# File collection and discovery

You can directly pass _Black_ files, but you can also pass directories and _Black_ will walk them, collecting files to format. It determines what files to format or skip automatically using the inclusion and exclusion regexes and as well their modification time.

## Ignoring unmodified files

_Black_ remembers files it has already formatted, unless the –diff flag is used or code is passed via standard input. This information is stored per-user. The exact location of the file depends on the _Black_ version and the system on which _Black_ is run. The file is non-portable. The standard location on common operating systems is:

- Windows: C:\Users<username>AppDataLocalblackblackCache<version>cache.<line-length>.<file-mode>.pickle

- macOS: /Users/<username>/Library/Caches/black/<version>/cache.<line-length>.<file-mode>.pickle

- Linux: /home/<username>/.cache/black/<version>/cache.<line-length>.<file-mode>.pickle

file-mode is an int flag that determines whether the file was formatted as 3.6+ only, as .pyi, and whether string normalization was omitted.

To override the location of these files on all systems, set the environment variable BLACK_CACHE_DIR to the preferred location. Alternatively on macOS and Linux, set XDG_CACHE_HOME to your preferred location. For example, if you want to put the cache in the directory you’re running _Black_ from, set BLACK_CACHE_DIR=.cache/black. _Black_ will then write the above files to .cache/black. Note that BLACK_CACHE_DIR will take precedence over XDG_CACHE_HOME if both are set.

## .gitignore

If –exclude is not set, _Black_ will automatically ignore files and directories in .gitignore file(s), if present.

If you want _Black_ to continue using .gitignore while also configuring the exclusion rules, please use –extend-exclude.