Add DISTUTILS_EXTRA_CONFIG option for passing setup.cfg overrides dur…

…ing build

distutils/dist.py

@@ -359,6 +359,10 @@ def find_config_files(self):
         if os.path.isfile(local_file):
             files.append(local_file)
 
+        extra_file = os.getenv("DISTUTILS_EXTRA_CONFIG")
+        if extra_file and os.path.isfile(extra_file):
+            files.append(extra_file)
+
         if DEBUG:
             self.announce("using config files: %s" % ', '.join(files))
 

distutils/tests/test_dist.py

@@ -492,6 +492,27 @@ def test_custom_pydistutils(self):
         finally:
             os.remove(user_filename)
 
+    def test_extra_pydistutils(self):
+        # make sure pydistutils.cfg is found
+        extra_filename = "overrides.cfg"
+
+        temp_dir = self.mkdtemp()
+        extra_filename = os.path.join(temp_dir, extra_filename)
+        with open(extra_filename, 'w') as f:
+            f.write('.')
+
+        # Testing will have been going terribly if this was set, but preserve
+        # it anyway (so it goes terribly but *consistently*)
+        old_extra_filename = os.environ.get("DISTUTILS_EXTRA_CONFIG")
+        os.environ["DISTUTILS_EXTRA_CONFIG"] = extra_filename
+        try:
+            dist = Distribution()
+            files = dist.find_config_files()
+            assert user_filename in files
+        finally:
+            os.remove(user_filename)
+            os.environ["DISTUTILS_EXTRA_CONFIG"] = old_extra_filename
+
     def test_fix_help_options(self):
         help_tuples = [('a', 'b', 'c', 'd'), (1, 2, 3, 4)]
         fancy_options = fix_help_options(help_tuples)

docs/distutils/configfile.rst

@@ -36,7 +36,8 @@ consequences:
   :file:`setup.py`
 
 * installers can override anything in :file:`setup.cfg` using the command-line
-  options to :file:`setup.py`
+  options to :file:`setup.py` or by pointing :envvar:`DISTUTILS_EXTRA_CONFIG`
+  to another configuration file
 
 The basic syntax of the configuration file is simple: