Comment by rtpg

In code where you really want to have these guarantees you turn on errors lke "no implicit any" in mypy and tighten the restrictions on the files you care about.

You still have the "garbage in/garbage out" problem on the boundaries but at the very least you can improve confidence. And if you're hardcore... turn that on all over, turn off explicit Any, write wrappers around all of your untyped dependencies etc etc. You can get what you want, just might be a lot of work