gh-108843: fix ast.unparse for f-string with many quotes (#108981)

1 files changed, 20 insertions, 1 deletions

diff --git a/Lib/ast.py b/Lib/ast.py
index 17ec7ff..1f54309 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -1236,17 +1236,36 @@ class _Unparser(NodeVisitor):
 
         new_fstring_parts = []
         quote_types = list(_ALL_QUOTES)
+        fallback_to_repr = False
         for value, is_constant in fstring_parts:
             if is_constant:
-                value, quote_types = self._str_literal_helper(
+                value, new_quote_types = self._str_literal_helper(
                     value,
                     quote_types=quote_types,
                     escape_special_whitespace=True,
                 )
+                if set(new_quote_types).isdisjoint(quote_types):
+                    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
             new_fstring_parts.append(value)
 
+        if fallback_to_repr:
+            # If we weren't able to find a quote type that works for all parts
+            # of the JoinedStr, fallback to using repr and triple single quotes.
+            quote_types = ["'''"]
+            new_fstring_parts.clear()
+            for value, is_constant in fstring_parts:
+                if is_constant:
+                    value = repr('"' + value)  # force repr to use single quotes
+                    expected_prefix = "'\""
+                    assert value.startswith(expected_prefix), repr(value)
+                    value = value[len(expected_prefix):-1]
+                new_fstring_parts.append(value)
+
         value = "".join(new_fstring_parts)
         quote_type = quote_types[0]
         self.write(f"{quote_type}{value}{quote_type}")