Add utility functions around filename matching

We add utils.fnmatch and utils.filenames_for in anticipation of our
checker manager creating file checkers for each file. We also include
tests for these functions and a couple previously untested utility
functions.

tests/unit/test_utils.py

@@ -1,5 +1,6 @@
 """Tests for flake8's utils module."""
 import os
+import mock
 
 import pytest
 
@@ -40,3 +41,48 @@ def test_normalize_path(value, expected):
 def test_normalize_paths(value, expected):
     """Verify we normalize comma-separated paths provided to the tool."""
     assert utils.normalize_paths(value) == expected
+
+
+def test_is_windows_checks_for_nt():
+    """Verify that we correctly detect Windows."""
+    with mock.patch.object(os, 'name', 'nt'):
+        assert utils.is_windows() is True
+
+    with mock.patch.object(os, 'name', 'posix'):
+        assert utils.is_windows() is False
+
+
+@pytest.mark.parametrize('filename,patterns,expected', [
+    ('foo.py', [], True),
+    ('foo.py', ['*.pyc'], False),
+    ('foo.pyc', ['*.pyc'], True),
+    ('foo.pyc', ['*.swp', '*.pyc', '*.py'], True),
+])
+def test_fnmatch(filename, patterns, expected):
+    """Verify that our fnmatch wrapper works as expected."""
+    assert utils.fnmatch(filename, patterns) is expected
+
+
+def test_filenames_from_a_directory():
+    """Verify that filenames_from walks a directory."""
+    filenames = list(utils.filenames_from('flake8/'))
+    assert len(filenames) > 2
+    assert 'flake8/__init__.py' in filenames
+
+
+def test_filenames_from_a_directory_with_a_predicate():
+    """Verify that predicates filter filenames_from."""
+    filenames = list(utils.filenames_from(
+        arg='flake8/',
+        predicate=lambda filename: filename == 'flake8/__init__.py',
+    ))
+    assert len(filenames) > 2
+    assert 'flake8/__init__.py' not in filenames
+
+
+def test_filenames_from_a_single_file():
+    """Verify that we simply yield that filename."""
+    filenames = list(utils.filenames_from('flake8/__init__.py'))
+
+    assert len(filenames) == 1
+    assert ['flake8/__init__.py'] == filenames