adkosm / flake8-black

flake8 plugin to run black for checking Python coding style

flake8-black

Introduction

This is an MIT licensed flake8 plugin for validating Python code style with the command line code formatting tool black. It is available to install from the Python Package Index (PyPI):

• https://pypi.python.org/pypi/flake8-black

Black, "The Uncompromising Code Formatter", is normally run to edit your Python code in place to match their coding style, a strict subset of the PEP 8 style guide.

The point of this plugin is to be able to run black --check ... from within the flake8 plugin ecosystem. You might use this via a git pre-commit hook, or as part of your continuous integration testing.

Flake8 Validation codes

Early versions of flake8 assumed a single character prefix for the validation codes, which became problematic with collisions in the plugin ecosystem. Since v3.0, flake8 has supported longer prefixes, therefore this plugin uses BLK as its prefix.

Code	Description
BLK100	Black would make changes.
BLK997	Conflicting configurations.
BLK9##	Internal error (various).

Note that if your Python code has a syntax error, black --check ... would report this as an error. Likewise flake8 ... will by default report the syntax error, but importantly it does not seem to then call the plugins, so you will not get an additional BLK error.

Installation

Python 3.6 or later is required to run black, so that is recommended, but black can be used on Python code written for older versions of Python.

Install flake8-black using pip, which should install flake8 and black as well if not already present:

$ pip install flake8-black

The new validator should be automatically included when using flake8 which may now report additional validation codes starting with BLK (as defined above). For example:

$ flake8 example.py

You can request only the BLK codes be shown using:

$ flake8 --select BLK example.py

Configuration

We recommend using the following settings in your flake8 configuration, for example in your .flake8 file:

[flake8]
# Recommend matching the black default line length of 88,
# rather than the flake8 default of 79:
max-line-length = 88
extend-ignore =
    # See https://github.com/PyCQA/pycodestyle/issues/373
    E203,

In order not to trigger flake8's E501 line too long errors, the plugin passes the flake8 maximum line length when it calls black, equivalent to doing black -l 88 --check *.py at the command line.

Note currently pycodestyle gives false positives on the spaces black uses for slices, which flake8 reports as E203: whitespace before ':'. Until pyflakes issue 373 is fixed, and flake8 is updated, we suggest disabling this style check.

If you are using custom value of maximum line length parameter, check that black configuration (pyproject.toml) and flake8 configuration (.flake8) use the same value. Otherwise, you will get BLK997 error.

Ignoring validation codes

Using the flake8 no-quality-assurance pragma comment is not recommended (e.g. adding # noqa: BLK100 to the first line black would change). Instead use the black pragma comments # fmt: off at the start, and # fmt: on at the end, of any region of your code which should not be changed.

Version History

Version	Release date	Changes
v0.0.1	2019-01-10	Initial public release.
v0.0.2	2019-02-15	Document syntax error behaviour (no BLK error reported).
v0.0.3	2019-02-21	Bug fix when W292 no newline at end of file applies, contribution from Sapphire Becker.
v0.0.4	2019-03-15	Supports black 19.3b0 which changed a function call.

Developers

This plugin is on GitHub at https://github.com/peterjc/flake8-black

To make a new release once tested locally and on TravisCI:

$ git tag vX.Y.Z
$ python setup.py sdist --formats=gztar
$ twine upload dist/flake8-black-X.Y.Z.tar.gz
$ git push origin master --tags