Before I commit my staged Python files, black formats my code and flake8 checks my compliance to PEP8. If everything passes, the commit is made. If not, then I the perform necessary edits and commit again. Less time is spent on code formatting so I can focus more on code logic.

I enjoy doing code reviews. They enable me to learn from other’s code while providing an opportunity to teach what I know. However, there are still some things I wish to improve when facilitating reviews in my open-source projects:

  • Less time commenting on code format, and more time discussing code logic
  • Less hassle spotting format errors (“can you really see that trailing whitespace on Line 76?”)
  • Stop sounding nitpicky (“Please put two blank lines between function definitions”)

If I could automate the processes above and remove the human-in-the-loop, we can focus more on code logic and implementation. Good thing, I learned about Git hooks, specifically pre-commit hooks. It enables you to automatically run a short script before committing. This script can be a checking tool or a formatter. If the script passes, then the commit is made, else, the commit is denied.

In this short post, I’ll describe how I created a pre-commit pipeline in PySwarms using the black code formatter, flake8 checker, and the pre-commit Python framework. The entire pipeline looks like this:

Figure: Pre-commit pipeline with black and flake8 for checking my .py files

I’ll first discuss the pre-commit framework, then add components one-by-one: first is black, and then flake8. I will show the dotfiles present in my project, so feel free to adopt them into your own!

The pre-commit Python framework

We can run shell files all we want to dictate how our pre-commit process goes, but this pre-commit framework written in Python got us covered. It even comes with a set of pre-commit hooks out of the box (batteries included!). To adopt pre-commit into our system, we simply perform the following actions:

  1. Install pre-commit: pip install pre-commit
  2. Add pre-commit to requirements.txt (or requirements-dev.txt)
  3. Define .pre-commit-config.yaml with the hooks you want to include.
  4. Execute pre-commit install to install git hooks in your .git/ directory.

The YAML file configures the sources where the hooks will be taken from. In our case, flake8’s already been included in this framework so we just need to specify its id. On the other hand, we need to define where to source black using few lines of code. Below is a sample .pre-commit-config.yaml file that I use in my project:

-   repo:
    rev: stable
    - id: black
      language_version: python3.6
-   repo:
    rev: v1.2.3
    - id: flake8

In the next section, I will discuss my code formatter (black) and checker (flake8). As usual, I will provide my config files for each component.

Code Formatter: black

The black code formatter in Python is an opinionated tool that formats your code in the best way possible. You can check its design decisions in the repository itself. Some notable formatting decisions, in my opinion:

  • Unlike in PEP8, code length is 88 characters, not 79.
  • Use of double-quotes than single-quotes in strings.
  • If there are many function args, each arg will be wrapped per line.

I’d rather maintain the recommended 79 character length. Good thing, they have an option to do so. I just need to configure my pyproject.toml to line-length=79 and everything is all set. Here’s my .toml file for configuring black:

line-length = 79
include = '\.pyi?$'
exclude = '''
  | \.hg
  | \.mypy_cache
  | \.tox
  | \.venv
  | _build
  | buck-out
  | build
  | dist

If you are not a fan of black, there’s always autopep8— a formatter more faithful to PEP8. Good thing, the pre-commit framework already has a hook on this tool, so there’s no need to source from another repository.

Flake8 checker

Flake8 is a powerful tool that checks our code’s compliance to PEP8. In order for black to work nicely with flake8 (or prevent it from spewing out various errors and warnings), we need to list down some error codes to ignore. You can check my .flake8 configuration below:

ignore = E203, E266, E501, W503, F403, F401
max-line-length = 79
max-complexity = 18
select = B,C,E,F,W,T4,B9


So what we have is a pipeline that safeguards my project against wrongly-formatted code. In my project’s CONTRIBUTING page, I explicitly mentioned to use pre-commits (or run flake8 and black on their code manually) before submitting a Pull Request.

Figure: Pre-commit pipeline with black and flake8 for checking my .py files

Now that we have a pre-commit framework set-up with black and flake8, let’s see it in action! Here we’ll see how black formats a Python file automagically:

Figure: Short demo on pre-commit hooks

Awesome right? Hope you learned something new today!


  • 09-15-2018: This post was featured in Episode #95 of Python Bytes! Thank you Mike and Brian!
  • 06-05-2018: Update pre-commit config filename (by @asottile)