[3.12] gh-109114: Relax the check for invalid lambdas inside f-strings to avoid false positives (GH-109121). (#109155)

pablogsal · web-flow · commit 21f4e6d6cb53 · 2023-09-12T15:52:25.000Z
(cherry picked from commit 5bda2f6) Signed-off-by: Pablo Galindo <pablogsal@gmail.com>
diff --git a/Grammar/python.gram b/Grammar/python.gram
@@ -1170,7 +1170,7 @@ invalid_expression:
         _PyPegen_check_legacy_stmt(p, a) ? NULL : p->tokens[p->mark-1]->level == 0 ? NULL :
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
-   | a='lambda' [lambda_params] b=':' &(FSTRING_MIDDLE | fstring_replacement_field)  {
+   | a='lambda' [lambda_params] b=':' &FSTRING_MIDDLE  {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "f-string: lambda expressions are not allowed without parentheses") }
 
 invalid_named_expression(memo):
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
@@ -1027,6 +1027,10 @@ def test_lambda(self):
                              "f'{lambda x:}'",
                              "f'{lambda :}'",
                              ])
+        # Ensure the detection of invalid lambdas doesn't trigger detection
+        # for valid lambdas in the second error pass
+        with self.assertRaisesRegex(SyntaxError, "invalid syntax"):
+            compile("lambda name_3=f'{name_4}': {name_3}\n1 $ 1", "<string>", "exec")
 
         # but don't emit the paren warning in general cases
         with self.assertRaisesRegex(SyntaxError, "f-string: expecting a valid expression after '{'"):
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-09-08-01-50-41.gh-issue-109114.adqgtb.rst b/Misc/NEWS.d/next/Core and Builtins/2023-09-08-01-50-41.gh-issue-109114.adqgtb.rst
@@ -0,0 +1,3 @@
+Relax the detection of the error message for invalid lambdas inside
+f-strings to not search for arbitrary replacement fields to avoid false
+positives. Patch by Pablo Galindo
diff --git a/Parser/parser.c b/Parser/parser.c

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+Relax the detection of the error message for invalid lambdas inside`
	`2`	`+f-strings to not search for arbitrary replacement fields to avoid false`
	`3`	`+positives. Patch by Pablo Galindo`