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Walrus Operator in list index #86482

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Brando753 mannequin opened this issue Nov 10, 2020 · 4 comments
Closed

Walrus Operator in list index #86482

Brando753 mannequin opened this issue Nov 10, 2020 · 4 comments
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@JelleZijlstra
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3.10 only security fixes docs Documentation in the Doc dir interpreter-core (Objects, Python, Grammar, and Parser dirs)

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@Brando753
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Brando753 mannequin commented Nov 10, 2020

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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    GitHub fields:

    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
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    Brando753 mannequin commented Nov 10, 2020

    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.

    @Brando753 Brando753 mannequin added topic-asyncio 3.8 only security fixes labels Nov 10, 2020
    @terryjreedy
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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.

    @terryjreedy terryjreedy added docs Documentation in the Doc dir interpreter-core (Objects, Python, Grammar, and Parser dirs) 3.10 only security fixes and removed topic-asyncio 3.8 only security fixes labels Nov 14, 2020
    @terryjreedy terryjreedy added docs Documentation in the Doc dir interpreter-core (Objects, Python, Grammar, and Parser dirs) 3.10 only security fixes and removed topic-asyncio 3.8 only security fixes labels Nov 14, 2020
    @lysnikolaou
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    New changeset cae6018 by Lysandros Nikolaou in branch 'master':
    bpo-42316: Allow unparenthesized walrus operator in indexes (GH-23317)
    cae6018

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @bedevere-bot bedevere-bot mentioned this issue Aug 9, 2022
    Merged
    JelleZijlstra added a commit that referenced this issue Oct 6, 2022
    @terryjreedy @JelleZijlstra
    gh-86482: Document assignment expression need for ()s (#23291)
    2b5f136 
    Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
    miss-islington pushed a commit to miss-islington/cpython that referenced this issue Oct 6, 2022
    @terryjreedy @miss-islington
    pythongh-86482: Document assignment expression need for ()s (pythonGH…
    cc41183 
    …-23291)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 2b5f136)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
    This was referenced Oct 6, 2022
    Merged
    Merged
    miss-islington pushed a commit to miss-islington/cpython that referenced this issue Oct 6, 2022
    @terryjreedy @miss-islington
    pythongh-86482: Document assignment expression need for ()s (pythonGH…
    90ebcbc 
    …-23291)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 2b5f136)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
    @JelleZijlstra JelleZijlstra self-assigned this Oct 6, 2022
    miss-islington added a commit that referenced this issue Oct 6, 2022
    @miss-islington @terryjreedy
    gh-86482: Document assignment expression need for ()s (GH-23291)
    4aa2ebc 
    Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 2b5f136)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
    miss-islington added a commit that referenced this issue Oct 6, 2022
    @miss-islington @terryjreedy
    gh-86482: Document assignment expression need for ()s (GH-23291)
    b7487df 
    Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 2b5f136)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
    carljm added a commit to carljm/cpython that referenced this issue Oct 6, 2022
    @carljm
    Merge branch 'main' into dictwatch
    e5d2169 
    * main:
  fixes pythongh-96078: os.sched_yield release the GIL while calling sched_yield(2). (pythongh-97965)
  pythongh-65961: Do not rely solely on `__cached__` (pythonGH-97990)
  pythongh-97850: Remove the open issues section from the import reference (python#97935)
  Docs: pin sphinx-lint (pythonGH-97992)
  pythongh-94590: add signatures to operator itemgetter, attrgetter, methodcaller (python#94591)
  Add Pynche's move to the What's new in 3.11 (python#97974)
  pythongh-97781: Apply changes from importlib_metadata 5. (pythonGH-97785)
  pythongh-86482: Document assignment expression need for ()s (python#23291)
  pythongh-97943: PyFunction_GetAnnotations should return a borrowed reference. (python#97949)
  pythongh-94808: Coverage: Test that maximum indentation level is handled (python#95926)
    carljm added a commit to carljm/cpython that referenced this issue Oct 8, 2022
    @carljm
    Merge branch 'main' into typewatch
    415ed49 
    * main:
  pythonGH-97002: Prevent `_PyInterpreterFrame`s from backing more than one `PyFrameObject` (pythonGH-97996)
  pythongh-97973: Return all necessary information from the tokenizer (pythonGH-97984)
  fixes pythongh-96078: os.sched_yield release the GIL while calling sched_yield(2). (pythongh-97965)
  pythongh-65961: Do not rely solely on `__cached__` (pythonGH-97990)
  pythongh-97850: Remove the open issues section from the import reference (python#97935)
  Docs: pin sphinx-lint (pythonGH-97992)
  pythongh-94590: add signatures to operator itemgetter, attrgetter, methodcaller (python#94591)
  Add Pynche's move to the What's new in 3.11 (python#97974)
  pythongh-97781: Apply changes from importlib_metadata 5. (pythonGH-97785)
  pythongh-86482: Document assignment expression need for ()s (python#23291)
  pythongh-97943: PyFunction_GetAnnotations should return a borrowed reference. (python#97949)
    mpage pushed a commit to mpage/cpython that referenced this issue Oct 11, 2022
    @terryjreedy @JelleZijlstra @mpage
    pythongh-86482: Document assignment expression need for ()s (python#2…
    ff0d621 
    …3291)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
    pablogsal pushed a commit that referenced this issue Oct 22, 2022
    @miss-islington @terryjreedy @pablogsal
    gh-86482: Document assignment expression need for ()s (GH-23291)
    cc10a8e 
    Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 2b5f136)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
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    Thanks, looks like we've completed this

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