feat: add --cache-dir command-line option#5201
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Add a --cache-dir option to set the directory where Black reads and writes its cache. It takes precedence over the BLACK_CACHE_DIR environment variable, mirroring how --workers relates to BLACK_NUM_WORKERS. This helps when an environment variable can't be set for the invocation, such as running Black as a pre-commit hook in a monorepo where the cache should be written outside the project root. Closes psf#5174
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Description
Adds a
--cache-dircommand-line option that sets the directory where Black readsand writes its cache. It takes precedence over the
BLACK_CACHE_DIRenvironmentvariable, mirroring how
--workersrelates toBLACK_NUM_WORKERS.Resolution precedence (highest first):
--cache-dir→BLACK_CACHE_DIR→XDG_CACHE_HOME→ platform-specific user cache directory.This is implemented by reassigning the module-level
black.cache.CACHE_DIRconstantonce in
main()before files are dispatched — the same global the cache read/writehelpers (and the existing test fixture) already consult at runtime. The single- and
multi-file paths both read/write the cache in the parent process, so one override
covers both.
Motivation
Closes #5174. The reporter runs Black as a pre-commit hook in a monorepo. Pre-commit
deliberately won't expose an
envknob at the hook level, soBLACK_CACHE_DIRcan't beset for that invocation and the cache lands in the project root. A CLI option lets the
cache location be configured directly and committed to VCS via the hook args.
Scope is intentionally narrow: this only relocates the formatting cache. It does not
overload an empty path as a cache-disable switch — the existing
--no-cacheflagalready serves that purpose.
Checklist - did you ...
CHANGES.mdif necessary?