Description

assignee = None
closed_at = None
created_at = <Date 2020-11-10.17:28:35.243>
labels = ['interpreter-core', '3.10', 'docs']
title = 'Walrus Operator in list index'
updated_at = <Date 2020-11-16.23:09:41.621>
user = 'https://bugs.python.org/Brando753'

activity = <Date 2020-11-16.23:09:41.621>
actor = 'lys.nikolaou'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Interpreter Core']
creation = <Date 2020-11-10.17:28:35.243>
creator = 'Brando753'
dependencies = []
files = []
hgrepos = []
issue_num = 42316
keywords = ['patch']
message_count = 3.0
messages = ['380691', '380970', '381179']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'docs@python', 'lys.nikolaou', 'Brando753']
pr_nums = ['23291', '23317']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue42316'
versions = ['Python 3.10']

Activity

1. Brando753 commented on Nov 10, 2020

   

   Brando753mannequin

   on Nov 10, 2020

   MannequinAuthor

   More actions

   Reading the PEP-572 document I don't see anything stating that Walrus operator in list indexes must be enclosed in parenthesis.

   Minimal Example:
   '''
   In [1]: a = list(range(10))

   In [2]: idx = -1

   In [3]: a[idx := idx +1]
   File "<ipython-input-3-65fc931e77dd>", line 1
   a[idx := idx +1]
   ^
   SyntaxError: invalid syntax

   '''

   If you enclose in parenthesis it works as expected:
   '''
   In [4]: a[(idx := idx +1)]
   Out[4]: 0

   In [5]: a[(idx := idx +1)]
   Out[5]: 1

   In [6]: a[(idx := idx +1)]
   Out[6]: 2
   '''

   Is this a bug or am I misreading the PEP-572 document somewhere? It's not a top-level assignment nor is it a list comprehension so I would expect it to work in the first example.
2. Brando753

   added

   topic-asyncio

   3.8 (EOL)end of lifeend of life

   on Nov 10, 2020
3. terryjreedy commented on Nov 14, 2020

   

   terryjreedy

   on Nov 14, 2020

   Member

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   PEP-572 does not saw much of anything about when parens are needed. Nor does the low priority itself. Looking through the grammar of expressions, an assignment_expression is also a starred_expression, a positional_item (in calls), and the first part of a comprehension. It is not itself an plain expression unless and until wrapped in parentheses. Subscription requires an expression list, which may be a single expression.

   To put it another way: an expression is an assignment expression but an assignment expression with a "name :=" prefix is not an expression, so parens are required anywhere an expression is required. That includes subscripts, slicings, displays, call items other than positional items, comprehension parts other than the initial expression (maybe, check), parts of conditional expressions (maybe, check), and lambda expression bodies. After checking, I would like a sentence added to the doc before the 'go see the PEP' bit.

   This has nothing to do with asyncio.
4. terryjreedy

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   3.10 (EOL)end of lifeend of life

   and removed

   topic-asyncio

   3.8 (EOL)end of lifeend of life

   on Nov 14, 2020
5. terryjreedy

   assigned

   docspython

   on Nov 14, 2020
6. terryjreedy

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   and removed

   topic-asyncio

   on Nov 14, 2020
7. 2 remaining items
8. terryjreedy

   assigned

   docspython

   on Nov 14, 2020
9. lysnikolaou commented on Nov 16, 2020

   

   lysnikolaou

   on Nov 16, 2020

   Member

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   New changeset cae6018 by Lysandros Nikolaou in branch 'master':
   bpo-42316: Allow unparenthesized walrus operator in indexes (GH-23317)
   cae6018
10. ezio-melotti

    transferred this issue fromon Apr 10, 2022
11. JelleZijlstra

    added a commit that references this issue on Oct 6, 2022

    gh-86482: Document assignment expression need for ()s (#23291)

    2b5f136
12. miss-islington

    added 2 commits that reference this issue on Oct 6, 2022

    pythongh-86482: Document assignment expression need for ()s (pythonGH…

    cc41183

    pythongh-86482: Document assignment expression need for ()s (pythonGH…

    90ebcbc
13. JelleZijlstra

    self-assigned this

    on Oct 6, 2022
14. miss-islington

    added 2 commits that reference this issue on Oct 6, 2022

    gh-86482: Document assignment expression need for ()s (GH-23291)

    4aa2ebc

    gh-86482: Document assignment expression need for ()s (GH-23291)

    b7487df
15. carljm

    added 2 commits that reference this issue on Oct 6, 2022

    Merge branch 'main' into dictwatch

    e5d2169

    Merge branch 'main' into typewatch

    415ed49
16. mpage

    added a commit that references this issue on Oct 11, 2022

    pythongh-86482: Document assignment expression need for ()s (python#2…

    ff0d621
17. pablogsal

    added a commit that references this issue on Oct 22, 2022

    gh-86482: Document assignment expression need for ()s (GH-23291)

    cc10a8e
18. hauntsaninja commented on Oct 28, 2022

    

    hauntsaninja

    on Oct 28, 2022

    Contributor

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    Thanks, looks like we've completed this
19. hauntsaninja

    closed this as completedon Oct 28, 2022

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