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Safe, atomic formatting with black and µsort

Project description


Safe, atomic formatting with black and µsort

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µfmt is a safe, atomic code formatter for Python built on top of black and µsort:

Black makes code review faster by producing the smallest diffs possible. Blackened code looks the same regardless of the project you’re reading.

μsort is a safe, minimal import sorter. Its primary goal is to make no “dangerous” changes to code, and to make no changes on code style.

µfmt formats files in-memory, first with µsort and then with black, before writing any changes back to disk. This enables a combined, atomic step in CI/CD workflows for checking or formatting files, without any chance of conflict or intermediate changes between the import sorter and the code formatter.


µfmt requires Python 3.8 or newer. You can install it from PyPI:

$ pip install ufmt

If you want to prevent unexpected formatting changes that can break your CI workflow, make sure to pin your transitive dependencies–including black, µsort, and µfmt–to your preferred versions.

If you use requirements.txt, this might look like:



To format one or more files or directories in place:

$ ufmt format <path> [<path> ...]

To validate files are formatted correctly, like for CI workflows:

$ ufmt check <path> [<path> ...]

To validate formatting and generate a diff of necessary changes:

$ ufmt diff <path> [<path> ...]


GitHub Actions

µfmt provides a GitHub Action that can be added to an existing workflow, or as a separate workflow or job, to enforce proper formatting in pull requests:

    runs-on: ubuntu-latest
      - uses: omnilib/ufmt@action-v1
          path: <PATH TO CHECK>
          requirements: requirements-fmt.txt
          python-version: "3.x"

See the user guide for details.

pre-commit hook

µfmt provides a pre-commit hook. To format your diff before every commit, add the following to your .pre-commit-config.yaml file:

  - repo:
    rev: v2.0.0
      - id: ufmt
          - black == 22.6.0
          - usort == 1.0.4

See the user guide for details.

Visual Studio Code

µfmt has an official VS Code extension to use µfmt as a formatter for Python files. Once installed, µfmt can be set as the default formatter with the following settings:

"[python]": {
  "editor.defaultFormatter": "omnilib.ufmt"

µfmt can automatically format when saving with the following settings:

"[python]": {
  "editor.defaultFormatter": "omnilib.ufmt",
  "editor.formatOnSave": true

For more details, or to install the extension, see the Visual Studio Marketplace page:

VS Code extension marketplace Install VS Code extension now


µfmt is copyright Amethyst Reese, and licensed under the MIT license. I am providing code in this repository to you under an open source license. This is my personal repository; the license you receive to my code is from me and not from my employer. See the LICENSE file for details.

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