Fixit 2: Meta’s Next-Generation Auto-Fixing Linter

Let’s talk about my newest open source project, built as part of my effort to improve the linting ecosystem at Meta:

This year, we have been building a new linter, Fixit 2, designed from the ground up to make developers more efficient and capable, both in open source projects and the diverse landscape of our internal monorepo. At Meta, we are using Fixit 2 with a few early adopters, and plan to roll it out to the rest of our monorepo soon. But any developer can use it to perform auto-fixing more efficiently and make faster improvements to their own codebases.

Something I don’t get to talk about enough is just how much I truly appreciate the ability to spend my time at Meta solving problems with open source, and that I can share my work with the rest of the world. Fixit is the third open source project that I’ve had the privilege of releasing at Meta, and I believe Fixit makes it easier than ever before to build new lint rules for Python, a powerful tool for any Python team.

Writing a new lint rule can be done with less than a dozen lines of code, and test cases are defined inline. You can even place it right next to the code that it will be linting:

# teambread/rules/
import fixit
import libcst

class HollywoodName(fixit.LintRule):
    VALID = [...] # no lint errors here
    INVALID = [...] # bad code samples here
    def visit_SimpleString(self, node: libcst.SimpleString):
        if node.value in ('"Paul"', "'Paul'"):
  , "It's underbaked!")

Unlike other new linters on the market, Fixit is uniquely focused on reducing the barriers to writing high quality, auto-fixing lint rules in pure Python, without needing to learn Rust or any other language. We believe that having targeted lint rules with auto-fixes is key to improving developer productivity even more than having the fastest possible linter:

When running Fixit 2 with auto-fixing lint rules, any code that triggers the lint rule is an opportunity to get an automatic replacement, improving the codebase with less effort from the developer. Applied more broadly, Fixit 2 can even be used as a tool to enact sweeping codemods against a large codebase while leaving a lint rule in place to handle any matching code in the future.

Fixit 2 is already available on PyPI. You can pip install fixit and start using it with zero configuration.

We have a roadmap with plans for future improvements and features, and a rich set of documentation and user guides to help you get started with Fixit 2 in your own projects or repositories.