Allow for's target expression to be starred

[isidentical]

Fixes #2878

[github-actions]

diff-shades reports zero changes comparing this PR (71b5708) to main (07a2e6f).

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[JelleZijlstra]

Thanks, some minor feedback.

Also, I noticed that [x for x in *a, *b] isn't allowed. Just curious, do you know why that is?

[JelleZijlstra]

I don't think this needs to be 310-specific

[isidentical]

Since the PEP 617 was opt-out in 3.9, the given examples are rejected by the official AST when the user starts the python interpreter with -Xoldparser. It is why I've added tests to 3.10 group.

[isidentical]

@JelleZijlstra this seems to be something that was introduced during the PEG parser merge, but there is no official reference in anywhere beside the grammar. I have no knowledge on the comprehensions, but perhaps @pablogsal can shed more light in that.

[pablogsal]

@JelleZijlstra this seems to be something that was introduced during the PEG parser merge, but there is no official reference in anywhere beside the grammar. I have no knowledge on the comprehensions, but perhaps @pablogsal can shed more light in that.

Hummm, maybe I am misunderstanding this, but this syntax has never been allowed:

Python 3.8.12 (default, Nov  1 2021, 12:21:59)
>>> [y for y in *x, *y]
  File "<stdin>", line 1
    [y for y in *x, *y]
                ^
SyntaxError: invalid syntax

Python 3.10.2 (main, Jan 17 2022, 12:00:35)
>>> [y for y in *x, *y]
  File "<stdin>", line 1
    [y for y in *x, *y]
                ^
SyntaxError: invalid syntax

[pablogsal]

This is In the same spirit as [x,y for x,y in range(10)] being invalid. We have an error for this in 3.10:

>>> [x,y for x,y in x]
  File "<stdin>", line 1
    [x,y for x,y in x]
     ^^^
SyntaxError: did you forget parentheses around the comprehension target?

[JelleZijlstra]

My thinking process was just "hey, I didn't know this was allowed, I wonder where else it could be allowed". But it does make sense that we're a bit stricter in comprehensions than in standalone for loops. Thanks for the extra information!

[isidentical]

@pablogsal no, I have meant the regular for syntax:

$ python -c "for a in *[], *[]: pass"
$ python -Xoldparser -c "for a in *[], *[]: pass"
  File "<string>", line 1
    for a in *[], *[]: pass
             ^
SyntaxError: invalid syntax

The git bisect tells me that this behavior initially got introduced in the PEP 617 merge. Since with 3.9/3.10 this is now supported, what we wonder is why these two are inconsistent.

[pablogsal]

The git bisect tells me that this behavior initially got introduced in the PEP 617 merge.

Ah, sorry, I misunderstood your comment. Yes, that was introduced on PEP 617 merge but I am not actually sure that was on purpose as I cannot find any previous discussion.

Maybe we need to revert that :(

[isidentical]

Maybe we need to revert that :(

It might be a bit too late, no? (it was part of both 3.9/3.10 stable). But yea, let's create a bpo ticket to discuss further.

[pablogsal]

It might be a bit too late, no? (it was part of both 3.9/3.10 stable). But yea, let's create a bpo ticket to discuss further.

It may be, but if we treat it like a bug, you are technically not allowed to rely on the bug :)

[pablogsal]

Opened https://bugs.python.org/issue46725

[JelleZijlstra]

Let's hold off on merging this until the BPO is resolved. If CPython's decision is to remove the syntax, I think it makes sense for Black to disallow it too.

[ichard26]

Has been documented in python/cpython#31481. Thanks for fixing this Batuhan!

[AIvinAt169922K]

Good luck