Comment by andenacitelli

You can also use something like stubgen to generate function definition signatures for dependencies for mypy to validate, then make your own changes to those files with better types if you wish.

I don’t think it’s very scalable, and having the library itself or a stubs package come with types is the only “good”-feeling route, but you at least have a somewhat decent path to still getting it decent without any intervention on the library’s part. It may even be sufficient, if (like in most situations) you only use a few functions from a library (which may in turn call others, but you only care about the ones your code directly touches), and therefore only need to type those ones.