Released on the Python Package Index (PyPI) Released on Conda Testing with TravisCI PyPI downloads Code style: black


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.

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 (and notes)
BLK100 Black would make changes.
BLK9## Internal error (various, listed below):
BLK900 Failed to load file: ...
BLK901 Invalid input.
BLK998 Could not access flake8 line length setting (no longer used).
BLK999 Unexpected exception.

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.


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


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:

# 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

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 a pyproject.toml file is found, the plugin will look at the following black settings:

  • target_version
  • skip_string_normalization
  • line_length

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.

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

Version Release date Changes
v0.1.0 2019-06-03
  • Uses main black settings from pyproject.toml, contribution from Alex.
  • WARNING: Now ignores flake8 max-line-length setting.
v0.0.4 2019-03-15
  • Supports black 19.3b0 which changed a function call.
v0.0.3 2019-02-21
  • Bug fix when W292 no newline at end of file applies, contribution from Sapphire Becker.
v0.0.2 2019-02-15
  • Document syntax error behaviour (no BLK error reported).
v0.0.1 2019-01-10
  • Initial public release.
  • Passes flake8 max-line-length setting to black.


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.

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