Add --skip-string-normalization

Fixes #118

README.md

@@ -87,6 +87,8 @@ Options:
                               files.  This will put trailing commas in function
                               signatures and calls also after *args and
                               **kwargs.  [default: per-file auto-detection]
+  -S, --skip-string-normalization
+                              Don't normalize string quotes or prefixes.
   --version                   Show the version and exit.
   --help                      Show this message and exit.
 ```
@@ -346,6 +348,12 @@ a bit easier than double quotes.  The latter requires use of the Shift
 key.  My recommendation here is to keep using whatever is faster to type
 and let *Black* handle the transformation.
 
+If you are adopting *Black* in a large project with pre-existing string
+conventions (like the popular ["single quotes for data, double quotes for
+human-readable strings"](https://stackoverflow.com/a/56190)), you can
+pass `--skip-string-normalization` on the command line.  This is meant as
+an adoption helper, avoid using this for new projects.
+
 
 ### Line breaks & binary operators
 
@@ -688,6 +696,11 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 ## Change Log
 
+### 18.6b0
+
+* added `--skip-string-normalization` (#118)
+
+
 ### 18.5b1
 
 * added `--pyi` (#249)

black.py

@@ -126,6 +126,7 @@ class FileMode(Flag):
     AUTO_DETECT = 0
     PYTHON36 = 1
     PYI = 2
+    NO_STRING_NORMALIZATION = 4
 
 
 @click.command()
@@ -182,6 +183,12 @@ class FileMode(Flag):
         "**kwargs.  [default: per-file auto-detection]"
     ),
 )
+@click.option(
+    "-S",
+    "--skip-string-normalization",
+    is_flag=True,
+    help="Don't normalize string quotes or prefixes.",
+)
 @click.version_option(version=__version__)
 @click.argument(
     "src",
@@ -199,6 +206,7 @@ def main(
     fast: bool,
     pyi: bool,
     py36: bool,
+    skip_string_normalization: bool,
     quiet: bool,
     src: List[str],
 ) -> None:
@@ -227,6 +235,8 @@ def main(
         mode |= FileMode.PYTHON36
     if pyi:
         mode |= FileMode.PYI
+    if skip_string_normalization:
+        mode |= FileMode.NO_STRING_NORMALIZATION
     report = Report(check=check, quiet=quiet)
     if len(sources) == 0:
         out("No paths given. Nothing to do 😴")
@@ -487,8 +497,11 @@ def format_str(
     future_imports = get_future_imports(src_node)
     is_pyi = bool(mode & FileMode.PYI)
     py36 = bool(mode & FileMode.PYTHON36) or is_python36(src_node)
+    normalize_strings = not bool(mode & FileMode.NO_STRING_NORMALIZATION)
     lines = LineGenerator(
-        remove_u_prefix=py36 or "unicode_literals" in future_imports, is_pyi=is_pyi
+        remove_u_prefix=py36 or "unicode_literals" in future_imports,
+        is_pyi=is_pyi,
+        normalize_strings=normalize_strings,
     )
     elt = EmptyLineTracker(is_pyi=is_pyi)
     empty_line = Line()
@@ -1286,6 +1299,7 @@ class LineGenerator(Visitor[Line]):
     """
 
     is_pyi: bool = False
+    normalize_strings: bool = True
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False
 
@@ -1354,7 +1368,7 @@ def visit_default(self, node: LN) -> Iterator[Line]:
 
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
-                if node.type == token.STRING:
+                if self.normalize_strings and node.type == token.STRING:
                     normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                     normalize_string_quotes(node)
                 if node.type not in WHITESPACE:

tests/test_black.py

@@ -236,6 +236,11 @@ def test_string_quotes(self) -> None:
         self.assertFormatEqual(expected, actual)
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
+        mode = black.FileMode.NO_STRING_NORMALIZATION
+        not_normalized = fs(source, mode=mode)
+        self.assertFormatEqual(source, not_normalized)
+        black.assert_equivalent(source, not_normalized)
+        black.assert_stable(source, not_normalized, line_length=ll, mode=mode)
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_slices(self) -> None: