Add integration test for flake8 --diff

tests/integration/test_main.py

@@ -0,0 +1,43 @@
+"""Integration tests for the main entrypoint of flake8."""
+import mock
+
+from flake8 import utils
+from flake8.main import application
+
+
+def test_diff_option(tmpdir, capsys):
+    """Ensure that FileChecker can handle --diff."""
+    t_py_contents = '''\
+import os
+import sys  # unused but not part of diff
+
+print('(to avoid trailing whitespace in test)')
+print('(to avoid trailing whitespace in test)')
+print(os.path.join('foo', 'bar'))
+
+y  # part of the diff and an error
+'''
+
+    diff = '''\
+diff --git a/t.py b/t.py
+index d64ac39..7d943de 100644
+--- a/t.py
++++ b/t.py
+@@ -4,3 +4,5 @@ import sys  # unused but not part of diff
+ print('(to avoid trailing whitespace in test)')
+ print('(to avoid trailing whitespace in test)')
+ print(os.path.join('foo', 'bar'))
++
++y  # part of the diff and an error
+'''
+
+    with mock.patch.object(utils, 'stdin_get_value', return_value=diff):
+        with tmpdir.as_cwd():
+            tmpdir.join('t.py').write(t_py_contents)
+
+            app = application.Application()
+            app.run(['--diff'])
+
+    out, err = capsys.readouterr()
+    assert out == "t.py:8:1: F821 undefined name 'y'\n"
+    assert err == ''