Don't introduce quotes to f-string sub-expressions on string boundaries (#871)

README.md

@@ -1034,6 +1034,9 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 * fixed bug that led *Black* format some code with a line length target
   of 1 (#762)
 
+* *Black* no longer introduces quotes in f-string subexpressions on string
+  boundaries (#863)
+
 
 ### 19.3b0
 

black.py

@@ -2728,7 +2728,15 @@ def normalize_string_quotes(leaf: Leaf) -> None:
         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 "f" in prefix.casefold():
-        matches = re.findall(r"[^{]\{(.*?)\}[^}]", new_body)
+        matches = re.findall(
+            r"""
+            (?:[^{]|^)\{  # start of the string or a non-{ followed by a single {
+                ([^{].*?)  # contents of the brackets except if begins with {{
+            \}(?:[^}]|$)  # A } followed by end of the string or a non-}
+            """,
+            new_body,
+            re.VERBOSE,
+        )
         for m in matches:
             if "\\" in str(m):
                 # Do not introduce backslashes in interpolated expressions

tests/data/string_quotes.py

@@ -44,6 +44,12 @@
 '\\""'
 "\\''"
 'Lots of \\\\\\\\\'quotes\''
+f'{y * " "} \'{z}\''
+f'{{y * " "}} \'{z}\''
+f'\'{z}\' {y * " "}'
+f'{y * x} \'{z}\''
+'\'{z}\' {y * " "}'
+'{y * x} \'{z}\''
 
 # output
 
@@ -93,3 +99,9 @@
 '\\""'
 "\\''"
 "Lots of \\\\\\\\'quotes'"
+f'{y * " "} \'{z}\''
+f"{{y * \" \"}} '{z}'"
+f'\'{z}\' {y * " "}'
+f"{y * x} '{z}'"
+"'{z}' {y * \" \"}"
+"{y * x} '{z}'"