Print filenames in the diff

pre_commit_hooks/pretty_format_json.py

@@ -1,11 +1,11 @@
 from __future__ import print_function
 
 import argparse
-import difflib
 import io
 import json
 import sys
 from collections import OrderedDict
+from difflib import unified_diff
 from typing import List
 from typing import Mapping
 from typing import Optional
@@ -56,11 +56,11 @@ def parse_topkeys(s):  # type: (str) -> List[str]
     return s.split(',')
 
 
-def get_diff(source, target):  # type: (str, str) -> str
+def get_diff(source, target, file):  # type: (str, str, str) -> str
     source_lines = source.splitlines(True)
     target_lines = target.splitlines(True)
-    diff = ''.join(difflib.unified_diff(source_lines, target_lines))
-    return diff
+    diff = unified_diff(source_lines, target_lines, fromfile=file, tofile=file)
+    return ''.join(diff)
 
 
 def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
@@ -129,7 +129,13 @@ def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
                 if args.autofix:
                     _autofix(json_file, pretty_contents)
                 else:
-                    print(get_diff(''.join(contents), pretty_contents))
+                    print(
+                        get_diff(
+                            ''.join(contents),
+                            pretty_contents,
+                            json_file,
+                        ),
+                    )
 
                 status = 1
         except ValueError:

tests/pretty_format_json_test.py

@@ -1,3 +1,4 @@
+import os
 import shutil
 
 import pytest
@@ -110,9 +111,13 @@ def test_badfile_main():
 def test_diffing_output(capsys):
     resource_path = get_resource_path('not_pretty_formatted_json.json')
     expected_retval = 1
+    a = os.path.join('a', resource_path)
+    b = os.path.join('b', resource_path)
     expected_out = '''\
---- \n+++ \n@@ -1,6 +1,9 @@
- {
+--- {}
++++ {}
+@@ -1,6 +1,9 @@
+ {{
 -    "foo":
 -    "bar",
 -        "alist": [2, 34, 234],
@@ -124,9 +129,9 @@ def test_diffing_output(capsys):
 +  ],
 +  "blah": null,
 +  "foo": "bar"
- }
+ }}
 
-'''
+'''.format(a, b)
     expected_err = 'File {} is not pretty-formatted\n'.format(resource_path)
 
     actual_retval = main([resource_path])