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 3.6 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> ...]

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: https://github.com/omnilib/ufmt
  rev: v1.3.2
    - id: ufmt

You can change the rev to any version >= 1.3.0. To pin black and usort, use the additional_dependencies option:

  - id: ufmt
      - black == 22.3.0
      - usort == 1.0.2


µfmt is copyright John 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.