Unparenthesized walrus is not allowed in genexps

[lysnikolaou]

BPO	42374
Nosy	@gvanrossum, @terryjreedy, @lysnikolaou, @pablogsal, @miss-islington
PRs	bpo-42374: Allow unparenthesized walrus in genexps #23319 [3.9] bpo-42374: Allow unparenthesized walrus in genexps (GH-23319) #23329

^{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 2020-11-16.23:39:39.790>
created_at = <Date 2020-11-16.17:43:28.572>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10']
title = 'Unparenthesized walrus is not allowed in genexps'
updated_at = <Date 2020-11-16.23:39:39.789>
user = 'https://github.com/lysnikolaou'

bugs.python.org fields:

activity = <Date 2020-11-16.23:39:39.789>
actor = 'lys.nikolaou'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-16.23:39:39.790>
closer = 'lys.nikolaou'
components = ['Interpreter Core']
creation = <Date 2020-11-16.17:43:28.572>
creator = 'lys.nikolaou'
dependencies = []
files = []
hgrepos = []
issue_num = 42374
keywords = ['patch', '3.9regression']
message_count = 3.0
messages = ['381133', '381178', '381185']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'terry.reedy', 'lys.nikolaou', 'pablogsal', 'miss-islington']
pr_nums = ['23319', '23329']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42374'
versions = ['Python 3.9', 'Python 3.10']

[lysnikolaou]

While a walrus is valid withing generator expressions in 3.8, it's not in 3.9 and onward.

➜  cpython git:(master) ✗ python3.8           
Python 3.8.6 (default, Oct  3 2020, 16:26:47) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> i = 2
>>> a = [1, 2, 3, 4]
>>> (b := i + j for j in a)
<generator object <genexpr> at 0x7f231e78c740>


➜  cpython git:(master) python3.10              
Python 3.10.0a1+ (heads/master:b580ed1, Oct 19 2020, 15:52:12) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> i = 2
>>> a = [1, 2, 3, 4]
>>> (b := i + j for j in a)
  File "<stdin>", line 1
    (b := i + j for j in a)
                ^
SyntaxError: invalid syntax

This is a regression introduced by the new parser.

[miss-islington]

New changeset cb3e5ed by Lysandros Nikolaou in branch 'master':
bpo-42374: Allow unparenthesized walrus in genexps (GH-23319)
cb3e5ed

[lysnikolaou]

New changeset 2b800ef by Lysandros Nikolaou in branch '3.9':
bpo-42374: Allow unparenthesized walrus in genexps (GH-23319) (GH-23329)
2b800ef