Does this online Ruff tool send my code to a server?

No. It loads Ruff's official WebAssembly build from a CDN and then runs the linter and formatter entirely in your browser. Your source code is never uploaded — all analysis happens locally on your machine.

Can Ruff automatically fix the issues it finds?

Many rules are auto-fixable, and this tool tags those diagnostics as 'fixable'. In your own project you would apply them with 'ruff check --fix'. Use Format mode here to instantly reformat code the way 'ruff format' does, with your chosen line length and quote style.

Code Module

Ruff Linter & Formatter Online

Run Ruff online — the fast Rust linter & formatter that replaces Flake8, isort and Black. Free, no install, runs in your browser.

Why use this tool?

Use this when you want to lint or auto-format Python without installing anything. Ruff is the fast, modern replacement for Flake8, isort, pyupgrade, and Black, and here it runs entirely in your browser via WebAssembly — paste code, pick your rules or format settings, and get results instantly.

Rules:Line length:

import os, sys
import json

def add( x,y ):
    unused = 42
    result=x+y
    return result

print( add(1,2) )

Lint results will appear here.

Powered by Ruff 0.15.22 (WebAssembly) — the extremely fast Rust linter & formatter that replaces Flake8, isort, pyupgrade and Black. Everything runs locally in your browser. Need AST, tokens and a live editor? Try the Ruff Playground.

Tool facts

Supported Python: Python 3.7 – 3.14
Processing: Runs entirely in your browser (Ruff compiled to WASM)
Input limit: Limited only by your device
Last reviewed: 2026-08-24

What it can't do

•Execute your code — it is a static linter and formatter only.
•Replace a full type checker for deep type inference across modules.

About Ruff Linter & Formatter

Ruff has become the de-facto standard for Python linting and formatting because it is written in Rust and runs 10–100× faster than the tools it replaces. A single tool covers what previously required Flake8 (and dozens of its plugins), isort, pyupgrade, autoflake, and the Black formatter.

This online version runs the official Ruff WebAssembly build directly in your browser — there is no server, no upload, and no installation. Linting reports each issue with its rule code (for example F401 for an unused import or E701 for multiple statements on one line), its exact line and column, and whether Ruff can auto-fix it.

Switch to Format mode to reformat your code to a consistent style with configurable line length and quote preference, matching what ruff format would produce in your editor or CI. Choose a rule preset — from Ruff's default set to the full ALL ruleset — to control how strict the linting is.

For a deeper, interactive experience with a live editor, editable raw config, shareable links and AST / token / formatter-IR views, use the separate Ruff Playground. This tool stays focused on quick, one-click linting and formatting.

Deciding whether Ruff can replace Pylint in a full project? Read our tested Ruff vs Pylint comparison: the same 14 lint categories, default and expanded configurations, automatic-fix result, fresh-process timings, and the semantic checks Ruff missed.

For the bigger picture, read our guide to Python static code analysis — what linters catch, what they miss, and where type checking fits.

Frequently Asked Questions

Ruff is an extremely fast Python linter and formatter written in Rust. It consolidates Flake8 (plus its plugin ecosystem), isort, pyupgrade, and Black into one tool that runs tens to hundreds of times faster, so you get the same checks and formatting with far less configuration and waiting.

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