[wcdolphin]

Has anyone migrated a large code base between MyPy and Pyright? In particular would love to hear about experiences with Django. Mypy’s performance and the join based type merging are quite limiting. At the same time, the ecosystem support around MyPy seems strong.

[Mehdi2277]

Django is one library that has some apis that are beyond type system and likely difficult to ever fully describe in type system. Some of this it handles with a custom mypy plugin to overrule/extend some type rules. No other python type checker I'm aware of supports plugins so this is one place mypy will have an advantage. Usually when behavior is too dynamic to be described well the stubs will fallback to Any leaving some typing holes for other type checkers.

At same time this only affects a subset of Django behavior and I have used pyright with a Django codebase and it mostly worked well. The more you use very dynamic features of Django the more this matters.

One note for both is you do want to tune configuration settings. Pyright basic is fairly easy to satisfy, pyright strict is too hard for most codebases. Similarly mypy defaults are too lenient, but mypy strict is pretty hard (even mypy doesn't use strict to check itself). I roughly go for strict for both and then remove ~5 hardest rules and call that good enough. Strictest rules pretty much require all of your dependencies to be well typed/stubbed.

[davepeck]

Yes. I moved a very large/mature Django project from use of MyPy to use of Pyright. My experience was that pyright flagged a bunch of new conditions that MyPy seemed to have missed. The cost was that I needed to add (many!) explicit annotations or casts in places the Django MyPy plug-in would have just “known” about the types, for instance in foreign key relations in Models.

In general, we were very happy with the result and never looked back. Correctness and perf were both notably improved. (That pyright is also part of pylance, Microsoft’s proprietary LSP implementation for Python that plays well with VSCode, was an advantage for this particular team.)

This was about a year and a half ago so the experience may differ; both pyright and mypy + its plugins have gotten more capable and mature over time.