flake8-staged-diff

Run flake8 but report the findings only on staged files (identified with git diff --staged) This tool is primarily meant to be integrated through pre-commit but it also offers a CLI.

How it works?

This tool runs first git diff -U0 --staged -- and identify the files and lines that were modified, it then runs flake8 on the entire files and simply filters out all the findings that are not coming from the modified code.

If no files is staged in git, it will return immediately

Pre-commit integration

An example .pre-commit-config.yaml:

repos:
  - repo: https://github.com/bagerard/flake8-staged-diff
    rev: ''  # Use the sha / tag you want to point at
    hooks:
      - id: flake8-staged-diff

NOTE: This will only affect local usage of pre-commit, typically through git commit. When it runs for instance in a Github pipeline through pre-commit run -a, there will be no staged files and the tool will simply pass.

CLI Usage

Interface is the same as flake8, e.g.

flake8-staged-diff file1.py file2.py --select=E501

Rationale

This tool allows to introduce some flake8 rules in large/legacy codebases only on the code that is updated or inserted.

We use this at work for enforcing type annotations on modified code using the following config

  - repo: https://github.com/bagerard/flake8-staged-diff
    rev: ''  # Use the sha / tag you want to point at
    hooks:
      - id: flake8-staged-diff
        args:
          - "--select=AN"
        additional_dependencies:
          - flake8-annotations

Consideration

Use this consciously, not every flake8's rule is a good candidate for this tool, in fact sometimes a modified line can introduce errors in other part of the file but since those part of the file aren't in the diff, they wouldn't appear through this tool. Make sure to review pre-commit / Flake8's maintainer comment made here.