Using Black To Auto Format Your Python by@davisdavid

Using Black To Auto Format Your Python

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Davis David

Data Scientist | AI Practitioner | Software Developer. Giving talks, teaching, writing.

Writing python code is one thing and writing the code in a good format is another thing. Most of the time especially junior coders/programmers focus on making sure the codes are working and forget to format their codes.

If you write a small program (with 1000 lines of codes) you could get away without formatting your code, but as programs get more and more complex, they get harder and harder to understand and at some point (around 15,000 lines of code), it becomes harder to understand the code that you yourself wrote.

The difference between working on well-formatted code and working on badly formatted code is like the difference between living in a palace and living in a dirty house.

Why formatting your python code is important?

1. Readability

Formatting your code will help in reading code efficiently. It looks more organized and when someone looks at your code, this will also help in giving a good impression of yours.

2.Help in your Coding Interview

Even while giving a coding interview, sometimes the interviewers will take care if you’re giving proper formatting for your code. If you forget to do formatting you might lose your job, just because of this reason.

3.Team support

Format your code becomes more important when you are working in a team where several people are working on the same software project and the code you write must be understood by your teammates otherwise it becomes harder to work together.

4.Easy to spot Bugs

Badly formatted code can make it really, really hard to spot bugs or even to work on a program and is also just really horrible to look at. It’s an offense to your eyes.

Pylint and Flake8

Most Python developers enjoy using Pylint or Flake8 to check their code for errors and style guides.

Pylint is a tool that checks for errors in the python programming language, it tries to enforce a coding standard and looks for code smells. It can also look for certain type errors, it can recommend suggestions about how particular blocks can be refactored and can offer you details about the code’s complexity.

Flake8 is a Python library that wraps PyFlakespycodestyle and Ned Batchelder’s McCabe script. It is a great toolkit for checking your code base against coding style (PEP8), programming errors like “library imported but unused”, “Undefined name” and codes that are not indented.

The problem is that these tools only report the problems they identify in the source code and let the burden to the Python developers to fix them!. But what if we have a tool that can identify and solve the problem at the same time. Black is the right tool for you to identify errors and format your python code at the same time and make you more productive.

Introduction To Black


From the project README:

By using Black, you agree to cede control over minutiae of hand-formatting. In return, Black gives you speed, determinism, and freedom from pycodestyle nagging about formatting. You will save time and mental energy for more important matters.

Black can reformat your entire file in place according to the Black code style. It helps your brain to focus on the problem you want to solve and code solutions, rather than get distracted by code structure and minor stylistic differences.

Install Black

Black can be installed by running 

pip install black
. It requires Python 3.6.0+ to run. Once Black is installed, you will have a new command-line tool called black available to you in your shell, and you’re ready to start!.

To get started right away with sensible defaults, choose the python file you want to format and then write black in the terminal, then the black will format your python file.

Format Single File

Let's look at this simple example, here is my two python functions in my python file called


Then you can use

in the terminal to change the format. After running the black, you will observe the following output.


Then you can open to see formatted python codes.


The Python code is now formatted and it’s more readable.

Format Multiple Files

To format more than one python file, write 

black folder_name/
 in the terminal.


Three python files within the folder named python_with_black have been reformatted.

Checking Files for Formatting

If you don’t want Black to change your file, but you want to know if Black thinks a file should be changed, you can use one of the following commands:

black --check . 
This will check which python file(s) can be formatted in the current folder but doesn’t actually modify the python file(s).


black --check --diff 
This shows what needs to be done to the file but doesn’t modify the file.


Change Number of Characters per Line

Note that Black defaults to 88 characters for its line length, but you can change that using the “-l” or “- -line-length” option.

Example change to 60 characters 

black -l 60 

Black in Jupyter Notebook

For Jupyter notebook users, you can still auto-format your python code with this simple extension called Jupyter Black. This extension reformats/prettifies code in a notebook’s code cell by black.

The Jupyter Black extension provides

  • A toolbar button.
  • A keyboard shortcut for reformatting the current code-cell (default: Ctrl-B).
  • A keyboard shortcut for reformatting whole code-cells (default: Ctrl-Shift-B).

Install Jupyter Black

First make sure you have installed jupyter-contrib-nbextensions and black, then run the following commands.

jupyter nbextension install — user

Then enable the extension by running.

jupyter nbextension enable jupyter-black-master/jupyter-black

Now you can start formatting your python code in each notebook cell. First, select the notebook cell you want to format your python code then click the extension button called Black.


Then Click the Jupyter Black button.


Editor Integration

You can integrate Black with your favorite editors. Currently Black support PyCharm/IntelliJ IDEA, Wing IDE, Vim, Visual Studio Code, Sublime Text 3, Atom/Nuclide, Kakoune, Thonny. Follow the instruction here to integrate Black with your favorite editor.

If you want to learn more about Black, I recommend watching the PyCon 2019 talk by Łukasz Langa.

If you learned something new or enjoyed reading this article, please share it so that others can see it. Till then, see you in the next post!

Also published on Medium.


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