Matt Segal Dev

Never think about Python formatting again

Fri 24 April 2020, by Matthew Segal
Category: Programming

At some point you realise that formatting your Python code is important. You want your code to be readable, but what's the right way to format it? You recognise that it's much harder to read this:

some_things = {"carrots": [1,2 ],
3,3, 3
], "pears": [] }

than it is to read this:

some_things = {
    "carrots": [1, 2],
    "apples": [3, 3, 3],
    "pears": [],

or... wait should it be like this instead? Hmm...

some_things = {
    "carrots": [1, 2],
    "apples":  [3, 3, 3],
    "pears":   [],

nah, nah, wait a sec maybe would be better if we kept in on one line to save space...

some_things = {"carrots": [1, 2], "apples":  [3, 3, 3], "pears": []}

Umm, is that line too long though? We could do this for hours.

Formatting your code is important, but it's easy to get lost in the details. You want your code to look professional, but it can be a time-sink. It's easy to:

This is all just incidental bullshit though. It's a distraction from your real work: laying out brackets one way or another isn't going to make your software run any better (but if the closing bracket isn't on its own new line then I'll gut you like the dog you are!).

Is there a way to avoid this mess? How can we get rid of all this incidental work?

Give black a try

Black is a tool that auto-formats your Python code. You run black over all your .py files and the correct formatting is applied for you. It's like prettier, but for Python instead of JavaScript.

Importantly, Black has minimal configuration. You basically only get to choose the maximum line length that you want, and everything else is decided by the formatter. It's the "uncompromising Python code formatter". This means you don't get to choose what formatting style you use, but it also means you don't need to decide either: once you've adopted Black, you never need to think about Python formatting again. No more config files, no more arguing with your coworkers. Spend your time on more valuable things, like what your code is doing.

Is it safe to just run your whole codebase through this tool? I think so. Black compares the Python abstract syntax tree of the code before and after the changes, just to make sure it didn't change or break anything. In the last few jobs I've worked, I've walked in, made the case for Black (politely), and run it over the whole codebase. It's never caused any issues.

Here's some of the other benefits of Black:

Running Black

You have to install it.

pip install black

Then you run it with a path as an argument

black .

Then it mangles all of your code!

reformatted /home/matt/code/redbubble/
reformatted /home/matt/code/redbubble/
reformatted /home/matt/code/redbubble/
reformatted /home/matt/code/redbubble/
All done! ✨ 🍰 ✨
4 files reformatted, 2 files left unchanged.

You can mess around a little bit with the line length config, or using pyproject.toml, but that's basically it.

If you're running CI and you want to check for correct formatting, you can use

black --check .

It returns exit code 0 if the formatting is correct, and exit code 1 if it's not.

Format on save

Format on save is incredible, it's been a big productivity boost for me. In VSCode you can add the following settings to format on save with black:

  "python.formatting.provider": "black",
  "editor.formatOnSave": true

I don't know about other editors, but I've set this up in PyCharm as well. Once that's done then any save will format the document. Here's an example:


Black is a just formatter, not a linter, so it does not do some linting functions. It will not complain about unused variables, imports and other linty stuff.

It will also not do import sorting like isort. In fact, Black and isort can fight over how imports should be formatted, if you're running both of them. You can resolve it by running isort then black, or vice versa, but it can make CI tests a little awkward.

Finally, it's "in beta", which as far as I can tell just means "you should expect some formatting to change in the future".


Black is awesome, it'll save you time and brain cycles, go forth and use it on all your Python code.

If you have any feedback or questions email me at [email protected]