PEP 572 : assignment expression to a global variable in a comprehension

[PierreQuentel]

BPO	38469
Nosy	@gvanrossum, @ncoghlan, @ambv, @PierreQuentel, @emilyemorehouse, @pablogsal
PRs	bpo-38469: Handle named expression scope whith global/nonlocal keywords #16755 [3.8] bpo-38469: Handle named expression scope with global/nonlocal keywords (GH-16755) #16760

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-10-14.18:34:35.368>
created_at = <Date 2019-10-13.20:54:11.116>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9']
title = 'PEP 572 : assignment expression to a global variable in a comprehension'
updated_at = <Date 2019-10-14.21:42:18.909>
user = 'https://github.com/PierreQuentel'

bugs.python.org fields:

activity = <Date 2019-10-14.21:42:18.909>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-14.18:34:35.368>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2019-10-13.20:54:11.116>
creator = 'quentel'
dependencies = []
files = []
hgrepos = []
issue_num = 38469
keywords = ['patch']
message_count = 10.0
messages = ['354601', '354603', '354604', '354606', '354607', '354608', '354615', '354628', '354644', '354664']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'ncoghlan', 'lukasz.langa', 'quentel', 'emilyemorehouse', 'pablogsal']
pr_nums = ['16755', '16760']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38469'
versions = ['Python 3.8', 'Python 3.9']

[PierreQuentel]

PEP-572 says that "an assignment expression occurring in a (...) comprehension (...) binds the target in the containing scope, honoring a nonlocal or global declaration for the target in that scope, if one exists."

In Appendix B, the PEP shows this example :

def f():
    global TARGET
    a = [TARGET := EXPR for VAR in ITERABLE]

So I don't understand why this fails:

Python 3.8.0rc1 (tags/v3.8.0rc1:34214de, Oct  1 2019, 18:42:37) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 0
>>> def f():
...     global x
...     [x := i for i in range(5)]
...
  File "<stdin>", line 3
SyntaxError: no binding for nonlocal 'x' found
>>>

Is this a bug or am I missing something ?

[pablogsal]

I think this is a bug in the symtable_extend_namedexpr_scope function because when visiting the stack in reverse order and finding a function block is always assigning the scope as DEF_NONLOCAL.

Marking also as a release blocker unless Nick, Guido or Emily confirm otherwise.

[gvanrossum]

It's a bug, but it's honestly such an odd corner case that I don't think this should hold up the 3.8.0 release. Let's fix it in 3.9 and backport it to 3.8.1.

[pablogsal]

I am working on a fix now, se we can merge it before the final release

[gvanrossum]

Thanks for working on a fix. But is it really important enough to violate the desired "rc1 == release" invariant? It's up to the release manager.

[pablogsal]

But is it really important enough to violate the desired "rc1 == release" invariant?

I created the fix because many other bug fixes have been merged since rc1 already so apologies if it seemed a bit precipitate. Let's see what the release manager thinks (in case the bugfix is correct) :)

[pablogsal]

New changeset fd5c414 by Pablo Galindo in branch 'master':
bpo-38469: Handle named expression scope with global/nonlocal keywords (GH-16755)
fd5c414

[ambv]

New changeset 927f07c by Łukasz Langa (Miss Islington (bot)) in branch '3.8':
bpo-38469: Handle named expression scope with global/nonlocal keywords (GH-16755) (bpo-16760)
927f07c

[PierreQuentel]

That was a quick fix, thanks !

[ambv]

(FTR the fix is part of Python 3.8.0)