# 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

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

```{program-output} black-primer –help

```