py
index 8efb629487..89d1f8e568 100644
--- a/var/spack/repos/builtin/packages/scotch/package.py
+++ b/var/spack/repos/builtin/packages/scotch/package.py
@@ -22,7 +22,6 @@
 # License along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import os
 from spack import *
 
 
@@ -165,8 +164,8 @@ class Scotch(Package):
 
         # General Features #
 
-        flex_path = os.path.join(self.spec['flex'].prefix.bin, 'flex')
-        bison_path = os.path.join(self.spec['bison'].prefix.bin, 'bison')
+        flex_path = self.spec['flex'].command.path
+        bison_path = self.spec['bison'].command.path
         makefile_inc.extend([
             'EXE       =',
             'OBJ       = .o',
diff --git a/var/spack/repos/builtin/packages/shiny-server/package.py b/var/spack/repos/builtin/packages/shiny-server/package.py
index e8a899115f..a3a2b2b160 100644
--- a/var/spack/repos/builtin/packages/shiny-server/package.py
+++ b/var/spack/repos/builtin/packages/shiny-server/package.py
@@ -54,9 +54,7 @@ class ShinyServer(CMakePackage):
         spec = self.spec
         options = []
 
-        options.extend([
-            "-DPYTHON=%s" % join_path(spec['python'].prefix.bin, 'python'),
-        ])
+        options.append("-DPYTHON=%s" % spec['python'].command.path)
 
         return options
 
diff --git a/var/spack/repos/builtin/packages/spark/package.py b/var/spack/repos/builtin/packages/spark/package.py
index 24dca5571d..3bc6abd234 100644
--- a/var/spack/repos/builtin/packages/spark/package.py
+++ b/var/spack/repos/builtin/packages/spark/package.py
@@ -69,8 +69,7 @@ class Spark(Package):
         env['JAVA_HOME'] = self.spec['jdk'].prefix
         # spack_env.set('JAVA_HOME', self.spec['jdk'].prefix)
 
-        hadoop_bin_path = join_path(self.spec['hadoop'].prefix.bin, 'hadoop')
-        hadoop_bin = Executable(hadoop_bin_path)
-        hadoop_classpath = hadoop_bin('classpath', return_output=True)
+        hadoop = self.spec['hadoop'].command
+        hadoop_classpath = hadoop('classpath', return_output=True)
 
         run_env.set('SPARK_DIST_CLASSPATH', hadoop_classpath)
diff --git a/var/spack/repos/builtin/packages/stat/package.py b/var/spack/repos/builtin/packages/stat/package.py
index 6fe82baa2f..59d110330a 100644
--- a/var/spack/repos/builtin/packages/stat/package.py
+++ b/var/spack/repos/builtin/packages/stat/package.py
@@ -69,7 +69,7 @@ class Stat(AutotoolsPackage):
             "--with-graphlib=%s"    % spec['graphlib'].prefix,
             "--with-stackwalker=%s" % spec['dyninst'].prefix,
             "--with-libdwarf=%s"    % spec['libdwarf'].prefix,
-            "--with-python=%s/bin/python" % spec['python'].prefix
+            "--with-python=%s"      % spec['python'].command.path,
         ]
         if '+dysect' in spec:
             args.append('--enable-dysectapi')
diff --git a/var/spack/repos/builtin/packages/subversion/package.py b/var/spack/repos/builtin/packages/subversion/package.py
index c7057a51df..628dd26d24 100644
--- a/var/spack/repos/builtin/packages/subversion/package.py
+++ b/var/spack/repos/builtin/packages/subversion/package.py
@@ -72,8 +72,7 @@ class Subversion(Package):
         if 'swig' in spec:
             options.append('--with-swig=%s' % spec['swig'].prefix)
         if 'perl' in spec:
-            options.append(
-                'PERL=%s' % join_path(spec['perl'].prefix.bin, 'perl'))
+            options.append('PERL=%s' % spec['perl'].command.path)
 
         configure(*options)
         make()
diff --git a/var/spack/repos/builtin/packages/superlu-dist/package.py b/var/spack/repos/builtin/packages/superlu-dist/package.py
index e19a33a900..e7d7b1587d 100644
--- a/var/spack/repos/builtin/packages/superlu-dist/package.py
+++ b/var/spack/repos/builtin/packages/superlu-dist/package.py
@@ -72,8 +72,8 @@ class SuperluDist(Package):
             'RANLIB       = true',
             'CC           = {0}'.format(self.spec['mpi'].mpicc),
             'CFLAGS       = -fPIC -std=c99 -O2 %s %s %s' % (
-                spec['parmetis'].cppflags,
-                spec['metis'].cppflags,
+                spec['parmetis'].headers.cpp_flags,
+                spec['metis'].headers.cpp_flags,
                 '-D_LONGINT' if '+int64' in spec else ''),
             'NOOPTS       = -fPIC -std=c99',
             'FORTRAN      = {0}'.format(self.spec['mpi'].mpif77),
diff --git a/var/spack/repos/builtin/packages/visit/package.py b/var/spack/repos/builtin/packages/visit/package.py
index 48a3762a2a..3c8505db97 100644
--- a/var/spack/repos/builtin/packages/visit/package.py
+++ b/var/spack/repos/builtin/packages/visit/package.py
@@ -52,7 +52,7 @@ class Visit(Package):
                 '-DVTK_MINOR_VERSION=1',
                 '-DVISIT_USE_GLEW=OFF',
                 '-DVISIT_LOC_QMAKE_EXE:FILEPATH={0}/qmake-qt4'.format(qt_bin),
-                '-DPYTHON_DIR:PATH={0}'.format(spec['python'].prefix),
+                '-DPYTHON_DIR:PATH={0}'.format(spec['python'].home),
                 '-DVISIT_SILO_DIR:PATH={0}'.format(spec['silo'].prefix),
                 '-DVISIT_HDF5_DIR:PATH={0}'.format(spec['hdf5'].prefix),
                 '-DVISIT_VTK_DIR:PATH={0}'.format(spec['vtk'].prefix),