Flake8 Black Plugin

Warnings

• gotcha Black's formatting for slices (e.g., `foo[1:]`) includes a space before the colon, which conflicts with `pycodestyle`'s `E203` error (`whitespace before ':'`). This will cause `flake8` to fail unless `E203` is explicitly ignored in your Flake8 configuration.
  Fix: Add `extend-ignore = E203` to your `[flake8]` section in `.flake8`, `setup.cfg`, or `tox.ini`.
• gotcha Black formats line breaks before binary operators, which aligns with PEP 8 as of April 2016. However, `flake8` has a disabled-by-default warning `W503` (`line break before binary operator`) that conflicts with this. Enabling `W503` will lead to false positives.
  Fix: Ensure `W503` is not enabled in your `flake8` configuration. You may consider using `W504` (`line break after binary operator`) instead if you need a similar check that is compatible with Black. If Black produces empty classes as `class Foo: ...`, it can trigger `E701`. You might need to ignore `E701` as well.
• breaking Earlier versions of `flake8-black` (before v0.1.2, which implicitly required `black>=19.3b0`) could fail with errors like `__call__() got an unexpected keyword argument 'target_versions'` if used with incompatible `black` versions due to `black` API changes.
  Fix: Upgrade `flake8-black` to the latest version and ensure `black` is at version `19.3b0` or newer. It is recommended to pin `black`'s version in your project to avoid unexpected formatting changes from future `black` updates.
• gotcha `flake8-black` does not automatically read or respect `black`'s `include` or `exclude` settings from `pyproject.toml`. If you have files or directories that `black` ignores, you must configure `flake8` to ignore them separately using `exclude` or `per-file-ignores` in your `flake8` configuration.
  Fix: Duplicate `black`'s `include` and `exclude` patterns in your `flake8` configuration file (`.flake8`, `setup.cfg`, or `tox.ini`).
• gotcha Flake8 itself does not currently use `pyproject.toml` for its configuration. All `flake8` related settings, including those for `flake8-black` or ignoring specific codes, must be placed in a `.flake8`, `setup.cfg`, or `tox.ini` file.
  Fix: Ensure `flake8` configurations are in one of the supported dedicated `flake8` configuration files.
• gotcha The `BLK100` error code from `flake8-black` signifies that `black` would make changes to the file. It does not reformat the file; it only reports that reformatting is needed. If you encounter `BLK100`, it means the file is not formatted according to `black`'s rules.
  Fix: Run `black <file_or_directory>` to reformat the code. Avoid using `# noqa: BLK100` and instead use `black`'s native pragmas like `# fmt: off`, `# fmt: on`, or `# fmt: skip` to disable formatting for specific sections or lines if absolutely necessary.

Install

• pip install flake8-black Install with pip

Quickstart

Install flake8-black along with flake8 and black. Create a .flake8 configuration file in your project root to align Flake8's settings with Black's, particularly the `max-line-length` to 88 and ignoring conflicting style checks like `E203` (whitespace before ':'), `W503` (line break before binary operator), and `E701` (multiple statements on one line, for empty classes). Then, simply run `flake8` as usual.

# my_module.py

def my_function(  # This line is intentionally long to trigger Black's reformatting
    argument_one: str,
    argument_two: int,
) -> None:
    print(f"Hello {argument_one}, your number is {argument_two}.")


# .flake8 (configuration file in project root)
# [flake8]
# max-line-length = 88
# extend-ignore = E203, W503, E701

# To run flake8 with flake8-black:
# flake8 my_module.py