pythongh-127975: Avoid reusing quote types in ast.unparse if not needed (python#127980)

Author: hauntsaninja

Lib/ast.py

@@ -1196,9 +1196,14 @@ def visit_JoinedStr(self, node):
                     fallback_to_repr = True
                     break
                 quote_types = new_quote_types
-            elif "\n" in value:
-                quote_types = [q for q in quote_types if q in _MULTI_QUOTES]
-                assert quote_types
+            else:
+                if "\n" in value:
+                    quote_types = [q for q in quote_types if q in _MULTI_QUOTES]
+                    assert quote_types
+
+                new_quote_types = [q for q in quote_types if q not in value]
+                if new_quote_types:
+                    quote_types = new_quote_types
             new_fstring_parts.append(value)
 
         if fallback_to_repr:

Lib/test/test_unparse.py

@@ -513,11 +513,13 @@ def test_class_bases_and_keywords(self):
         self.check_src_roundtrip("class X(*args, **kwargs):\n    pass")
 
     def test_fstrings(self):
-        self.check_src_roundtrip("f'-{f'*{f'+{f'.{x}.'}+'}*'}-'")
-        self.check_src_roundtrip("f'\\u2028{'x'}'")
+        self.check_src_roundtrip('''f\'\'\'-{f"""*{f"+{f'.{x}.'}+"}*"""}-\'\'\'''')
+        self.check_src_roundtrip('''f\'-{f\'\'\'*{f"""+{f".{f'{x}'}."}+"""}*\'\'\'}-\'''')
+        self.check_src_roundtrip('''f\'-{f\'*{f\'\'\'+{f""".{f"{f'{x}'}"}."""}+\'\'\'}*\'}-\'''')
+        self.check_src_roundtrip('''f"\\u2028{'x'}"''')
         self.check_src_roundtrip(r"f'{x}\n'")
-        self.check_src_roundtrip("f'{'\\n'}\\n'")
-        self.check_src_roundtrip("f'{f'{x}\\n'}\\n'")
+        self.check_src_roundtrip('''f"{'\\n'}\\n"''')
+        self.check_src_roundtrip('''f"{f'{x}\\n'}\\n"''')
 
     def test_docstrings(self):
         docstrings = (

Misc/NEWS.d/next/Library/2024-12-20-08-44-12.gh-issue-127975.8HJwu9.rst

@@ -0,0 +1 @@
+Avoid reusing quote types in :func:`ast.unparse` if not needed.