I wanted to migrate one of my Python packages from mypy to ty and pyrefly. I handed this task over to Claude, and at the end I asked it to write out some guidance on how to perform it most efficiently. So what follows is AI-generated 'slop'.
This guide is not about using fastkml. It documents how fastkml itself was migrated from mypy to Astral's ty and Meta's pyrefly, so the same playbook can be replayed on other codebases with less trial and error. Keep it here because the next migration (human- or agent-driven) should start from findings, not from zero.
Running two checkers instead of one is deliberate, not incidental. ty and pyrefly disagree with each other and with mypy often enough that running only one gives a false sense of completeness. Budget for both, and expect them to catch different subsets of the same bugs.
TL;DR
Get a raw error-count baseline for both tools before touching any source. Categorize by error kind, not by file.
