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

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

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 ...

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).

> 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.

Not in anything dict please.

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

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

> 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

Okay.

New changeset b0aba1fcdc3da952698d99aec2334faa79a8b68c by Pablo Galindo in branch 'master':
bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332)
https://github.com/python/cpython/commit/b0aba1fcdc3da952698d99aec2334faa79a8b68c

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

New changeset 87c87b5bd6f6a5924b485398f353308410f9d8c1 by Pablo Galindo in branch '3.9':
[3.9] bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332) (GH-23333)
https://github.com/python/cpython/commit/87c87b5bd6f6a5924b485398f353308410f9d8c1

New changeset a85fefe71b3bf56b1b3e72e752e0f962c3b0b487 by Lysandros Nikolaou in branch 'master':
bpo-42381: Document walrus-related syntax changes in whatsnew (GH-23382)
https://github.com/python/cpython/commit/a85fefe71b3bf56b1b3e72e752e0f962c3b0b487