Output a message when a hook fails due to file modification

pre_commit/commands/run.py

@@ -89,8 +89,10 @@ def _run_single_hook(hook, repo, args, write, skips=frozenset()):
     retcode, stdout, stderr = repo.run_hook(hook, filenames)
     diff_after = cmd_output('git', 'diff', retcode=None, encoding=None)
 
+    file_modifications = diff_before != diff_after
+
     # If the hook makes changes, fail the commit
-    if diff_before != diff_after:
+    if file_modifications:
         retcode = 1
 
     if retcode:
@@ -104,9 +106,19 @@ def _run_single_hook(hook, repo, args, write, skips=frozenset()):
 
     write(color.format_color(pass_fail, print_color, args.color) + '\n')
 
-    if (stdout or stderr) and (retcode or args.verbose):
+    if (stdout or stderr or file_modifications) and (retcode or args.verbose):
         write('hookid: {0}\n'.format(hook['id']))
         write('\n')
+
+        # Print a message if failing due to file modifications
+        if file_modifications:
+            write('Files were modified by this hook.')
+
+            if stdout or stderr:
+                write(' Additional output:\n')
+
+            write('\n')
+
         for output in (stdout, stderr):
             assert type(output) is bytes, type(output)
             if output.strip():

testing/resources/modified_file_returns_zero_repo/bin/hook3.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+for f in $@; do
+    # Non UTF-8 bytes
+    echo -e '\x01\x97' > "$f"
+done

testing/resources/modified_file_returns_zero_repo/hooks.yaml

@@ -2,9 +2,14 @@
     name: Bash hook
     entry: bin/hook.sh
     language: script
-    files: ''
+    files: 'foo.py'
 -   id: bash_hook2
     name: Bash hook
     entry: bin/hook2.sh
     language: script
     files: ''
+-   id: bash_hook3
+    name: Bash hook
+    entry: bin/hook3.sh
+    language: script
+    files: 'bar.py'

tests/commands/run_test.py

@@ -40,9 +40,9 @@ def repo_with_failing_hook(tempdir_factory):
         yield git_path
 
 
-def stage_a_file():
-    cmd_output('touch', 'foo.py')
-    cmd_output('git', 'add', 'foo.py')
+def stage_a_file(filename='foo.py'):
+    cmd_output('touch', filename)
+    cmd_output('git', 'add', filename)
 
 
 def get_write_mock_output(write_mock):
@@ -127,16 +127,22 @@ def test_hook_that_modifies_but_returns_zero(
         tempdir_factory, 'modified_file_returns_zero_repo',
     )
     with cwd(git_path):
+        stage_a_file('bar.py')
         _test_run(
             git_path,
             {},
             (
                 # The first should fail
                 b'Failed',
-                # With a modified file (the hook's output)
+                # With a modified file (default message + the hook's output)
+                b'Files were modified by this hook. Additional output:\n\n'
                 b'Modified: foo.py',
                 # The next hook should pass despite the first modifying
                 b'Passed',
+                # The next hook should fail
+                b'Failed',
+                # bar.py was modified, but provides no additional output
+                b'Files were modified by this hook.\n',
             ),
             1,
             True,