Allow specifying --workers via environment variable

[hauntsaninja]

Fixes #3742

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• Add new / update outdated documentation?

[JelleZijlstra]

Looks good, but this should also be mentioned in the documentation, maybe somewhere in https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html (which I think is rather different on main, I need to do a release sometime).

[github-actions]

diff-shades reports zero changes comparing this PR (9b9ee61) to main (c732a1f).

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

[hauntsaninja]

It looks like it will automatically have something added to that file from the --help:

black/docs/usage_and_configuration/the_basics.md

Line 35 in 9b2b048

<summary>CLI reference</summary>

  -W, --workers INTEGER RANGE     Number of parallel workers [default:
                                  BLACK_NUM_WORKERS environment variable or
                                  number of CPUs in the system]  [x>=1]

Should I document this somewhere beyond that? I don't see any other documentation about --workers and I'm not sure if there's a good existing section in the_basics.md

[hauntsaninja]

I guess I could make a new section below that called "environment variable options" and mention both this and BLACK_CACHE_DIR?

[JelleZijlstra]

I guess I could make a new section below that called "environment variable options" and mention both this and BLACK_CACHE_DIR?

Yes, sounds good!