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).

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.

If you are using pre-commit configure it to call black and/or flake8 directly - you do not need flake8-black at all.

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.

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.

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

$ pip install flake8-black

Alternatively, if you are using the Anaconda packaging system, the following command will install the plugin with its dependencies:

$ conda install -c conda-forge 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

Python package management

We covered using pip or conda by hand above. If you are using a PyPI based Python dependency system like pipenv or poetry, you may run into complications because at the time of writing all the black releases to PyPI have been tagged as pre-releases (beta code). PEP440 Handling of pre-releases could be more explicit here.

For pipenv, flake8-black v0.2.0 onwards should just work.

For poetry, include this in your pyproject.toml configuration file:

[tool.poetry.dev-dependencies]
...
black = { version = "*", allow-prereleases = true }
...

In either case, for large projects you should consider pinning the exact version of black you want to use as their updates do sometimes introduce changes which would show up as new BLK100 violations via flake8.

Configuration

We assume you are familiar with flake8 configuration and black configuration.

We recommend using the following settings in your flake8 configuration, for example in your .flake8, setup.cfg, or tox.ini file:

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

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.

Separately pyproject.toml is used for black configuration - if this file is found, the plugin will look at the following black settings:

• target_version
• skip_string_normalization
• line_length

You can specify a particular path for the pyproject.toml file (e.g. global development settings) using --black-config FILENAME at the command line, or using black-config = FILENAME in your flake8 configuration file.

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. Or, exlude the entire file by name (see below).

Ignoring files

The plugin does NOT currently consider the black settings include and exclude, so if you have certain Python files which you do not use with black and have told it to ignore, you will also need to tell flake8 to ignore them (e.g. using exclude or per-file-ignores).

Version History

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

The PyPI upload should trigger an automated pull request updating the flake8-black conda-forge recipe.