config: Change ConfigFileFinder._read_config() to accept variadic args

This simplifies `._read_config()` by removing a conditional branch in
the situation where it is called with one file to process.  Now the
contract accepts any number of arguments of the same type.

Where callers invoke `._read_config()` with a `Sequence`, the call site
has been changed to unpack arguments (i.e., `*`).

The tests in `test_merged_config_parser.py` needed to return a string
for the user configuration path instead of an empty list since
`ConfigFileFinder.user_config_file()` returns a string.

src/flake8/options/config.py

@@ -4,7 +4,7 @@ import configparser
 import logging
 import os.path
 import sys
-from typing import Dict, List, Sequence, Tuple, Union
+from typing import Dict, List, Tuple
 
 from flake8 import utils
 
@@ -55,11 +55,9 @@ class ConfigFileFinder(object):
         # fmt: on
 
     @staticmethod
-    def _read_config(files):
-        # type: (Union[Sequence[str], str]) -> Tuple[configparser.RawConfigParser, List[str]]  # noqa: E501
+    def _read_config(*files):
+        # type: (*str) -> Tuple[configparser.RawConfigParser, List[str]]
         config = configparser.RawConfigParser()
-        if isinstance(files, (str, type(u""))):
-            files = [files]
 
         found_files = []
         for filename in files:
@@ -129,7 +127,9 @@ class ConfigFileFinder(object):
         Return (config, found_config_files) tuple.
         """
         if self._local_configs is None:
-            config, found_files = self._read_config(self.local_config_files())
+            config, found_files = self._read_config(
+                *self.local_config_files()
+            )
             if found_files:
                 LOG.debug("Found local configuration files: %s", found_files)
             self._local_configs = config

tests/unit/test_config_file_finder.py

@@ -113,7 +113,7 @@ def test_local_configs_double_read():
 ])
 def test_read_config_catches_broken_config_files(files):
     """Verify that we do not allow the exception to bubble up."""
-    _, parsed = config.ConfigFileFinder._read_config(files)
+    _, parsed = config.ConfigFileFinder._read_config(*files)
     assert BROKEN_CONFIG_PATH not in parsed
 
 

tests/unit/test_merged_config_parser.py

@@ -183,7 +183,7 @@ def test_parsed_configs_are_equivalent(
         localcfs.return_value = [config_fixture_path]
         with mock.patch.object(config_finder,
                                'user_config_file') as usercf:
-            usercf.return_value = []
+            usercf.return_value = ''
             parsed_config = parser.merge_user_and_local_config()
 
     assert parsed_config['ignore'] == ['E123', 'W234', 'E111']
@@ -215,7 +215,7 @@ def test_parsed_hyphenated_and_underscored_names(
         localcfs.return_value = [config_file]
         with mock.patch.object(config_finder,
                                'user_config_file') as usercf:
-            usercf.return_value = []
+            usercf.return_value = ''
             parsed_config = parser.merge_user_and_local_config()
 
     assert parsed_config['max_line_length'] == 110