lambda expression no longer valid at comp_if in Python 3.9

[gousaiyang]

BPO	43755
Nosy	@gvanrossum, @lysnikolaou, @pablogsal, @miss-islington, @tirkarthi, @gousaiyang
PRs	bpo-43755: Update docs to reflect that lambda is not allowed in comp_if since 3.9 #25231 [3.9] bpo-43755: Update docs to reflect that lambda is not allowed in comp_if since 3.9 (GH-25231) #25233

^{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 2021-04-06.22:17:20.933>
created_at = <Date 2021-04-06.20:07:25.211>
labels = ['type-bug', '3.9', '3.10']
title = 'lambda expression no longer valid at comp_if in Python 3.9'
updated_at = <Date 2021-04-07.18:18:03.202>
user = 'https://github.com/gousaiyang'

bugs.python.org fields:

activity = <Date 2021-04-07.18:18:03.202>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-06.22:17:20.933>
closer = 'pablogsal'
components = []
creation = <Date 2021-04-06.20:07:25.211>
creator = 'gousaiyang'
dependencies = []
files = []
hgrepos = []
issue_num = 43755
keywords = ['patch']
message_count = 8.0
messages = ['390369', '390370', '390371', '390376', '390377', '390380', '390381', '390459']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'lys.nikolaou', 'pablogsal', 'miss-islington', 'xtreak', 'gousaiyang']
pr_nums = ['25231', '25233']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43755'
versions = ['Python 3.9', 'Python 3.10']

[gousaiyang]

According to the documentation, a lambda expression at the comp_if position of a comprehension is allowed (can be parsed as lambda_expr_nocond). But this seems broken in Python 3.9 PEG parser. Example:

user@host:/$ python3.8
Python 3.8.9 (default, Apr  3 2021, 01:00:00)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for x in range(10) if lambda: 1]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>
user@host:/$ python3.9
Python 3.9.3 (default, Apr  3 2021, 00:51:37)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for x in range(10) if lambda: 1]
  File "<stdin>", line 1
    [x for x in range(10) if lambda: 1]
                             ^
SyntaxError: invalid syntax

[tirkarthi]

This looks like a duplicate of https://bugs.python.org/issue41848

[gousaiyang]

OK I didn't find this duplicate issue. After reading that, I think we should update the documentation (https://docs.python.org/3/reference/expressions.html). comp_if ::= "if" expression_nocond [comp_iter] can become comp_if ::= "if" or_test [comp_iter], and all nocond stuffs can be removed.

[gvanrossum]

Oh, nice improvement suggestion for the docs! Do you feel comfortable submitting a small PR for that?

[gousaiyang]

I'll work on it soon.

[pablogsal]

New changeset 0fdf11e by Saiyang Gou in branch 'master':
bpo-43755: Update docs to reflect that lambda is not allowed in comp_if since 3.9 (GH-25231)
0fdf11e

[pablogsal]

Thanks for the quick fix, Saiyang Gou