black: add a single-quotes option to prefer single quotes

Add support for `black -s|--single-quotes`.

Fixes #594

README.md

@@ -79,6 +79,8 @@ Options:
   --pyi                       Format all input files like typing stubs
                               regardless of file extension (useful when piping
                               source on standard input).
+  -s, --single-quotes
+                              Use single quotes during string normalization.
   -S, --skip-string-normalization
                               Don't normalize string quotes or prefixes.
   -N, --skip-numeric-underscore-normalization
@@ -805,6 +807,7 @@ request is rejected with `HTTP 501` (Not Implemented).
 The headers controlling how code is formatted are:
 
  - `X-Line-Length`: corresponds to the `--line-length` command line flag.
+ - `X-Single-Quotes`: corresponds to the `--single-quotes` command line flag.
  - `X-Skip-String-Normalization`: corresponds to the `--skip-string-normalization`
     command line flag. If present and its value is not the empty string, no string
     normalization will be performed.

black.py

@@ -117,13 +117,15 @@ class FileMode(Flag):
     PYI = 2
     NO_STRING_NORMALIZATION = 4
     NO_NUMERIC_UNDERSCORE_NORMALIZATION = 8
+    SINGLE_QUOTES = 16
 
     @classmethod
     def from_configuration(
         cls,
         *,
         py36: bool,
         pyi: bool,
+        single_quotes: bool,
         skip_string_normalization: bool,
         skip_numeric_underscore_normalization: bool,
     ) -> "FileMode":
@@ -132,6 +134,8 @@ def from_configuration(
             mode |= cls.PYTHON36
         if pyi:
             mode |= cls.PYI
+        if single_quotes:
+            mode |= cls.SINGLE_QUOTES
         if skip_string_normalization:
             mode |= cls.NO_STRING_NORMALIZATION
         if skip_numeric_underscore_normalization:
@@ -199,6 +203,12 @@ def read_pyproject_toml(
         "(useful when piping source on standard input)."
     ),
 )
+@click.option(
+    "-s",
+    "--single-quotes",
+    is_flag=True,
+    help="Use single quotes during string normalization.",
+)
 @click.option(
     "-S",
     "--skip-string-normalization",
@@ -300,6 +310,7 @@ def main(
     fast: bool,
     pyi: bool,
     py36: bool,
+    single_quotes: bool,
     skip_string_normalization: bool,
     skip_numeric_underscore_normalization: bool,
     quiet: bool,
@@ -314,6 +325,7 @@ def main(
     mode = FileMode.from_configuration(
         py36=py36,
         pyi=pyi,
+        single_quotes=single_quotes,
         skip_string_normalization=skip_string_normalization,
         skip_numeric_underscore_normalization=skip_numeric_underscore_normalization,
     )
@@ -631,11 +643,13 @@ 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)
+    single_quotes = bool(mode & FileMode.SINGLE_QUOTES)
     normalize_strings = not bool(mode & FileMode.NO_STRING_NORMALIZATION)
     normalize_fmt_off(src_node)
     lines = LineGenerator(
         remove_u_prefix=py36 or "unicode_literals" in future_imports,
         is_pyi=is_pyi,
+        single_quotes=single_quotes,
         normalize_strings=normalize_strings,
         allow_underscores=py36
         and not bool(mode & FileMode.NO_NUMERIC_UNDERSCORE_NORMALIZATION),
@@ -1425,6 +1439,7 @@ class LineGenerator(Visitor[Line]):
 
     is_pyi: bool = False
     normalize_strings: bool = True
+    single_quotes: bool = False
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False
     allow_underscores: bool = False
@@ -1468,7 +1483,7 @@ def visit_default(self, node: LN) -> Iterator[Line]:
             normalize_prefix(node, inside_brackets=any_open_brackets)
             if self.normalize_strings and node.type == token.STRING:
                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
-                normalize_string_quotes(node)
+                normalize_string_quotes(node, self.single_quotes)
             if node.type == token.NUMBER:
                 normalize_numeric_literal(node, self.allow_underscores)
             if node.type not in WHITESPACE:
@@ -2494,27 +2509,28 @@ def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
     leaf.value = f"{new_prefix}{match.group(2)}"
 
 
-def normalize_string_quotes(leaf: Leaf) -> None:
-    """Prefer double quotes but only if it doesn't cause more escaping.
+def normalize_string_quotes(leaf: Leaf, single_quotes: bool) -> None:
+    """Normalize quotes but only if it doesn't cause more escaping.
 
     Adds or removes backslashes as appropriate. Doesn't parse and fix
     strings nested in f-strings (yet).
 
     Note: Mutates its argument.
     """
+    preferred_quote = "'" if single_quotes else '"'
+    other_quote = '"' if single_quotes else "'"
     value = leaf.value.lstrip("furbFURB")
     if value[:3] == '"""':
         return
-
     elif value[:3] == "'''":
         orig_quote = "'''"
         new_quote = '"""'
-    elif value[0] == '"':
-        orig_quote = '"'
-        new_quote = "'"
+    elif value[0] == preferred_quote:
+        orig_quote = preferred_quote
+        new_quote = other_quote
     else:
-        orig_quote = "'"
-        new_quote = '"'
+        orig_quote = other_quote
+        new_quote = preferred_quote
     first_quote_pos = leaf.value.find(orig_quote)
     if first_quote_pos == -1:
         return  # There's an internal error
@@ -2555,8 +2571,8 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     if new_escape_count > orig_escape_count:
         return  # Do not introduce more escaping
 
