Add minimal code to handle --prepend-config

Add code to handle config files specified with the --prepend-config
argument.

Options specified in prepend files get a higher precedence than flake8
defaults but a lower one than user configs. This allows to override
the defaults without affecting the behaviour of user and local config
files.

This implements a solution for issue #349

tests/integration/test_aggregator.py

@@ -26,7 +26,7 @@ def test_aggregate_options_with_config(optmanager):
     """Verify we aggregate options and config values appropriately."""
     arguments = ['flake8', '--config', CLI_SPECIFIED_CONFIG, '--select',
                  'E11,E34,E402,W,F', '--exclude', 'tests/*']
-    config_finder = config.ConfigFileFinder('flake8', arguments, [])
+    config_finder = config.ConfigFileFinder('flake8', arguments, [], [])
     options, args = aggregator.aggregate_options(
         optmanager, config_finder, arguments)
 
@@ -40,7 +40,7 @@ def test_aggregate_options_when_isolated(optmanager):
     """Verify we aggregate options and config values appropriately."""
     arguments = ['flake8', '--isolated', '--select', 'E11,E34,E402,W,F',
                  '--exclude', 'tests/*']
-    config_finder = config.ConfigFileFinder('flake8', arguments, [])
+    config_finder = config.ConfigFileFinder('flake8', arguments, [], [])
     optmanager.extend_default_ignore(['E8'])
     options, args = aggregator.aggregate_options(
         optmanager, config_finder, arguments)

tests/unit/test_config_file_finder.py

@@ -15,7 +15,7 @@ BROKEN_CONFIG_PATH = 'tests/fixtures/config_files/broken.ini'
 
 def test_uses_default_args():
     """Show that we default the args value."""
-    finder = config.ConfigFileFinder('flake8', None, [])
+    finder = config.ConfigFileFinder('flake8', None, [], [])
     assert finder.parent == os.path.abspath('.')
 
 
@@ -27,14 +27,14 @@ def test_uses_default_args():
 def test_windows_detection(platform, is_windows):
     """Verify we detect Windows to the best of our knowledge."""
     with mock.patch.object(sys, 'platform', platform):
-        finder = config.ConfigFileFinder('flake8', None, [])
+        finder = config.ConfigFileFinder('flake8', None, [], [])
     assert finder.is_windows is is_windows
 
 
 def test_cli_config():
     """Verify opening and reading the file specified via the cli."""
     cli_filepath = CLI_SPECIFIED_FILEPATH
-    finder = config.ConfigFileFinder('flake8', None, [])
+    finder = config.ConfigFileFinder('flake8', None, [], [])
 
     parsed_config = finder.cli_config(cli_filepath)
     assert parsed_config.has_section('flake8')
@@ -42,7 +42,7 @@ def test_cli_config():
 
 def test_cli_config_double_read():
     """Second request for CLI config is cached."""
-    finder = config.ConfigFileFinder('flake8', None, [])
+    finder = config.ConfigFileFinder('flake8', None, [], [])
 
     parsed_config = finder.cli_config(CLI_SPECIFIED_FILEPATH)
     boom = Exception("second request for CLI config not cached")
@@ -68,59 +68,82 @@ def test_cli_config_double_read():
 ])
 def test_generate_possible_local_files(args, expected):
     """Verify generation of all possible config paths."""
-    finder = config.ConfigFileFinder('flake8', args, [])
+    finder = config.ConfigFileFinder('flake8', args, [], [])
 
     assert (list(finder.generate_possible_local_files()) ==
             expected)
 
