Gauging changes

A lot of the time, your change will affect formatting and/or performance. Quantifying these changes is hard, so we have tooling to help make it easier.

It’s recommended you evaluate the quantifiable changes your Black formatting modification causes before submitting a PR. Think about if the change seems disruptive enough to cause frustration to projects that are already “black formatted”.


black-primer is a tool built for CI (and humans) to have Black --check a number of Git accessible projects in parallel. (configured in primer.json) (A PR will be accepted to add Mercurial support.)

Run flow

  • Ensure we have a black + git in PATH

  • Load projects from primer.json

  • Run projects in parallel with --worker workers (defaults to CPU count / 2)

    • Checkout projects

    • Run black and record result

    • Clean up repository checkout (can optionally be disabled via --keep)

  • Display results summary to screen

  • Default to cleaning up --work-dir (which defaults to tempfile schemantics)

  • Return

    • 0 for successful run

    • < 0 for environment / internal error

    • > 0 for each project with an error

Speed up runs 🏎

If you’re running locally yourself to test black on lots of code try:

  • Using -k / --keep + -w / --work-dir so you don’t have to re-checkout the repo each run

CLI arguments

Usage: black-primer [OPTIONS]

  primer - prime projects for blackening... 🏴

  -c, --config PATH      JSON config file path  [default: /home/docs/checkouts
  --debug                Turn on debug logging  [default: False]
  -k, --keep             Keep workdir + repos post run  [default: False]
  -L, --long-checkouts   Pull big projects to test  [default: False]
  --no-diff              Disable showing source file changes in black output
                         [default: False]
  --projects TEXT        Comma separated list of projects to run  [default: ST
  -R, --rebase           Rebase project if already checked out  [default:
  -w, --workdir PATH     Directory path for repo checkouts  [default:
  -W, --workers INTEGER  Number of parallel worker coroutines  [default: 2]
  -h, --help             Show this message and exit.