Add hook to enable monkeypatching in Debian and unblock distutils adoption in Setuptools.

distutils/__init__.py

@@ -9,5 +9,15 @@
 """
 
 import sys
+import importlib
 
 __version__ = sys.version[:sys.version.index(' ')]
+
+
+try:
+    # Allow Debian (only) to customize system behavior.
+    # Ref pypa/distutils#2. This hook is deprecated and
+    # no other environments should use it.
+    importlib.import_module('_distutils_system_mod')
+except ImportError:
+    pass

distutils/command/install_egg_info.py

@@ -19,14 +19,21 @@ class install_egg_info(Command):
     def initialize_options(self):
         self.install_dir = None
 
-    def finalize_options(self):
-        self.set_undefined_options('install_lib',('install_dir','install_dir'))
-        basename = "%s-%s-py%d.%d.egg-info" % (
+    @property
+    def basename(self):
+        """
+        Allow basename to be overridden by child class.
+        Ref pypa/distutils#2.
+        """
+        return "%s-%s-py%d.%d.egg-info" % (
             to_filename(safe_name(self.distribution.get_name())),
             to_filename(safe_version(self.distribution.get_version())),
             *sys.version_info[:2]
         )
-        self.target = os.path.join(self.install_dir, basename)
+
+    def finalize_options(self):
+        self.set_undefined_options('install_lib',('install_dir','install_dir'))
+        self.target = os.path.join(self.install_dir, self.basename)
         self.outputs = [self.target]
 
     def run(self):

distutils/sysconfig.py

@@ -129,6 +129,14 @@ def get_python_inc(plat_specific=0, prefix=None):
             "on platform '%s'" % os.name)
 
 
+# allow this behavior to be monkey-patched. Ref pypa/distutils#2.
+def _posix_lib(standard_lib, libpython, early_prefix, prefix):
+    if standard_lib:
+        return libpython
+    else:
+        return os.path.join(libpython, "site-packages")
+
+
 def get_python_lib(plat_specific=0, standard_lib=0, prefix=None):
     """Return the directory containing the Python library (standard or
     site additions).
@@ -152,6 +160,8 @@ def get_python_lib(plat_specific=0, standard_lib=0, prefix=None):
             return os.path.join(prefix, "lib-python", sys.version[0])
         return os.path.join(prefix, 'site-packages')
 
+    early_prefix = prefix
+
     if prefix is None:
         if standard_lib:
             prefix = plat_specific and BASE_EXEC_PREFIX or BASE_PREFIX
@@ -169,10 +179,7 @@ def get_python_lib(plat_specific=0, standard_lib=0, prefix=None):
         implementation = 'pypy' if IS_PYPY else 'python'
         libpython = os.path.join(prefix, libdir,
                                  implementation + get_python_version())
-        if standard_lib:
-            return libpython
-        else:
-            return os.path.join(libpython, "site-packages")
+        return _posix_lib(standard_lib, libpython, early_prefix, prefix)
     elif os.name == "nt":
         if standard_lib:
             return os.path.join(prefix, "Lib")