Python library maintainers should prioritize type-checking tests over source code

Summary

This article addresses the growing challenge for Python library maintainers who feel overwhelmed by the increasing number of type checkers (Mypy, Pyrefly, Pyright, ty, Zuban, and others). The author argues that the common practice of running type checkers on source code while leaving tests untyped is backwards. Instead, maintainers should prioritize running as many type checkers as possible on their test suite, and at least one on their source code. The key insight is that type-checking tests provides more value for catching integration issues and ensuring library correctness.