Avoid removing whitespace for walrus operators within subscripts

[charliermarsh]

Description

At present, Black removes spaces around walrus operators within subscripts. For example, this is stable style:

x[y:=1]

While rare in practice, it appears that this behavior is unintentional, and falls out from Black's removal of spaces around colons in slices (e.g., x[y:z]).

This PR removes that behavior, instead requiring spaces around walrus operators in such cases.

Closes #3820.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[charliermarsh]

A related question -- Black's stable style uses:

a[:(x := 13)]

Perhaps this should instead be:

a[: (x := 13)]

(But I don't feel at all strongly here -- more asking if the stable style is intentional.)

[github-actions]

diff-shades reports zero changes comparing this PR (4aca4b3) to main (74d3009).

---

What is this? | Workflow run | diff-shades documentation

[JelleZijlstra]

A related question -- Black's stable style uses:

a[:(x := 13)]

Perhaps this should instead be:

a[: (x := 13)]

(But I don't feel at all strongly here -- more asking if the stable style is intentional.)

I think the latter would be more consistent, since we add spaces to e.g. 'x[:a+4]' and the walrus should generally be treated like any other operator.

[JelleZijlstra]

This looks good, thanks! However, the change should be made only in the preview style: we'll promote it to stable in January.

[hauntsaninja]

Thanks, this looks good! I made some minor changes to preview handling, tests and reformatted the source

[charliermarsh]

Thanks!

[JelleZijlstra]

Tests are failing on 3.8/3.9 because of a change noted in https://docs.python.org/3.10/whatsnew/3.10.html#other-language-changes: "Assignment expressions can now be used unparenthesized within set literals and set comprehensions, as well as in sequence indexes (but not slices)."

We should split out the offending tests into a separate test file that gets run only on 3.10+.