--extend-exclude not working when in pre-commit

[jsh9]

Describe the bug

When Black is used as a pre-commit hook, the --extend-exclude option has no effect.

To Reproduce

• Take a piece of Python code that you know will be formatted by Black
• Add the path of this file to --extend-exclude
• Run Black directly in the command line (like black /folder/that/contains/this/file)
• Run Black in pre-commit

Expected behavior

When Black is run directly in the command line, that file is not formatted.

When Black is run via pre-commit, that file should also not be formatted.

Actual behavior

When Black is run directly in the command line, that file is not formatted.

When Black is run via pre-commit, that file is formatted.

Environment

• Black's version: the main branch
• OS and Python version: Python 3.9, macOS

Root cause

I did some debugging found the root cause: when Black is run directly in the command line, this branch is entered:

black/src/black/__init__.py

Line 676 in cf4cc29

elif p.is_dir():

But when Black is run via pre-commit, this branch is entered because p.is_file() is True:

black/src/black/__init__.py

Line 646 in cf4cc29

if is_stdin or p.is_file():

In the if is_stdin or p.is_file() branch, there's no checking of extend_exclude or exclude patterns, only force_exclude. That's why --extend-exclude has no effect.

Proposed solution

Given the root cause above, the solution should be straightforward.

[hauntsaninja]

I think you're looking for --force-exclude https://github.com/psf/black/blob/main/docs/usage_and_configuration/the_basics.md#--force-exclude
You can also use the pre-commit exclude options.

See #395 and #438

[jsh9]

I would argue that the documentation around exclude, extend-exclude, and force-exclude is very confusing.

Does force-exclude replace exclude? Or does it add on top of exclude?

And if users have to use force-exclude in pre-commit and force-exclude replaces exclude, does it mean that we now have to look into Black's code base to find out what the default exclude pattern is?

The code logic may be fine, but I would appreciate additional clarity in the documentation.