Add a --show-diff-on-failure option

pre_commit/commands/run.py

@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 
 import logging
 import os
+import subprocess
 import sys
 
 from pre_commit import color
@@ -152,6 +153,13 @@ def _run_hooks(repo_hooks, args, environ):
     retval = 0
     for repo, hook in repo_hooks:
         retval |= _run_single_hook(hook, repo, args, skips, cols)
+    if (
+            retval and
+            args.show_diff_on_failure and
+            subprocess.call(('git', 'diff', '--quiet')) != 0
+    ):
+        print('All changes made by hooks:')
+        subprocess.call(('git', 'diff'))
     return retval
 
 

pre_commit/main.py

@@ -149,6 +149,10 @@ def main(argv=None):
         '--hook-stage', choices=('commit', 'push'), default='commit',
         help='The stage during which the hook is fired e.g. commit or push.',
     )
+    run_parser.add_argument(
+        '--show-diff-on-failure', action='store_true',
+        help='When hooks fail, run `git diff` directly afterward.',
+    )
     run_mutex_group = run_parser.add_mutually_exclusive_group(required=False)
     run_mutex_group.add_argument(
         '--all-files', '-a', action='store_true', default=False,

tests/commands/run_test.py

@@ -58,6 +58,7 @@ def _get_opts(
         source='',
         allow_unstaged_config=False,
         hook_stage='commit',
+        show_diff_on_failure=False,
 ):
     # These are mutually exclusive
     assert not (all_files and files)
@@ -67,11 +68,12 @@ def _get_opts(
         color=color,
         verbose=verbose,
         hook=hook,
-        hook_stage=hook_stage,
         no_stash=no_stash,
         origin=origin,
         source=source,
         allow_unstaged_config=allow_unstaged_config,
+        hook_stage=hook_stage,
+        show_diff_on_failure=show_diff_on_failure,
     )
 
 
@@ -151,6 +153,23 @@ def test_hook_that_modifies_but_returns_zero(
         )
 
 
+def test_show_diff_on_failure(
+        capfd, cap_out, tempdir_factory, mock_out_store_directory,
+):
+    git_path = make_consuming_repo(
+        tempdir_factory, 'modified_file_returns_zero_repo',
+    )
+    with cwd(git_path):
+        stage_a_file('bar.py')
+        _test_run(
+            cap_out, git_path, {'show_diff_on_failure': True},
+            # we're only testing the output after running
+            (), 1, True,
+        )
+    out, _ = capfd.readouterr()
+    assert 'diff --git' in out
+
+
 @pytest.mark.parametrize(
     ('options', 'outputs', 'expected_ret', 'stage'),
     (