implemented a basic double quote string fixer

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