Improve syntax error for assignment expr inside conditional expr

Author: brianschubert

Grammar/python.gram

@@ -1264,6 +1264,8 @@ invalid_expression:
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         _PyPegen_raise_error_for_missing_comma(p, a, b) }
+   | disjunction 'if' a=disjunction ':=' b=disjunction {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "assignment expression must be parenthesized inside conditional expression" ) }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
    | a=disjunction 'if' b=disjunction 'else' !expression {
         RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("expected expression after 'else', but statement is given") }

Lib/test/test_syntax.py

@@ -180,6 +180,10 @@
 Traceback (most recent call last):
 SyntaxError: expected expression before 'if', but statement is given
 
+>>> 1 if x := True else 2
+Traceback (most recent call last):
+SyntaxError: assignment expression must be parenthesized inside conditional expression
+
 >>> if True:
 ...     print("Hello"
 ...

Misc/NEWS.d/next/Core_and_Builtins/2026-03-22-14-49-48.gh-issue-146260.s8PzJW.rst

@@ -0,0 +1,3 @@
+Improve :exc:`SyntaxError` message when an :ref:`assignment expression
+<assignment-expressions>` is used inside the ``if`` clause of a
+:ref:`conditional expression <if_expr>`. Patch by Brian Schubert.