def get_cache_dir() -> Path:
    """Get the cache directory used by black.

    Users can customize this directory on all systems using `BLACK_CACHE_DIR`
    environment variable. By default, the cache directory is the user cache directory
    under the black application.

    This result is immediately set to a constant `black.cache.CACHE_DIR` as to avoid
    repeated calls.
    """
    # NOTE: Function mostly exists as a clean way to test getting the cache directory.
    default_cache_dir = user_cache_dir("black")
    cache_dir = Path(os.environ.get("BLACK_CACHE_DIR", default_cache_dir))
    cache_dir = cache_dir / __version__
    return cache_dir
Get the cache directory used by black.

    Users can customize this directory on all systems using `BLACK_CACHE_DIR`
    environment variable. By default, the cache directory is the user cache directory
    under the black application.

    This result is immediately set to a constant `black.cache.CACHE_DIR` as to avoid
    repeated calls.
    
src/black/cache.py