-    if new_escape_count == orig_escape_count and orig_quote == '"':
-        return  # Prefer double quotes
+    if new_escape_count == orig_escape_count and orig_quote == preferred_quote:
+        return  # Quote is already as desired -> nothing to do
 
     leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"
 

blackd.py

@@ -13,6 +13,7 @@
 VERSION_HEADER = "X-Protocol-Version"
 LINE_LENGTH_HEADER = "X-Line-Length"
 PYTHON_VARIANT_HEADER = "X-Python-Variant"
+SINGLE_QUOTES_HEADER = "X-Single-Quotes"
 SKIP_STRING_NORMALIZATION_HEADER = "X-Skip-String-Normalization"
 SKIP_NUMERIC_UNDERSCORE_NORMALIZATION_HEADER = "X-Skip-Numeric-Underscore-Normalization"
 FAST_OR_SAFE_HEADER = "X-Fast-Or-Safe"
@@ -67,6 +68,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
                     return web.Response(
                         status=400, text=f"Invalid value for {PYTHON_VARIANT_HEADER}"
                     )
+        single_quotes = bool(request.headers.get(SINGLE_QUOTES_HEADER, False))
         skip_string_normalization = bool(
             request.headers.get(SKIP_STRING_NORMALIZATION_HEADER, False)
         )
@@ -79,6 +81,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         mode = black.FileMode.from_configuration(
             py36=py36,
             pyi=pyi,
+            single_quotes=single_quotes,
             skip_string_normalization=skip_string_normalization,
             skip_numeric_underscore_normalization=skip_numeric_underscore_normalization,
         )

plugin/black.vim

@@ -32,6 +32,9 @@ endif
 if !exists("g:black_linelength")
   let g:black_linelength = 88
 endif
+if !exists("g:black_single_quotes")
+  let g:black_single_quotes = 0
+endif
 if !exists("g:black_skip_string_normalization")
   let g:black_skip_string_normalization = 0
 endif
@@ -100,6 +103,8 @@ def Black():
   fast = bool(int(vim.eval("g:black_fast")))
   line_length = int(vim.eval("g:black_linelength"))
   mode = black.FileMode.AUTO_DETECT
+  if bool(int(vim.eval("g:black_single_quotes"))):
+    mode |= black.FileMode.SINGLE_QUOTES
   if bool(int(vim.eval("g:black_skip_string_normalization"))):
     mode |= black.FileMode.NO_STRING_NORMALIZATION
   buffer_str = '\n'.join(vim.current.buffer) + '\n'

tests/data/string_single_quotes.py

@@ -0,0 +1,97 @@
+''''''
+'\''
+'"'
+"'"
+"\""
+"Hello"
+"Don't do that"
+'Here is a "'
+'What\'s the deal here?'
+"What's the deal \"here\"?"
+"And \"here\"?"
+"""Strings with "" in them"""
+'''Strings with "" in them'''
+'''Here's a "'''
+'''Here's a " '''
+'''Just a normal triple
+quote'''
+f"just a normal {f} string"
+f'''This is a triple-quoted {f}-string'''
+f'MOAR {" ".join([])}'
+f"MOAR {' '.join([])}"
+r"raw string ftw"
+r'Date d\'expiration:(.*)'
+r'Tricky "quote'
+r'Not-so-tricky \"quote'
+r'Not-so-tricky \'single-quote'
+rf'{yay}'
+'\n\
+The \"quick\"\n\
+brown fox\n\
+jumps over\n\
+the \'lazy\' dog.\n\
+'
+re.compile(r'[\\"]')
+"x = ''; y = \"\""
+"x = '''; y = \"\""
+"x = ''''; y = \"\""
+"x = '' ''; y = \"\""
+"x = ''; y = \"\"\""
+"x = '''; y = \"\"\"\""
+"x = ''''; y = \"\"\"\"\""
+"x = '' ''; y = \"\"\"\"\""
+'unnecessary \"\"escaping'
+"unnecessary \'\'escaping"
+'\\""'
+"\\''"
+'Lots of \\\\\\\\\'quotes\''
+
+# output
+
+""""""
+"'"
+'"'
+"'"
+'"'
+'Hello'
+"Don't do that"
+'Here is a "'
+"What's the deal here?"
+'What\'s the deal "here"?'
+'And "here"?'
+"""Strings with "" in them"""
+"""Strings with "" in them"""
+'''Here's a "'''
+"""Here's a " """
+"""Just a normal triple
+quote"""
+f'just a normal {f} string'
+f"""This is a triple-quoted {f}-string"""
+f'MOAR {" ".join([])}'
+f"MOAR {' '.join([])}"
+r'raw string ftw'
+r'Date d\'expiration:(.*)'
+r'Tricky "quote'
+r'Not-so-tricky \"quote'
+r'Not-so-tricky \'single-quote'
+rf'{yay}'
+'\n\
+The "quick"\n\
+brown fox\n\
+jumps over\n\
+the \'lazy\' dog.\n\
+'
+re.compile(r'[\\"]')
+'x = \'\'; y = ""'
+"x = '''; y = \"\""
+"x = ''''; y = \"\""
+"x = '' ''; y = \"\""
+'x = \'\'; y = """'
+'x = \'\'\'; y = """"'
+'x = \'\'\'\'; y = """""'
+'x = \'\' \'\'; y = """""'
+'unnecessary ""escaping'
+"unnecessary ''escaping"
+'\\""'
+"\\''"
+"Lots of \\\\\\\\'quotes'"

tests/test_black.py

@@ -314,6 +314,15 @@ def test_string_quotes(self) -> None:
         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_string_single_quotes(self) -> None:
+        source, expected = read_data("string_single_quotes")
+        mode = black.FileMode.SINGLE_QUOTES
+        actual = fs(source, mode=mode)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, line_length=ll, mode=mode)
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_slices(self) -> None:
         source, expected = read_data("slices")