bpo-41979: Accept star-unpacking on with-item targets

[isidentical]

https://bugs.python.org/issue41979

[lysnikolaou]

FWIW there are some differences here in the SyntaxErrors displayed.

[pablogsal]

FWIW there are some differences here in the SyntaxErrors displayed.

What syntax error are you referring to?

[lysnikolaou]

➜  cpython git:([bpo-41979](https://bugs.python.org/issue41979)) python3.8
Python 3.8.6 (default, Oct  3 2020, 16:26:47) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> with a as *b:
  File "<stdin>", line 1
    with a as *b:
              ^
SyntaxError: invalid syntax
>>> with a as (*b):
...     pass
... 
  File "<stdin>", line 1
SyntaxError: starred assignment target must be in a list or tuple
>>> 
➜  cpython git:([bpo-41979](https://bugs.python.org/issue41979)) ./python
Python 3.10.0a1+ (heads/[bpo-41979](https://bugs.python.org/issue41979):bc2fd55f74, Oct  9 2020, 11:30:38) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> with a as *b:
...     pass
... 
  File "<stdin>", line 1
SyntaxError: starred assignment target must be in a list or tuple
>>> with a as (*b):
  File "<stdin>", line 1
    with a as (*b):
               ^
SyntaxError: can't use starred expression here

I'm not saying we should change something here. I'm just mentioning the differences, so that we know about them. Thoughts?

[pablogsal]

I'm not saying we should change something here. I'm just mentioning the differences, so that we know about them. Thoughts?

Oh, thanks for the example! I think the second one may be a bit confusing to users (you can use star expressions there but as the first error mentions, must be in a list or a tuple). I will try to create a PR today for this, but I think we can treat it separately from this one, what do you think?

[isidentical]

FWIW, this behavior is the exact same thing with other assignments;

(3.10)
>>> *a = [1,2]
  File "<stdin>", line 1
SyntaxError: starred assignment target must be in a list or tuple
>>> (*a) = [1,2]
  File "<stdin>", line 1
    (*a) = [1,2]
     ^
SyntaxError: can't use starred expression here

[pablogsal]

FWIW, this behavior is the exact same thing with other assignments;

Yeah, that's why I proposed to treat this independently of this PR. I will create a patch for this syntax error today if I find the time

[lysnikolaou]

FWIW, this behavior is the exact same thing with other assignments;

Yeah, that's why I proposed to treat this independently of this PR. I will create a patch for this syntax error today if I find the time

Agreed.

What bugs me most is that in the with a as *b example, I'm not immediately getting an error like in the old parser. I must complete the whole with statement before I get to the SyntaxError.

[pablogsal]

I must complete the whole with statement before I get to the SyntaxError.

But that only makes sense in the REPL no? We can look together later how to make this fail before if you wish. Likely is a matter of restricting some of that rule or checking the order. But I would be cautious of this as it probably will raise the complexity of the grammar a bit.

[miss-islington]

Thanks @isidentical for the PR, and @pablogsal for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[miss-islington]

Sorry, @isidentical and @pablogsal, I could not cleanly backport this to 3.9 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 48f305fd122080a9181cbda33bdb42ea36a0136f 3.9

[pablogsal]

@isidentical Can you do the backport, please?

[lysnikolaou]

Agreed, let's merge this and come back to it later.

[bedevere-bot]

GH-22612 is a backport of this pull request to the 3.9 branch.