# Linting, formatting, and type checking for Python


## Prerequisites

Complete [Develop your app](/guides/python/lint-format-typing/develop/). This topic requires a local Python
installation because the tools and Git hooks introduced here run on your
host. If you don't want to install Python locally, skip this topic. The same
checks run in CI in the [next topic](/guides/python/lint-format-typing/configure-github-actions/).

## Overview

Linting, formatting, and type checking are automated ways to catch bugs,
enforce style, and spot type errors before code runs. Running them on every
commit, in CI, and in your editor catches problems early when they're cheap
to fix.

In this section, you'll configure three tools for your Python application.
Ruff handles linting and formatting in a single fast pass. Pyright statically
checks your code for type errors. Pre-commit hooks run both of these
automatically before each Git commit so problems are caught locally before
they're committed.

## Linting and formatting with Ruff

Ruff is an extremely fast Python linter and formatter written in Rust. It replaces multiple tools like flake8, isort, and black with a single unified tool.

Create a `pyproject.toml` file in your `python-docker-example` directory:

**`python-docker-example/`**

`pyproject.toml` (new):

```toml
# Configuration for code-quality tools.
# - [tool.ruff]: linting and formatting (https://docs.astral.sh/ruff/)
# - [tool.pyright]: static type checking (https://microsoft.github.io/pyright/)

[tool.ruff]
target-version = "py312"

[tool.ruff.lint]
select = [
    "E",  # pycodestyle errors
    "W",  # pycodestyle warnings
    "F",  # pyflakes
    "I",  # isort
    "B",  # flake8-bugbear
    "C4",  # flake8-comprehensions
    "UP",  # pyupgrade
    "ARG001", # unused arguments in functions
]
ignore = [
    "E501",  # line too long, handled by black
    "B008",  # do not perform function calls in argument defaults
    "W191",  # indentation contains tabs
    "B904",  # Allow raising exceptions without from e, for HTTPException
]
```



Install Ruff:

```console
$ pip install ruff
```

If you're using a virtual environment, make sure it is activated so the `ruff`
command is available.

Run these commands to check and format your code:

```console
# Check for errors
$ ruff check .

# Automatically fix fixable errors
$ ruff check --fix .

# Format code
$ ruff format .
```

## Type checking with Pyright

Pyright is a fast static type checker for Python that works well with modern Python features.

Update `pyproject.toml` to add the Pyright configuration at the bottom.

**`python-docker-example/`**

`pyproject.toml` (modified):

```toml
# Configuration for code-quality tools.
# - [tool.ruff]: linting and formatting (https://docs.astral.sh/ruff/)
# - [tool.pyright]: static type checking (https://microsoft.github.io/pyright/)

[tool.ruff]
target-version = "py312"

[tool.ruff.lint]
select = [
    "E",  # pycodestyle errors
    "W",  # pycodestyle warnings
    "F",  # pyflakes
    "I",  # isort
    "B",  # flake8-bugbear
    "C4",  # flake8-comprehensions
    "UP",  # pyupgrade
    "ARG001", # unused arguments in functions
]
ignore = [
    "E501",  # line too long, handled by black
    "B008",  # do not perform function calls in argument defaults
    "W191",  # indentation contains tabs
    "B904",  # Allow raising exceptions without from e, for HTTPException
]

[tool.pyright]
typeCheckingMode = "strict"
pythonVersion = "3.12"
exclude = [".venv"]
```



Install Pyright and run it:

```console
$ pip install pyright
$ pyright
```

## Setting up pre-commit hooks

Pre-commit hooks run checks automatically before each commit on your local
machine. Create a `.pre-commit-config.yaml` file in your `python-docker-example`
directory to set up Ruff hooks:

**`python-docker-example/`**

`.pre-commit-config.yaml` (new):

```yaml
# Pre-commit hook configuration. Runs Ruff (lint + format) on every
# `git commit`. See https://pre-commit.com/

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.15.15
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format
```



To install and use:

```console
$ pip install pre-commit
$ pre-commit install
$ git commit -m "Test commit"  # Automatically runs checks
```

## Summary

In this section, you learned how to:

- Configure and use Ruff for linting and formatting
- Set up Pyright for static type checking
- Automate checks with pre-commit hooks

These tools help maintain code quality and catch errors early in development.

Related information:

- [Ruff documentation](https://docs.astral.sh/ruff/)
- [Pyright documentation](https://microsoft.github.io/pyright/)
- [pre-commit framework](https://pre-commit.com/)

## Next steps

- [Configure GitHub Actions](/guides/python/lint-format-typing/configure-github-actions/) to run these checks automatically
- Customize linting rules to match your team's style preferences
- Explore advanced type checking features