Add --check support to EOF and Trailing Whitespace fixers

This change adds an advisory mode via `--check` that only warns of
formatting issues with files, but does not address them.

This support is desirable because--while I don't mind the automagic
changes when done in a mechanical way--some individuals who I
described the current behavior of these fixers to were a bit uneasy
about the magic that went along with them. Adding `--check` so
others can opt out (similar to `black --check`) is a compromise on
this front.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>

pre_commit_hooks/end_of_file_fixer.py

@@ -9,7 +9,7 @@ from typing import Optional
 from typing import Sequence
 
 
-def fix_file(file_obj):  # type: (IO[bytes]) -> int
+def fix_file(file_obj, apply_fixes=True):  # type: (IO[bytes], bool) -> int
     # Test for newline at end of file
     # Empty files will throw IOError here
     try:
@@ -20,8 +20,9 @@ def fix_file(file_obj):  # type: (IO[bytes]) -> int
     # last_character will be '' for an empty file
     if last_character not in {b'\n', b'\r'} and last_character != b'':
         # Needs this seek for windows, otherwise IOError
-        file_obj.seek(0, os.SEEK_END)
+        if apply_fixes:
-        file_obj.write(b'\n')
+            file_obj.seek(0, os.SEEK_END)
+            file_obj.write(b'\n')
         return 1
 
     while last_character in {b'\n', b'\r'}:
@@ -29,8 +30,9 @@ def fix_file(file_obj):  # type: (IO[bytes]) -> int
         if file_obj.tell() == 1:
             # If we've reached the beginning of the file and it is all
             # linebreaks then we can make this file empty
-            file_obj.seek(0)
+            if apply_fixes:
-            file_obj.truncate()
+                file_obj.seek(0)
+                file_obj.truncate()
             return 1
 
         # Go back two bytes and read a character
@@ -45,8 +47,9 @@ def fix_file(file_obj):  # type: (IO[bytes]) -> int
         if remaining == sequence:
             return 0
         elif remaining.startswith(sequence):
-            file_obj.seek(position + len(sequence))
+            if apply_fixes:
-            file_obj.truncate()
+                file_obj.seek(position + len(sequence))
+                file_obj.truncate()
             return 1
 
     return 0
@@ -54,6 +57,11 @@ def fix_file(file_obj):  # type: (IO[bytes]) -> int
 
 def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
+    parser.add_argument(
+        '--check', action='store_true',
+        help="Don't write the files back. Returns a non-zero code if changes "
+             'would be applied. Returns zero if no changes are required.',
+    )
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
 
@@ -62,9 +70,12 @@ def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     for filename in args.filenames:
         # Read as binary so we can read byte-by-byte
         with open(filename, 'rb+') as file_obj:
-            ret_for_file = fix_file(file_obj)
+            ret_for_file = fix_file(file_obj, apply_fixes=not args.check)
             if ret_for_file:
-                print('Fixing {}'.format(filename))
+                if args.check:
+                    print('Would fix {}'.format(filename))
+                else:
+                    print('Fixing {}'.format(filename))
             retv |= ret_for_file
 
     return retv

pre_commit_hooks/trailing_whitespace_fixer.py

@@ -7,21 +7,20 @@ from typing import Optional
 from typing import Sequence
 
 
-def _fix_file(filename, is_markdown, chars):
+def _fix_file(filename, is_markdown, chars=None, apply_fixes=True):
-    # type: (str, bool, Optional[bytes]) -> bool
+    # type: (str, bool, Optional[bytes], 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:
+        if apply_fixes:
-            for line in newlines:
+            with open(filename, mode='wb') as file_processed:
-                file_processed.write(line)
+                for line in newlines:
-        return True
+                    file_processed.write(line)
-    else:
+    return newlines != lines
-        return False
 
 
-def _process_line(line, is_markdown, chars):
+def _process_line(line, is_markdown, chars=None):
     # type: (bytes, bool, Optional[bytes]) -> bytes
     if line[-2:] == b'\r\n':
         eol = b'\r\n'
@@ -39,6 +38,12 @@ def _process_line(line, is_markdown, chars):
 
 def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
+    parser.add_argument(
+        '--check',
+        action='store_true',
+        help="Don't write the files back. Returns a non-zero code if changes "
+             'would be applied. Returns zero if no changes are required.',
+    )
     parser.add_argument(
         '--no-markdown-linebreak-ext',
         action='store_true',
@@ -89,8 +94,11 @@ def main(argv=None):  # type: (Optional[Sequence[str]]) -> 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):
+        if _fix_file(filename, md, chars, apply_fixes=not args.check):
-            print('Fixing {}'.format(filename))
+            if args.check:
+                print('Would fix {}'.format(filename))
+            else:
+                print('Fixing {}'.format(filename))
             return_code = 1
     return return_code
 

tests/end_of_file_fixer_test.py

@@ -40,3 +40,15 @@ def test_integration(input_s, expected_retval, output, tmpdir):
 
     assert file_output == output
     assert ret == expected_retval
+
+
+@pytest.mark.parametrize(('input_s', 'expected_retval', 'output'), TESTS)
+def test_integration_check(input_s, expected_retval, output, tmpdir):
+    path = tmpdir.join('file.txt')
+    path.write_binary(input_s)
+
+    ret = main(['--check', path.strpath])
+    file_output = path.read_binary()
+
+    assert file_output == input_s
+    assert ret == expected_retval

tests/trailing_whitespace_fixer_test.py

@@ -20,6 +20,20 @@ def test_fixes_trailing_whitespace(input_s, expected, tmpdir):
     assert path.read() == expected
 
 
+@pytest.mark.parametrize(
+    ('input_s'),
+    (
+        ('foo \nbar \n'),
+        ('bar\t\nbaz\t\n'),
+    ),
+)
+def test_check(input_s, tmpdir):
+    path = tmpdir.join('file.md')
+    path.write(input_s)
+    assert main(('--check', path.strpath,)) == 1
+    assert path.read() == input_s
+
+
 def test_ok_no_newline_end_of_file(tmpdir):
     filename = tmpdir.join('f')
     filename.write_binary(b'foo\nbar')