 
-@pytest.mark.parametrize('args,extra_config_files,expected', [
-    # No arguments, common prefix of abspath('.')
-    ([],
-        [],
-        [os.path.abspath('setup.cfg'),
-            os.path.abspath('tox.ini')]),
-    # Common prefix of "flake8/"
-    (['flake8/options', 'flake8/'],
-        [],
-        [os.path.abspath('setup.cfg'),
-            os.path.abspath('tox.ini')]),
-    # Common prefix of "flake8/options"
-    (['flake8/options', 'flake8/options/sub'],
-        [],
-        [os.path.abspath('setup.cfg'),
-            os.path.abspath('tox.ini')]),
-    # Common prefix of "flake8/" with extra config files specified
-    (['flake8/'],
-        [CLI_SPECIFIED_FILEPATH],
-        [os.path.abspath('setup.cfg'),
-            os.path.abspath('tox.ini'),
-            os.path.abspath(CLI_SPECIFIED_FILEPATH)]),
-    # Common prefix of "flake8/" with missing extra config files specified
-    (['flake8/'],
-        [CLI_SPECIFIED_FILEPATH,
-            'tests/fixtures/config_files/missing.ini'],
-        [os.path.abspath('setup.cfg'),
-            os.path.abspath('tox.ini'),
-            os.path.abspath(CLI_SPECIFIED_FILEPATH)]),
-])
-def test_local_config_files(args, extra_config_files, expected):
+@pytest.mark.parametrize(
+    'args,prepend_config_files,extra_config_files,expected', [
+        # No arguments, common prefix of abspath('.')
+        ([],
+            [],
+            [],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini')]),
+        # Common prefix of "flake8/"
+        (['flake8/options', 'flake8/'],
+            [],
+            [],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini')]),
+        # Common prefix of "flake8/options"
+        (['flake8/options', 'flake8/options/sub'],
+            [],
+            [],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini')]),
+        # Common prefix of "flake8/" with extra config files specified
+        (['flake8/'],
+            [],
+            [CLI_SPECIFIED_FILEPATH],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini'),
+                os.path.abspath(CLI_SPECIFIED_FILEPATH)]),
+        # Common prefix of "flake8/" with missing extra config files specified
+        (['flake8/'],
+            [],
+            [CLI_SPECIFIED_FILEPATH,
+                'tests/fixtures/config_files/missing.ini'],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini'),
+                os.path.abspath(CLI_SPECIFIED_FILEPATH)]),
+        # Common prefix of "flake8/" with prepend config files specified
+        (['flake8/'],
+            [CLI_SPECIFIED_FILEPATH],
+            [],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini')]),
+        # Common prefix of "flake8/" with prepend and extra config files
+        (['flake8/'],
+            [CLI_SPECIFIED_FILEPATH],
+            [CLI_SPECIFIED_FILEPATH],
+            [os.path.abspath('setup.cfg'),
+                os.path.abspath('tox.ini'),
+                os.path.abspath(CLI_SPECIFIED_FILEPATH)]),
+    ])
+def test_local_config_files(
+        args, prepend_config_files, extra_config_files, expected
+):
     """Verify discovery of local config files."""
-    finder = config.ConfigFileFinder('flake8', args, extra_config_files)
+    finder = config.ConfigFileFinder(
+        'flake8', args, prepend_config_files, extra_config_files
+    )
 
     assert list(finder.local_config_files()) == expected
 
 
 def test_local_configs():
     """Verify we return a ConfigParser."""
-    finder = config.ConfigFileFinder('flake8', None, [])
+    finder = config.ConfigFileFinder('flake8', None, [], [])
 
     assert isinstance(finder.local_configs(), configparser.RawConfigParser)
 
 
 def test_local_configs_double_read():
     """Second request for local configs is cached."""
-    finder = config.ConfigFileFinder('flake8', None, [])
+    finder = config.ConfigFileFinder('flake8', None, [], [])
 
     first_read = finder.local_configs()
     boom = Exception("second request for local configs not cached")

tests/unit/test_get_local_plugins.py

@@ -31,7 +31,7 @@ def test_get_local_plugins_uses_cli_config():
 def test_get_local_plugins():
     """Verify get_local_plugins returns expected plugins."""
     config_fixture_path = 'tests/fixtures/config_files/local-plugin.ini'
-    config_finder = config.ConfigFileFinder('flake8', [], [])
+    config_finder = config.ConfigFileFinder('flake8', [], [], [])
 
     with mock.patch.object(config_finder, 'local_config_files') as localcfs:
         localcfs.return_value = [config_fixture_path]

tests/unit/test_merged_config_parser.py

@@ -17,7 +17,7 @@ def optmanager():
 @pytest.fixture
 def config_finder():
     """Generate a simple ConfigFileFinder."""
-    return config.ConfigFileFinder('flake8', [], [])
+    return config.ConfigFileFinder('flake8', [], [], [])
 
 
 def test_parse_cli_config(optmanager, config_finder):