ruff/docs/tutorial.md

Tutorial

This tutorial will walk you through the process of integrating Ruff's linter and formatter into your project. For a more detailed overview, see Configuring Ruff.

Getting Started

To start, we'll install Ruff through PyPI (or with your preferred package manager):

pip install ruff

Let's then assume that our project structure looks like:

numbers
  ├── __init__.py
  └── numbers.py

...where numbers.py contains the following code:

from typing import Iterable

import os


def sum_even_numbers(numbers: Iterable[int]) -> int:
    """Given an iterable of integers, return the sum of all even numbers in the iterable."""
    return sum(
        num for num in numbers
        if num % 2 == 0
    )

We can run the Ruff linter over our project via ruff check:

❯ ruff check .
numbers/numbers.py:3:8: F401 [*] `os` imported but unused
Found 1 error.
[*] 1 fixable with the `--fix` option.

Ruff identified an unused import, which is a common error in Python code. Ruff considers this a "fixable" error, so we can resolve the issue automatically by running ruff check --fix:

❯ ruff check --fix .
Found 1 error (1 fixed, 0 remaining).

Running git diff shows the following:

--- a/numbers/numbers.py
+++ b/numbers/numbers.py
@@ -1,7 +1,5 @@
 from typing import Iterable

-import os
-

def sum_even_numbers(numbers: Iterable[int]) -> int:
    """Given an iterable of integers, return the sum of all even numbers in the iterable."""
    return sum(
        num for num in numbers
        if num % 2 == 0
    )

Now that our project is passing ruff check, we can run the Ruff formatter via ruff format:

❯ ruff format .
1 file reformatted

Running git diff shows that the sum call was reformatted to fit within the default 88-character line length limit:

--- a/numbers.py
+++ b/numbers.py
@@ -3,7 +3,4 @@ from typing import Iterable

 def sum_even_numbers(numbers: Iterable[int]) -> int:
     """Given an iterable of integers, return the sum of all even numbers in the iterable."""
-    return sum(
-        num for num in numbers
-        if num % 2 == 0
-    )
+    return sum(num for num in numbers if num % 2 == 0)

Thus far, we've been using Ruff's default configuration. Let's take a look at how we can customize Ruff's behavior.

Configuration

To determine the appropriate settings for each Python file, Ruff looks for the first pyproject.toml, ruff.toml, or .ruff.toml file in the file's directory or any parent directory.

To configure Ruff, let's create a pyproject.toml file in our project's root directory:

[tool.ruff]
# Set the maximum line length to 79 characters.
line-length = 79

[tool.ruff.linter]
# Add the `line-too-long` rule to the enforced rule set. By default, Ruff omits rules that
# overlap with the use of a formatter, like Black, but we can override this behavior by
# explicitly adding the rule.
extend-select = ["E501"]

Running Ruff again, we see that it now enforces a maximum line width, with a limit of 79 characters:

❯ ruff check .
numbers/numbers.py:5:80: E501 Line too long (90 > 79 characters)
Found 1 error.

For a full enumeration of the supported settings, see Settings. For our project specifically, we'll want to make note of the minimum supported Python version:

[project]
# Support Python 3.10+.
requires-python = ">=3.10"

[tool.ruff]
# Set the maximum line length to 79 characters.
line-length = 79

[tool.ruff.linter]
# Add the `line-too-long` rule to the enforced rule set.
extend-select = ["E501"]

Rule Selection

Ruff supports over 700 lint rules split across over 50 built-in plugins, but determining the right set of rules will depend on your project's needs: some rules may be too strict, some are framework-specific, and so on.

By default, Ruff enables Flake8's F rules, along with a subset of the E rules, omitting any stylistic rules that overlap with the use of a formatter, like ruff format or Black.

If you're introducing a linter for the first time, the default rule set is a great place to start: it's narrow and focused while catching a wide variety of common errors (like unused imports) with zero configuration.

