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.

3 files changed, 24 insertions, 38 deletions

diff --git a/src/flake8/options/config.py b/src/flake8/options/config.py
index 0adb957..5884873 100644
--- a/src/flake8/options/config.py
+++ b/src/flake8/options/config.py
@@ -3,7 +3,6 @@ import collections
 import configparser
 import logging
 import os.path
-import sys
 from typing import List, Optional, Tuple
 
 from flake8 import utils
@@ -46,22 +45,30 @@ class ConfigFileFinder(object):
         # The value of --isolated from the CLI.
         self.ignore_config_files = ignore_config_files
 
-        # Platform specific settings
-        self.is_windows = sys.platform == "win32"
-        self.xdg_home = os.environ.get(
-            "XDG_CONFIG_HOME", os.path.expanduser("~/.config")
-        )
-
-        # Look for '.<program_name>' files
-        self.program_config = "." + program_name
+        # User configuration file.
         self.program_name = program_name
+        self.user_config_file = self._user_config_file(program_name)
 
         # List of filenames to find in the local/project directory
-        self.project_filenames = ("setup.cfg", "tox.ini", self.program_config)
+        self.project_filenames = ("setup.cfg", "tox.ini", "." + program_name)
 
         self.local_directory = os.path.abspath(os.curdir)
 
     @staticmethod
+    def _user_config_file(program_name):
+        # type: (str) -> str
+        if utils.is_windows():
+            home_dir = os.path.expanduser("~")
+            config_file_basename = "." + program_name
+        else:
+            home_dir = os.environ.get(
+                "XDG_CONFIG_HOME", os.path.expanduser("~/.config")
+            )
+            config_file_basename = program_name
+
+        return os.path.join(home_dir, config_file_basename)
+
+    @staticmethod
     def _read_config(*files):
         # type: (*str) -> Tuple[configparser.RawConfigParser, List[str]]
         config = configparser.RawConfigParser()
@@ -139,15 +146,9 @@ class ConfigFileFinder(object):
         """Parse all local config files into one config object."""
         return self.local_configs_with_files()[0]
 
-    def user_config_file(self):
-        """Find the user-level config file."""
-        if self.is_windows:
-            return os.path.expanduser("~\\" + self.program_config)
-        return os.path.join(self.xdg_home, self.program_name)
-
     def user_config(self):
         """Parse the user config file into a config object."""
-        config, found_files = self._read_config(self.user_config_file())
+        config, found_files = self._read_config(self.user_config_file)
         if found_files:
             LOG.debug("Found user configuration files: %s", found_files)
         return config
diff --git a/tests/unit/test_config_file_finder.py b/tests/unit/test_config_file_finder.py
index 29aacd4..d21b03d 100644
--- a/tests/unit/test_config_file_finder.py
+++ b/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
diff --git a/tests/unit/test_merged_config_parser.py b/tests/unit/test_merged_config_parser.py
index 2eaa4b3..41885e4 100644
--- a/tests/unit/test_merged_config_parser.py
+++ b/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': [