Add an option for --single-quote, but strongly prefer double quote in…

… readme.

README.md

@@ -68,6 +68,8 @@ black [OPTIONS] [SRC]...
 
 Options:
   -l, --line-length INTEGER   Where to wrap around.  [default: 88]
+  --single-quote              Use single quotes instead of double quotes in
+                              strings except for triple-quoted strings.
   --check                     Don't write the files back, just return the
                               status.  Return code 0 means nothing would
                               change.  Return code 1 means some files would be
@@ -341,6 +343,9 @@ a one double-quote regardless of fonts and syntax highlighting used.
 On top of this, double quotes for strings are consistent with C which
 Python interacts a lot with.
 
+> While we strongly recommend double quotes, we also provide a
+> `--single-quote` option if you prefer.
+
 On certain keyboard layouts like US English, typing single quotes is
 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

black.py

@@ -44,7 +44,7 @@
 from blib2to3.pgen2.parse import ParseError
 
 
-__version__ = "18.5b1"
+__version__ = "18.5z1"
 DEFAULT_LINE_LENGTH = 88
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))
 
@@ -126,6 +126,7 @@ class FileMode(Flag):
     AUTO_DETECT = 0
     PYTHON36 = 1
     PYI = 2
+    SINGLE_QUOTE = 32
 
 
 @click.command()
@@ -137,6 +138,11 @@ class FileMode(Flag):
     help="How many character per line to allow.",
     show_default=True,
 )
+@click.option(
+    "--single-quote",
+    is_flag=True,
+    help="Use single quotes instead of double quotes in strings.",
+)
 @click.option(
     "--check",
     is_flag=True,
@@ -195,6 +201,7 @@ def main(
     ctx: click.Context,
     line_length: int,
     check: bool,
+    single_quote: bool,
     diff: bool,
     fast: bool,
     pyi: bool,
@@ -227,6 +234,8 @@ def main(
         mode |= FileMode.PYTHON36
     if pyi:
         mode |= FileMode.PYI
+    if single_quote:
+        mode |= FileMode.SINGLE_QUOTE
     report = Report(check=check, quiet=quiet)
     if len(sources) == 0:
         out("No paths given. Nothing to do 😴")
@@ -487,8 +496,11 @@ 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)
+    single_quote = bool(mode & FileMode.SINGLE_QUOTE)
     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,
+        single_quote=single_quote,
     )
     elt = EmptyLineTracker(is_pyi=is_pyi)
     empty_line = Line()
@@ -1286,6 +1298,7 @@ class LineGenerator(Visitor[Line]):
     """
 
     is_pyi: bool = False
+    single_quote: bool = False
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False
 
@@ -1356,7 +1369,7 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
                 if node.type == token.STRING:
                     normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
-                    normalize_string_quotes(node)
+                    normalize_string_quotes(node, single_quote=self.single_quote)
                 if node.type not in WHITESPACE:
                     self.current_line.append(node)
         yield from super().visit_default(node)
@@ -2313,7 +2326,7 @@ 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:
+def normalize_string_quotes(leaf: Leaf, single_quote: bool = False) -> None:
     """Prefer double quotes but only if it doesn't cause more escaping.
 
     Adds or removes backslashes as appropriate. Doesn't parse and fix
@@ -2322,18 +2335,28 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     Note: Mutates its argument.
     """
     value = leaf.value.lstrip("furbFURB")
-    if value[:3] == '"""':
+
+    quote_char = '"'
+    alt_quote_char = "'"
+    triple_quote_chars = '"""'
+    alt_triple_quote_chars = "'''"
+
+    if single_quote:
+        quote_char = "'"
+        alt_quote_char = '"'
+
+    if value[:3] == triple_quote_chars:
         return
 
-    elif value[:3] == "'''":
-        orig_quote = "'''"
-        new_quote = '"""'
-    elif value[0] == '"':
-        orig_quote = '"'
-        new_quote = "'"
+    elif value[:3] == alt_triple_quote_chars:
+        orig_quote = alt_triple_quote_chars
+        new_quote = triple_quote_chars
+    elif value[0] == quote_char:
+        orig_quote = quote_char
+        new_quote = alt_quote_char
     else:
-        orig_quote = "'"
-        new_quote = '"'
+        orig_quote = alt_quote_char
+        new_quote = quote_char
     first_quote_pos = leaf.value.find(orig_quote)
     if first_quote_pos == -1:
         return  # There's an internal error
@@ -2360,16 +2383,16 @@ def normalize_string_quotes(leaf: Leaf) -> None:
             leaf.value = f"{prefix}{orig_quote}{body}{orig_quote}"
         new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
         new_body = sub_twice(unescaped_new_quote, rf"\1\\{new_quote}", new_body)
-    if new_quote == '"""' and new_body[-1] == '"':
+    if new_quote == triple_quote_chars and new_body[-1] == triple_quote_chars[0]:
         # edge case:
-        new_body = new_body[:-1] + '\\"'
+        new_body = new_body[:-1] + "\\" + triple_quote_chars[0]
     orig_escape_count = body.count("\\")
     new_escape_count = new_body.count("\\")
     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 == quote_char:
+        return
 
     leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"