Code formatting and linting¶

Introduction¶

Code formatting guidelines set a standard for the layout of your code, for stylistic elements such as use of line breaks and whitespace. Format doesn’t have any impact on how the code works, but using a consistent style makes your code more readable, and makes it more likely to be reused.

Linting tools check your code for errors such as a missing bracket or line indent. This can save time and frustration because you can catch errors in advance of running the code.

As a project grows and goes through various stages of development it becomes important to maintain code quality. Using a consistent format and linting your code ensures that it is consistent, readable, and easy to debug and maintain.

Set up Python tools¶

There are a variety of Python tools available to use with your Kedro projects. This guide shows you how to use black, flake8, and isort to format and lint your Kedro projects.

black is a PEP 8 compliant opinionated Python code formatter. black can check for styling inconsistencies and reformat your files in place. You can read more in the black documentation.
flake8 is a wrapper around pep8, pyflakes, and mccabe which can lint code and format it with respect to PEP 8, and check the cyclomatic complexity of your code base. You can read more in the flake8 documentation.
isort is a Python library used to reformat code by sorting imports alphabetically and automatically separating them into sections by type. You can read more in the isort documentation.

Install the tools¶

Install black, flake8, and isort by adding the following lines to your project’s src/requirements.txt file:

black # Used for formatting code
flake8 # Used for linting and formatting
isort # Used for formatting code (sorting module imports)

To install all the project-specific dependencies, including the linting tools, navigate to the root directory of the project and run:

pip install -r src/requirements.txt

Alternatively, you can individually install the linting tools using the following shell commands:

pip install black
pip install flake8
pip install isort

Configure `flake8`¶

Store your flake8 configuration in a file named setup.cfg within your project root. The Kedro starters use the following configuration:

[flake8]
max-line-length=88
extend-ignore=E203

Run the tools¶

Use the following commands to run lint checks:

black --check <project_root>
isort --profile black --check <project_root>

You can also have black and isort automatically format your code by omitting the --check flag. Since isort and black both format your imports, adding --profile black to the isort run helps avoid potential conflicts.

Use the following to invoke flake8:

flake8 <project_root>

Automated formatting and linting with `pre-commit` hooks¶

You can automate the process of formatting and linting with pre-commit hooks. These hooks are run before committing your code to your repositories to automatically point out formatting issues, making code reviews easier and less time-consuming.

Install `pre-commit`¶

You can install pre-commit along with other dependencies by including it in the src/requirements.txt file of your Kedro project by adding the following line:

pre-commit

You can also install pre-commit using the following command:

pip install pre-commit

Add `pre-commit` configuration file¶

Create a file named .pre-commit-config.yaml in your Kedro project root directory. You can add entries for the hooks you want to run before each commit. Below is a sample YAML file with entries for black,flake8, and isort:

repos:
  - repo: https://github.com/pycqa/isort
    rev: 5.10.1
    hooks:
      - id: isort
        name: isort (python)
        args: ["--profile", "black"]

  - repo: https://github.com/pycqa/flake8
    rev: ''  # pick a git hash / tag to point to
    hooks:
      - id: flake8

  - repo: https://github.com/psf/black
    rev: 22.8.0
    hooks:
      - id: black
        language_version: python3.9

Install git hook scripts¶

Run the following command to complete installation:

pre-commit install

This enables pre-commit hooks to run automatically every time you execute git commit.