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This approach doesn't work. Please read the discussion on the issue, particularly this comment:

#111488 (comment)

For example, this PR breaks this case:

>>> [x for a, b, (c + 1, d()) in y]
  File "<stdin>", line 1
    [x for a, b, (c + 1, d()) in y]
                  ^^^^^
SyntaxError: cannot assign to expressio

as it now shows:

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

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

fixed

➜  ~ python3.13
Python 3.13.0a2+ (heads/main-dirty:7b74e4141b, Jan  4 2024, 01:39:25) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for a, b, (c + 1, d()) in y]
  File "<stdin>", line 1
    [x for a, b, (c + 1, d()) in y]
                  ^^^^^
SyntaxError: cannot assign to expression
>>> [x for a, b, (c + 1, d()) if y]
  File "<stdin>", line 1
    [x for a, b, (c + 1, d()) if y]
                              ^^
SyntaxError: 'in' expected after for-loop variables
>>> 

Hello, I have made the requested changes; please review again :)

This is better but note lowering the grammar to not include expressions will still change existing errors. For example:

>>> [x for a>b if z else 42 in x]
  File "<stdin>", line 1
    [x for a>b if z else 42 in x]
SyntaxError: cannot assign to conditional expression

changes to

>>> [x for a>b if z else 42 in x]
  File "<stdin>", line 1
    [x for a>b if z else 42 in x]
            ^
SyntaxError: 'in' expected after for-loop variables

I think is not too bad because the error is still correct but this may affect other constructs, although I think the error will more or less be correct (maybe it will show AFTER some other real error, though). @lysnikolaou what do you think?

An example of what I mean:

>>> [x for (yield z) with y]
  File "<stdin>", line 1
    [x for (yield z) with y]
            ^^^^^^^
SyntaxError: cannot assign to yield expression

will change to

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

here are two errors and now we show the one that's further from the start of the comprehension. So after fixing that there is another error reported BEFORE it.

I am not sure if we should care about that, but this is a difference.

(@grigoriev-semyon don't change anything yet, let's check what @lysnikolaou thinks)

I'm okay with both, although I'm incline to say let's fix this one and accept that there might be cases that become suboptimal because of it.

Either way we have to accept that there are error messages that are very hard to get right, and this might be one of them. I'm also kind of inclined to try and have a more sophisticated approach to this and other error messages, where you have a priority system where you can try more than one invalid_ rules and the error with the highest priority wins. Don't know how much effort that would take though.

@pablogsal I tried to improve the result, but it seems difficult.

If you agree, I suggest merging this PR and creating a task to further improve the error in this place.
I am also ready to work on a future task.

Yeah I think we will merge this and check how it goes. Thanks for the patience @grigoriev-semyon and the good work!

Congrats @grigoriev-semyon! Thanks everyone :)