Add "--check" option support to trailing_whitespace hook

pre_commit_hooks/trailing_whitespace_fixer.py

@@ -9,14 +9,17 @@ def _fix_file(
         filename: str,
         is_markdown: bool,
         chars: bytes | None,
+        check_only: bool = False,
+        error_lines: list[int] = None,
 ) -> bool:
     with open(filename, mode='rb') as file_processed:
         lines = file_processed.readlines()
-    newlines = [_process_line(line, is_markdown, chars) for line in lines]
+    newlines = [_process_line(line, is_markdown, chars, line_num, error_lines) for line_num, line in enumerate(lines)]
     if newlines != lines:
-        with open(filename, mode='wb') as file_processed:
-            for line in newlines:
-                file_processed.write(line)
+        if not check_only:
+            with open(filename, mode='wb') as file_processed:
+                for line in newlines:
+                    file_processed.write(line)
         return True
     else:
         return False
@@ -26,7 +29,10 @@ def _process_line(
         line: bytes,
         is_markdown: bool,
         chars: bytes | None,
+        line_num: int,
+        error_lines: list[int] | None
 ) -> bytes:
+    org_line = line
     if line[-2:] == b'\r\n':
         eol = b'\r\n'
         line = line[:-2]
@@ -38,11 +44,15 @@ def _process_line(
     # preserve trailing two-space for non-blank lines in markdown files
     if is_markdown and (not line.isspace()) and line.endswith(b'  '):
         return line[:-2].rstrip(chars) + b'  ' + eol
-    return line.rstrip(chars) + eol
+    result = line.rstrip(chars) + eol
+    if error_lines is not None and org_line != result:
+        error_lines.append(line_num+1)
+    return result
 
 
 def main(argv: Sequence[str] | None = None) -> int:
     parser = argparse.ArgumentParser()
+    parser.add_argument('--check', action='store_true', help='Check without fixing')
     parser.add_argument(
         '--no-markdown-linebreak-ext',
         action='store_true',
@@ -93,8 +103,14 @@ def main(argv: Sequence[str] | None = None) -> int:
     for filename in args.filenames:
         _, extension = os.path.splitext(filename.lower())
         md = all_markdown or extension in md_exts
-        if _fix_file(filename, md, chars):
-            print(f'Fixing {filename}')
+        error_lines = []
+        if _fix_file(filename, md, chars, args.check, error_lines):
+            if args.check:
+                location = ",".join(map(str, error_lines[:4]))
+                location += "..." if len(error_lines) > 4 else ""
+                print(f'Trailing whitespace check failed: {filename} @ {location}')
+            else:
+                print(f'Fixing {filename}')
             return_code = 1
     return return_code
 

tests/trailing_whitespace_fixer_test.py

@@ -18,6 +18,23 @@ def test_fixes_trailing_whitespace(input_s, expected, tmpdir):
     assert main((str(path),)) == 1
     assert path.read() == expected
 
+@pytest.mark.parametrize(
+    ('input_s', 'exit_code', "lines"),
+    (
+            ('foo \nbar \n', 1, [1,2]),
+            ('bar\t\nbaz\t\n', 1, [1,2]),
+            ('bar\nbaz\t\n', 1, [2]),
+    ),
+)
+def test_fixes_trailing_whitespace_check_only(capsys, input_s, exit_code, lines, tmpdir):
+    path = tmpdir.join('file.md')
+    path.write(input_s)
+    assert main(('--check', str(path),)) == exit_code
+    assert path.read() == input_s
+    captured = capsys.readouterr()
+    location = "@ " + ','.join(map(str, lines))
+    assert location in captured.out
+
 
 def test_ok_no_newline_end_of_file(tmpdir):
     filename = tmpdir.join('f')