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Python formatter

Format Python in the Black style with Ruff’s formatter, compiled to WebAssembly and running in your browser. Choose line length, quotes and indentation; syntax errors are shown with their line.

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Common mistakes

  • Pasting from an interactive session

    Lines starting with >>> or In [1]: aren’t Python. Remove the prompts and the printed output first.

  • Mixing tabs and spaces

    Python rejects indentation that mixes the two inconsistently, so the formatter reports it as an error. Re-indent the block, then format.

  • Being surprised by the magic trailing comma

    A comma after the last item keeps a call, list or dict split one item per line. Remove the comma if you want it joined onto one line.

  • Expecting a linter

    Unused imports, undefined names and bugs pass through untouched. Formatting only changes layout.

How it works

  1. 1

    Paste Python

    A function, a script or a module. Open or drop a .py file; nothing is uploaded.

  2. 2

    Choose the options

    “Default” is 4 spaces, as PEP 8 recommends. Set the line length (80, 100 or 120; Black’s own default is 88) and single or double quotes.

  3. 3

    Read any error

    Code that doesn’t parse isn’t formatted: you get the error and a button that jumps to its line.

Questions

Is this the same as Black?
Ruff’s formatter is built as a drop-in replacement for Black and gives the same output on almost all code. The few differences are documented by the Ruff project and rarely show up in everyday code.
Does it sort imports or fix lint errors?
No. It only formats. Sorting imports and fixing lint findings are separate Ruff commands (ruff check --fix) that don’t run here.
Why is a line still longer than the limit?
Like Black, Ruff doesn’t split long strings or comments, and a single long name can’t be broken. Those lines stay over the limit.
Which Python syntax is supported?
Current Python 3: type hints, f-strings, async and await, decorators, dataclasses, match statements and the walrus operator.