Change how we apply lazy to the git hook

2 files changed, 37 insertions, 5 deletions

diff --git a/src/flake8/main/git.py b/src/flake8/main/git.py
index 642ff38..d0c87d3 100644
--- a/src/flake8/main/git.py
+++ b/src/flake8/main/git.py
@@ -140,11 +140,14 @@ def make_temporary_directory_from(destination, directory):
 
 
 def find_modified_files(lazy):
-    diff_index = piped_process(
-        ['git', 'diff-index', '' if lazy else '--cached', '--name-only',
-         '--diff-filter=ACMRTUXB', 'HEAD'],
-    )
-
+    diff_index_cmd = [
+        'git', 'diff-index', '--cached', '--name-only',
+        '--diff-filter=ACMRTUXB', 'HEAD'
+    ]
+    if lazy:
+        diff_index_cmd.remove('--cached')
+
+    diff_index = piped_process(diff_index_cmd)
     (stdout, _) = diff_index.communicate()
     stdout = to_text(stdout)
     return stdout.splitlines()
diff --git a/tests/unit/test_git.py b/tests/unit/test_git.py
new file mode 100644
index 0000000..24d4b5b
--- /dev/null
+++ b/tests/unit/test_git.py
@@ -0,0 +1,29 @@
+"""Tests around functionality in the git integration."""
+import mock
+import pytest
+
+from flake8.main import git
+
+
+@pytest.mark.parametrize('lazy', [True, False])
+def test_find_modified_files(lazy):
+    """Confirm our logic for listing modified files."""
+    if lazy:
+        # Here --cached is missing
+        call = [
+            'git', 'diff-index', '--name-only', '--diff-filter=ACMRTUXB',
+            'HEAD'
+        ]
+    else:
+        call = [
+            'git', 'diff-index', '--cached', '--name-only',
+            '--diff-filter=ACMRTUXB', 'HEAD'
+        ]
+    mocked_popen = mock.Mock()
+    mocked_popen.communicate.return_value = ('', '')
+
+    with mock.patch('flake8.main.git.piped_process') as piped_process:
+        piped_process.return_value = mocked_popen
+        git.find_modified_files(lazy)
+
+    piped_process.assert_called_once_with(call)