cc @pablogsal @lysnikolaou

Thanks for the issue! Will take a bite at this today at night

This works for me:

diff --git a/Grammar/python.gram b/Grammar/python.gram
index ec964d6783..98270fe77f 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -1282,6 +1282,8 @@ invalid_with_item:
         RAISE_SYNTAX_ERROR_INVALID_TARGET(STAR_TARGETS, a) }
 
 invalid_for_target:
+    | 'async'? 'for' star_targets !'in' {
+        RAISE_SYNTAX_ERROR("'in' expected after for-loop variables") }
     | 'async'? 'for' a=star_expressions {
         RAISE_SYNTAX_ERROR_INVALID_TARGET(FOR_TARGETS, a) }

Python 3.13.0a1+ (heads/main-dirty:4a929d432b, Oct 31 2023, 15:32:48) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for x if range(1)]
  File "<stdin>", line 1
    [x for x if range(1)]
             ^^
SyntaxError: 'in' expected after for-loop variables

Let me know if this worth a PR.

This sort of works, but it breaks other error messages, for instance this one:

 for a, b, (c + 1, d()): pass

That used to be: SyntaxError: cannot assign to expression but now will show 'in' expected after for-loop variables which is not correct. What is worse, adding in produces this:

>>> for a, b, (c + 1, d()) in P:
  File "<stdin>", line 1
    for a, b, (c + 1, d()) in P:
                           ^^
SyntaxError: 'in' expected after for-loop variables

which is hilariously incorrect. This happens as well with function calls:

for a, b() in c: pass

now shows:

        for a, b() in c: pass
                   ^^
    SyntaxError: 'in' expected after for-loop variable

instead of cannot assign to function call

Yep, wrong place, this should be restricted to comprehensions. test_syntax.py pass with this alternative at the end of for_if_clause[comprehension_ty].

this should be restricted to comprehensions.

You have the same problem in list comprehensions, is just that there are no tests for that:

>>> [x for x() in a]
  File "<stdin>", line 1
    [x for x() in a]
               ^^
SyntaxError: 'in' expected after for-loop variables

this currently shows:

>>> [x for x() in a]
  File "<stdin>", line 1
    [x for x() in a]
           ^^^
SyntaxError: cannot assign to function call

@skirpichev If you want a quick PR, you can add tests that cover this case for list/set/dict comprehensions in test_syntax