If you're migrating to Ruff from another linter, you can enable rules that are equivalent to those enforced in your previous configuration. For example, if we want to enforce the pyupgrade rules, we can set our pyproject.toml to the following:

[project]
requires-python = ">=3.10"

[tool.ruff.linter]
extend-select = [
  "UP",  # pyupgrade
]

If we run Ruff again, we'll see that it now enforces the pyupgrade rules. In particular, Ruff flags the use of the deprecated typing.Iterable instead of collections.abc.Iterable:

❯ ruff check .
numbers/numbers.py:1:1: UP035 [*] Import from `collections.abc` instead: `Iterable`
Found 1 error.
[*] 1 fixable with the `--fix` option.

Over time, we may choose to enforce additional rules. For example, we may want to enforce that all functions have docstrings:

[project]
requires-python = ">=3.10"

[tool.ruff.linter]
extend-select = [
  "UP",  # pyupgrade
  "D",   # pydocstyle
]

[tool.ruff.linter.pydocstyle]
convention = "google"

If we run Ruff again, we'll see that it now enforces the pydocstyle rules:

❯ ruff check .
numbers/__init__.py:1:1: D104 Missing docstring in public package
numbers/numbers.py:1:1: UP035 [*] Import from `collections.abc` instead: `Iterable`
numbers/numbers.py:1:1: D100 Missing docstring in public module
Found 3 errors.
[*] 1 fixable with the `--fix` option.

Ignoring Errors

Any lint rule can be ignored by adding a # noqa comment to the line in question. For example, let's ignore the UP035 rule for the Iterable import:

from typing import Iterable  # noqa: UP035


def sum_even_numbers(numbers: Iterable[int]) -> int:
    """Given an iterable of integers, return the sum of all even numbers in the iterable."""
    return sum(num for num in numbers if num % 2 == 0)

Running ruff check again, we'll see that it no longer flags the Iterable import:

❯ ruff check .
numbers/__init__.py:1:1: D104 Missing docstring in public package
numbers/numbers.py:1:1: D100 Missing docstring in public module
Found 3 errors.

If we want to ignore a rule for an entire file, we can add a # ruff: noqa comment to the top of the file:

# ruff: noqa: UP035
from typing import Iterable


def sum_even_numbers(numbers: Iterable[int]) -> int:
    """Given an iterable of integers, return the sum of all even numbers in the iterable."""
    return sum(num for num in numbers if num % 2 == 0)

For more in-depth instructions on ignoring errors, please see Error suppression.

Adding Rules

When enabling a new rule on an existing codebase, you may want to ignore all existing violations of that rule and instead focus on enforcing it going forward.

Ruff enables this workflow via the --add-noqa flag, which will adds a # noqa directive to each line based on its existing violations. We can combine --add-noqa with the --select command-line flag to add # noqa directives to all existing UP035 violations:

❯ ruff check --select UP035 --add-noqa .
Added 1 noqa directive.

Running git diff shows the following:

diff --git a/tutorial/src/main.py b/tutorial/src/main.py
index b9291c5ca..b9f15b8c1 100644
--- a/numbers/numbers.py
+++ b/numbers/numbers.py
@@ -1,4 +1,4 @@
-from typing import Iterable
+from typing import Iterable  # noqa: UP035


 def sum_even_numbers(numbers: Iterable[int]) -> int:

Integrations

This tutorial has focused on Ruff's command-line interface, but Ruff can also be used as a pre-commit hook via ruff-pre-commit:

# Run the Ruff linter.
- repo: https://github.com/astral-sh/ruff-pre-commit
  # Ruff version.
  rev: v0.1.1
  hooks:
    - id: ruff
# Run the Ruff formatter.
- repo: https://github.com/astral-sh/ruff-pre-commit
  # Ruff version.
  rev: v0.0.291
  hooks:
    - id: ruff-format

Ruff can also be used as a VS Code extension or alongside any other editor through the Ruff LSP.

For more, see Integrations.