gh-138294: Fix Misleading error for ((a) := 1)

Grammar/python.gram

@@ -1261,6 +1261,9 @@ invalid_expression:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "t-string: lambda expressions are not allowed without parentheses") }
 
 invalid_named_expression(memo):
+    | '(' a=NAME ')' ':=' expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot parenthesize target name in assignment expression") }
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }

Lib/test/test_syntax.py

@@ -2686,6 +2686,13 @@ def f(x: *b)
     >>> f(x = 5, *:)
     Traceback (most recent call last):
     SyntaxError: Invalid star expression
+
+Invalid assignment expressions with parenthesized targets:
+
+    >>> ((a) := 1)
+    Traceback (most recent call last):
+       ...
+    SyntaxError: cannot parenthesize target name in assignment expression
 """
 
 import re

Misc/NEWS.d/next/Core_and_Builtins/2025-09-08-12-30-00.gh-issue-138294.abcd12.rst

@@ -0,0 +1 @@
+Raise an improved :exc:`SyntaxError` when using parentheses around a walrus target.