trailing-whitespace: Add option to only lint files without fixing them

README.md

@@ -196,13 +196,14 @@ very specific format.  You must opt in to this by setting [`files`](https://pre-
 
 
 #### `trailing-whitespace`
-Trims trailing whitespace.
+Detects and optionally trims trailing whitespace.
+  - By default, this hook trims all whitespace from the ends of lines.
+    To specify a custom set of characters to trim instead, use `args: [--chars,"<chars to trim>"]`.
+    To only lint files without fixing them, specify `--no-fix`.
   - To preserve Markdown [hard linebreaks](https://github.github.com/gfm/#hard-line-break)
     use `args: [--markdown-linebreak-ext=md]` (or other extensions used
     by your markdownfiles).  If for some reason you want to treat all files
     as markdown, use `--markdown-linebreak-ext=*`.
-  - By default, this hook trims all whitespace from the ends of lines.
-    To specify a custom set of characters to trim instead, use `args: [--chars,"<chars to trim>"]`.
 
 ### Deprecated / replaced hooks
 

pre_commit_hooks/trailing_whitespace_fixer.py

@@ -5,18 +5,20 @@
 from typing import Sequence
 
 
-def _fix_file(
+def _process_file(
         filename: str,
         is_markdown: bool,
         chars: bytes | None,
+        fix: bool,
 ) -> bool:
     with open(filename, mode='rb') as file_processed:
         lines = file_processed.readlines()
     newlines = [_process_line(line, is_markdown, chars) for line in lines]
     if newlines != lines:
-        with open(filename, mode='wb') as file_processed:
-            for line in newlines:
-                file_processed.write(line)
+        if fix:
+            with open(filename, mode='wb') as file_processed:
+                for line in newlines:
+                    file_processed.write(line)
         return True
     else:
         return False
@@ -61,10 +63,29 @@ def main(argv: Sequence[str] | None = None) -> int:
     parser.add_argument(
         '--chars',
         help=(
-            'The set of characters to strip from the end of lines.  '
+            'The set of characters to consider as whitespace at the end of '
+            'lines.  '
             'Defaults to all whitespace characters.'
         ),
     )
+    parser.add_argument(
+        '--fix',
+        action='store_true',
+        help=(
+            'Fix files by stripping detected trailing whitespace.  '
+            'Default: On'
+        ),
+    )
+    parser.add_argument(
+        '--no-fix',
+        dest='fix',
+        action='store_false',
+        help=(
+            'Do not fix files by stripping detected trailing whitespace.  '
+            'Default: Off, meaning files do get fixed.'
+        ),
+    )
+    parser.set_defaults(fix=True)
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
 
@@ -93,8 +114,8 @@ 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}')
+        if _process_file(filename, md, chars, args.fix):
+            print(('Fixing ' if args.fix else 'Not fixing ') + filename)
             return_code = 1
     return return_code
 

tests/trailing_whitespace_fixer_test.py

@@ -19,6 +19,21 @@ def test_fixes_trailing_whitespace(input_s, expected, tmpdir):
     assert path.read() == expected
 
 
+@pytest.mark.parametrize(
+    ('arg', 'expected'),
+    (
+        ('--fix', 'foo\nbar\n'),
+        ('--no-fix', 'foo \nbar \n'),
+    ),
+)
+def test_fix_no_fix(tmpdir, arg, expected):
+    input_s = 'foo \nbar \n'
+    path = tmpdir.join('file.txt')
+    path.write(input_s)
+    assert main((str(path), arg)) == 1
+    assert path.read() == expected
+
+
 def test_ok_no_newline_end_of_file(tmpdir):
     filename = tmpdir.join('f')
     filename.write_binary(b'foo\nbar')