config: Determine path to user configuration immediately

Preemptively determine the path of the user configuration file during
the construction of the `ConfigFileFinder` object.  The user
configuration path will always be the same, regardless of when it gets
obtained by a run of `flake8`.

This isolates the logic of determining the user configuration path into
a static helper method to be called to set the `.user_config_file`
attribute.  The helper method leverages `utils.is_windows()`, instead of
reimplementing the check, and decomposes clearly the directory name and
the base name to construct the path with a single `return` path.
Additionally, this avoids reconstructing the path on demand of obtaining
the user configuration file path.

tests/unit/test_config_file_finder.py

@@ -2,7 +2,6 @@
 """Tests for the ConfigFileFinder."""
 import configparser
 import os
-import sys
 
 import mock
 import pytest
@@ -13,18 +12,6 @@ CLI_SPECIFIED_FILEPATH = 'tests/fixtures/config_files/cli-specified.ini'
 BROKEN_CONFIG_PATH = 'tests/fixtures/config_files/broken.ini'
 
 
-@pytest.mark.parametrize('platform,is_windows', [
-    ('win32', True),
-    ('linux', False),
-    ('darwin', False),
-])
-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')
-    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

tests/unit/test_merged_config_parser.py

@@ -69,9 +69,9 @@ def test_parse_user_config(optmanager, config_finder):
                           action='count')
     parser = config.MergedConfigParser(optmanager, config_finder)
 
-    with mock.patch.object(parser.config_finder, 'user_config_file') as usercf:
-        usercf.return_value = 'tests/fixtures/config_files/cli-specified.ini'
-        parsed_config = parser.parse_user_config()
+    config_finder.user_config_file = ('tests/fixtures/config_files/'
+                                      'cli-specified.ini')
+    parsed_config = parser.parse_user_config()
 
     assert parsed_config == {
         'ignore': ['E123', 'W234', 'E111'],
@@ -127,11 +127,9 @@ def test_merge_user_and_local_config(optmanager, config_finder):
         localcfs.return_value = [
             'tests/fixtures/config_files/local-config.ini'
         ]
-        with mock.patch.object(config_finder,
-                               'user_config_file') as usercf:
-            usercf.return_value = ('tests/fixtures/config_files/'
-                                   'user-config.ini')
-            parsed_config = parser.merge_user_and_local_config()
+        config_finder.user_config_file = ('tests/fixtures/config_files/'
+                                          'user-config.ini')
+        parsed_config = parser.merge_user_and_local_config()
 
     assert parsed_config == {
         'exclude': [