implemented a basic double quote string fixer

README.md

@@ -36,6 +36,7 @@ Add this to your `.pre-commit-config.yaml`
 - `name-tests-test` - Assert that files in tests/ end in _test.py
 - `pyflakes` - Run pyflakes on your python files
 - `requirements-txt-fixer` - Sorts entries in requirements.txt
+- `double-quote-string-fixer` - This hook replaces double quoted strings with single quoted strings
 - `trailing-whitespace` - Trims trailing whitespace.
 
 ### As a standalone package

hooks.yaml

@@ -79,6 +79,12 @@
     entry: requirements-txt-fixer
     language: python
     files: requirements.*\.txt$
+-   id: double-quote-string-fixer
+    name: Fix double quoted strings
+    description: This hook replaces double quoted strings with single quoted strings
+    entry: double-quote-string-fixer
+    language: python
+    files: \.py$
 -   id: trailing-whitespace
     name: Trim Trailing Whitespace
     description: This hook trims trailing whitespace.

pre_commit_hooks/string_fixer.py

@@ -0,0 +1,65 @@
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import re
+import tokenize
+
+
+double_quote_starts = tuple(s for s in tokenize.single_quoted if '"' in s)
+compiled_tokenize_string = re.compile(tokenize.String)
+
+
+def handle_match(m):
+    string = m.group(0)
+
+    for double_quote_start in double_quote_starts:
+        if string.startswith(double_quote_start):
+            meat = string[len(double_quote_start):-1]
+            if '"' in meat or "'" in meat:
+                break
+            return (
+                double_quote_start.replace('"', "'") +
+                string[len(double_quote_start):-1] +
+                "'"
+            )
+    return string
+
+
+def fix_strings(filename):
+    return_value = 0
+
+    lines = []
+
+    with open(filename, 'r') as read_handle:
+        for line in read_handle:
+            if '"""' in line:
+                # Docstrings are hard, fuck it
+                lines.append(line)
+            else:
+                result = re.sub(compiled_tokenize_string, handle_match, line)
+                lines.append(result)
+                return_value |= int(result != line)
+
+    with open(filename, 'w') as write_handle:
+        for line in lines:
+            write_handle.write(line)
+
+    return return_value
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to fix')
+    args = parser.parse_args(argv)
+
+    retv = 0
+
+    for filename in args.filenames:
+        return_value = fix_strings(filename)
+        if return_value != 0:
+            print('Fixing strings in {0}'.format(filename))
+        retv |= return_value
+
+    return retv

setup.py

@@ -46,6 +46,7 @@ setup(
             'debug-statement-hook = pre_commit_hooks.debug_statement_hook:debug_statement_hook',
             'end-of-file-fixer = pre_commit_hooks.end_of_file_fixer:end_of_file_fixer',
             'name-tests-test = pre_commit_hooks.tests_should_end_in_test:validate_files',
+            'double-quote-string-fixer = pre_commit_hooks.string_fixer:main',
             'requirements-txt-fixer = pre_commit_hooks.requirements_txt_fixer:fix_requirements_txt',
             'trailing-whitespace-fixer = pre_commit_hooks.trailing_whitespace_fixer:fix_trailing_whitespace',
         ],

tests/check_docstring_first_test.py

@@ -15,7 +15,7 @@ TESTS = (
     ('', 0, ''),
     # Acceptable
     ('"foo"', 0, ''),
-    # Docstrin after code
+    # Docstring after code
     (
         'from __future__ import unicode_literals\n'
         '"foo"\n',

tests/string_fixer_test.py

@@ -0,0 +1,80 @@
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import pytest
+
+from pre_commit_hooks.string_fixer import main
+
+TESTS = (
+    # Base cases
+    (
+        "''",
+        "''",
+        0
+    ),
+    (
+        '""',
+        "''",
+        1
+    ),
+    (
+        r'"\'"',
+        r'"\'"',
+        0
+    ),
+    (
+        r'"\""',
+        r'"\""',
+        0
+    ),
+    (
+        r"'\"\"'",
+        r"'\"\"'",
+        0
+    ),
+    # String somewhere in the line
+    (
+        'x = "foo"',
+        "x = 'foo'",
+        1
+    ),
+    # Test escaped characters
+    (
+        r'"\'"',
+        r'"\'"',
+        0
+    ),
+    # Docstring
+    (
+        '""" Foo """',
+        '""" Foo """',
+        0
+    ),
+    # Fuck it, won't even try to fix
+    (
+        """
+        x = " \\n
+        foo \\n
+        "\n
+        """,
+        """
+        x = " \\n
+        foo \\n
+        "\n
+        """,
+        0
+    ),
+)
+
+
+@pytest.mark.parametrize(('input_s', 'expected_output', 'expected_retval'), TESTS)
+def test_rewrite(input_s, expected_output, expected_retval, tmpdir):
+    tmpfile = tmpdir.join('file.txt')
+
+    with open(tmpfile.strpath, 'w') as f:
+        f.write(input_s)
+
+    retval = main([tmpfile.strpath])
+    assert tmpfile.read() == expected_output
+    assert retval == expected_retval