Allow unparenthesized walrus in set comprehensions

[lysnikolaou]

BPO	42381
Nosy	@gvanrossum, @lysnikolaou, @pablogsal, @miss-islington
PRs	bpo-42381: Allow walrus in set literals and set comprehensions #23332 [3.9] bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332) #23333 bpo-42381: Document walrus-related syntax changes in whatsnew #23382

^{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 2020-11-18.23:50:58.954>
created_at = <Date 2020-11-16.22:42:21.215>
labels = ['interpreter-core', 'type-bug', '3.10']
title = 'Allow unparenthesized walrus in set comprehensions'
updated_at = <Date 2020-11-18.23:50:58.953>
user = 'https://github.com/lysnikolaou'

bugs.python.org fields:

activity = <Date 2020-11-18.23:50:58.953>
actor = 'lys.nikolaou'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-18.23:50:58.954>
closer = 'lys.nikolaou'
components = ['Interpreter Core']
creation = <Date 2020-11-16.22:42:21.215>
creator = 'lys.nikolaou'
dependencies = []
files = []
hgrepos = []
issue_num = 42381
keywords = ['patch']
message_count = 13.0
messages = ['381173', '381177', '381187', '381188', '381190', '381191', '381192', '381194', '381195', '381201', '381203', '381383', '381384']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'lys.nikolaou', 'pablogsal', 'miss-islington']
pr_nums = ['23332', '23333', '23382']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42381'
versions = ['Python 3.10']

[lysnikolaou]

Since unparenthesized walruses are allowed in list comprehensions and in generator expressions, should we maybe allow them in set comprehensions as well?

[gvanrossum]

Yeah, I suppose the comprehensions should all have the same syntax inside their various brackets.

[pablogsal]

Apparently we currently don't allow walruses in set literals either:

>>> {y := 4, 4**2, 3**3}
  File "<stdin>", line 1
    {y := 4, 4**2, 3**3}
       ^
SyntaxError: invalid syntax

But they should be allowed as well per PEP-572 (as the pep mentions all comprehensions):

There is one special case: an assignment expression occurring in a list, set or dict comprehension or in a generator expression (below collectively referred to as "comprehensions") binds the target in ...

[pablogsal]

The only question left is that if we should allow this on dict literals and dict comprehensions as well (because these constructs have a ":" in the middle that can be noisy if there are walrus, the answer isn't immediately obvious to me).

[lysnikolaou]

dict comprehension

FWIW, unparenthesized walruses are still disallowed in dictcomps and to be honest, I'm not sure if we should allow them or not. I feel that allowing too many colons there would be too noisy.

{a:='a': b:=1} doesn't really look too beautiful.

[gvanrossum]

Not in anything dict please.

Nagging question — is there sufficient difference between {x := y} and {x: y} ?

[lysnikolaou]

While {(a:='a'): (b:=1)} looks somewhat cleaner to me.

[pablogsal]

Nagging question — is there sufficient difference between {x := y} and {x: y} ?

I think there is enough distinction, yes. It will also be a bit odd if is allowed in list and list comps but not set and set comps

[gvanrossum]

Okay.

[pablogsal]

New changeset b0aba1f by Pablo Galindo in branch 'master':
bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332)
b0aba1f

[gvanrossum]

Let’s make sure this is all written up in whatsnew.

[pablogsal]

New changeset 87c87b5 by Pablo Galindo in branch '3.9':
[3.9] bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332) (GH-23333)
87c87b5

[miss-islington]

New changeset a85fefe by Lysandros Nikolaou in branch 'master':
bpo-42381: Document walrus-related syntax changes in whatsnew (GH-23382)
a85fefe