New option to allow modified files

pre_commit/commands/run.py

@@ -129,15 +129,20 @@ def _run_single_hook(classifier, hook, args, skips, cols, use_color):
     diff_after = cmd_output_b('git', 'diff', '--no-ext-diff', retcode=None)
 
     file_modifications = diff_before != diff_after
+    modified_ok = args.modified_files_ok
 
     # If the hook makes changes, fail the commit
-    if file_modifications:
+    if not modified_ok and file_modifications:
         retcode = 1
 
-    if retcode:
+    if retcode and not modified_ok:
         retcode = 1
         print_color = color.RED
         pass_fail = 'Failed'
+    elif file_modifications and modified_ok:
+        retcode = 0
+        print_color = color.YELLOW
+        pass_fail = 'Edited'
     else:
         retcode = 0
         print_color = color.GREEN

pre_commit/main.py

@@ -101,6 +101,13 @@ def _add_run_options(parser):
         '--show-diff-on-failure', action='store_true',
         help='When hooks fail, run `git diff` directly afterward.',
     )
+    parser.add_argument(
+        '--modified-files-ok', '-m', action='store_true',
+        default='PRE_COMMIT_MODIFIED_FILES_OK' in os.environ,
+        help="File modification by hooks won't be equated with failure. "
+             'Equivalent to setting the environment variable '
+             'PRE_COMMIT_MODIFIED_FILES_OK=true.',
+    )
     mutex_group = parser.add_mutually_exclusive_group(required=False)
     mutex_group.add_argument(
         '--all-files', '-a', action='store_true', default=False,

testing/util.py

@@ -107,6 +107,7 @@ def run_opts(
         hook_stage='commit',
         show_diff_on_failure=False,
         commit_msg_filename='',
+        modified_files_ok=False,
 ):
     # These are mutually exclusive
     assert not (all_files and files)
@@ -121,6 +122,7 @@ def run_opts(
         hook_stage=hook_stage,
         show_diff_on_failure=show_diff_on_failure,
         commit_msg_filename=commit_msg_filename,
+        modified_files_ok=modified_files_ok,
     )