How to Check Python with Ruff Online (ruff check, Tested)

ruff check is Ruff's linter: it parses your Python and flags issues — unused imports (F401), un-sorted imports (I001), deprecated patterns (UP…), likely bugs (B…), and hundreds more — each tagged with a rule code so you can look it up or silence it. It's the counterpart to ruff format (which only restyles code). Ruff is written in Rust, so a check that takes Flake8 seconds finishes in milliseconds.

1. Open the Ruff Playground — it loads the official Ruff WebAssembly build in your browser.
2. Paste or type your Python into the editor. Linting runs live as you type.
3. Read the Diagnostics tab: every issue shows its rule code, line:column, and a [*] marker if Ruff can auto-fix it.
4. Open Settings to edit the config as JSON — the same fields as [tool.ruff] in pyproject.toml (select, ignore, line-length).
5. Switch tabs to also see the Formatted output (ruff format), the AST, Tokens, or the Formatter IR.

That's the whole loop. The check runs in your browser. Locally, the same command is ruff check path/to/file.py after pip install ruff (or uv add ruff).

Here's the file I used — intentionally sloppy, the kind of thing that accumulates in a real codebase:

import os, sys
import json
from typing import List

def load(path):
    f = open(path)
    data = json.load(f)
    names: List[str] = data['names']
    result = []
    for i in range(len(names)):
        result.append(names[i].upper())
    unused = 42
    return result

x=load('data.json')
print( x )

Running ruff check messy.py --select E,F,I,UP,B,SIM gives this — real, unedited output:

messy.py:1:1: E401 [*] Multiple imports on one line
messy.py:1:1: I001 [*] Import block is un-sorted or un-formatted
messy.py:1:8: F401 [*] `os` imported but unused
messy.py:1:12: F401 [*] `sys` imported but unused
messy.py:3:1: UP035 `typing.List` is deprecated, use `list` instead
messy.py:6:9: SIM115 Use a context manager for opening files
messy.py:8:12: UP006 [*] Use `list` instead of `List` for type annotation
messy.py:12:5: F841 Local variable `unused` is assigned to but never used
Found 8 errors.
[*] 5 fixable with the `--fix` option (1 hidden fix can be enabled with the `--unsafe-fixes` option).

Eight issues from sixteen lines. Notice the [*] markers — those five are auto-fixable. The playground shows the same list in its Diagnostics tab, with the [*] rendered as a fixable badge.

Add --fix and Ruff rewrites the safe issues in place. On my file:

Found 8 errors (6 fixed, 2 remaining).

Six gone in one pass. The two left (SIM115, the file opened without a context manager, and the hidden unsafe fix) need a human decision — Ruff won't guess there. The import block collapsed and sorted itself down to just what's used:

import json


def load(path):
    f = open(path)
    data = json.load(f)

ruff check finds problems; ruff format fixes *style*. They're separate commands (and separate tabs in the playground). Real before/after on a badly-spaced snippet:

def add(a,b) :
    return a+b
x = { "k":1,"y":2 }

def add(a, b):
    return a + b


x = {"k": 1, "y": 2}

Same output you'd get from Black on most files — see Ruff format vs Black for where they differ. In the playground, this is the Formatted tab.

Every diagnostic starts with a letter-prefixed code. The prefix tells you which family (and which flake8 plugin it replaces):

You control which families run with select/ignore in the config — E/F (pyflakes + pycodestyle) are the sane default; add I, UP, B, SIM for import sorting, upgrades, bug-bears and simplifications. Edit these live in the playground's Settings panel.

Checking Ruff online is perfect for a quick look — trying a rule, sharing a reproducible snippet, or checking code on a machine where you can't install anything. For day-to-day work you'll want it local (pip install ruff or uv add ruff) and wired into your editor and CI, so ruff check and ruff format --check run on every save and every push. The rules and output are identical; the online playground just skips the install.