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PEG parser doesn't allow lambda in for_if_clause #86014

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slebedev mannequin opened this issue Sep 23, 2020 · 5 comments
Closed

PEG parser doesn't allow lambda in for_if_clause #86014

slebedev mannequin opened this issue Sep 23, 2020 · 5 comments
Labels
3.9 only security fixes 3.10 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@slebedev
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slebedev mannequin commented Sep 23, 2020

BPO 41848
Nosy @gvanrossum, @terryjreedy, @benjaminp, @serhiy-storchaka, @lysnikolaou, @pablogsal, @isidentical
PRs
  • bpo-41848: Allow simple lambdas in if clauses of comprehensions #22394
    		•

    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 2020-09-26.23:36:40.147>
created_at = <Date 2020-09-23.23:33:02.167>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10']
title = "PEG parser doesn't allow lambda in for_if_clause"
updated_at = <Date 2020-09-26.23:36:40.145>
user = 'https://bugs.python.org/slebedev'

    bugs.python.org fields:

    activity = <Date 2020-09-26.23:36:40.145>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-26.23:36:40.147>
closer = 'gvanrossum'
components = ['Interpreter Core']
creation = <Date 2020-09-23.23:33:02.167>
creator = 'slebedev'
dependencies = []
files = []
hgrepos = []
issue_num = 41848
keywords = ['patch']
message_count = 5.0
messages = ['377435', '377511', '377514', '377545', '377546']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'terry.reedy', 'benjamin.peterson', 'serhiy.storchaka', 'lys.nikolaou', 'pablogsal', 'BTaskaya', 'slebedev']
pr_nums = ['22394']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41848'
versions = ['Python 3.9', 'Python 3.10']
    @slebedev
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    slebedev mannequin commented Sep 23, 2020

    Reproducer:

    [x for x in [] if lambda: x]

    This parses fine in 3.8, but doesn't parse in 3.9 because the grammar expects a disjunction after if in for_if_clause *. While this change has zero practical significance, I think it might be useful to maintain a list of such changes for tooling authors (who might need to support multiple Python versions).

    @slebedev slebedev mannequin added 3.9 only security fixes 3.10 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error labels Sep 23, 2020
    @terryjreedy
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    After adding parens,
>>> [x for x in [] if (lambda: x)]
[]

    @gvanrossum
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    I propose not to fix this, since it is pointless (a lambda is always truthy).

    We can call it out in What's New in 3.9 -- as a very small footnote. :-)

    @pablogsal
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    I concur with Guido: it makes the grammar slightly more complex for no gain. Also, the originally allowed rule does not allow *any* lambda but only the ones that are simple, which is a special case that makes it even more complex. In any case you can use parentheses to break this limitation.

    Like Guido, I propose to add a very small comment to What's New in 3.9

    @gvanrossum
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    Okay, let's close this.

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    da-woods added a commit to da-woods/cython that referenced this issue Aug 21, 2022
    @da-woods
    Remove p_lambdef_nocond from the parser.
    051bbc7 
    Note that it wasn't correct before since it didn't pass the
correct flag to `p_lambdef` and thus was equivalent to just
using `p_lambdef`.

Note also that there's a difference in behaviour between Python3.9+
and before. Python <3.9 allowed `[i for i in range(10) if lambda: i]`
while Python >=3.9 disallows this. Arguably it's pointless because
the lambda always evaluates to True. See
python/cpython#86014
for the Python issue.

With this change Cython will follow the Python 3.9 behaviour at
the cost of potentially breaking some code that does use the
pattern above. If that isn't desirable I can produce an
alternative change that fixes p_lambda_nocond instead.

Part of cleanup in cython#4595.
    da-woods added a commit to da-woods/cython that referenced this issue Aug 21, 2022
    @da-woods
    Remove p_lambdef_nocond from the parser.
    a2d46b3 
    Note that it wasn't correct before since it didn't pass the
correct flag to `p_lambdef` and thus was equivalent to just
using `p_lambdef`.

Note also that there's a difference in behaviour between Python3.9+
and before. Python <3.9 allowed `[i for i in range(10) if lambda: i]`
while Python >=3.9 disallows this. Arguably it's pointless because
the lambda always evaluates to True. See
python/cpython#86014
for the Python issue.

With this change Cython will follow the Python 3.9 behaviour at
the cost of potentially breaking some code that does use the
pattern above. If that isn't desirable I can produce an
alternative change that fixes p_lambda_nocond instead.

Part of cleanup in cython#4595.
    @da-woods da-woods mentioned this issue Aug 21, 2022
    Merged
    scoder pushed a commit to cython/cython that referenced this issue Sep 5, 2022
    @da-woods
    Remove p_lambdef_nocond from the parser, following Python 3.9+. (GH…
    85a29fb 
    …-4992)

Note that it wasn't correct before since it didn't pass the
correct flag to `p_lambdef` and thus was equivalent to just
using `p_lambdef`.

Note also that there's a difference in behaviour between Python3.9+
and before. Python <3.9 allowed `[i for i in range(10) if lambda: i]`
while Python >=3.9 disallows this. Arguably it's pointless because
the lambda always evaluates to True. See
python/cpython#86014
for the Python issue.

With this change Cython will follow the Python 3.9 behaviour at
the cost of potentially breaking some code that does use the
pattern above.

Part of the cleanup in #4595
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