Walrus Operator in list index

[Brando753]

BPO	42316
Nosy	@terryjreedy, @lysnikolaou
PRs	gh-86482: Document assignment expression need for ()s #23291 bpo-42316: Allow unparenthesized walrus operator in indexes #23317

^{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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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'

bugs.python.org fields:

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']

[Brando753]

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.

[terryjreedy]

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.