> I don't get this argument. Isn't this the case for all other code as well? Most code has not been formally verified to work 100% correct. So you assume always

> I don't get this argument. Isn't this the case for all other code as well? Most code has not been formally verified to work 100% correct. So you assume always all code is wrong? This doesn't make sense. It isn't that you have to assume the code is wrong (you always have to assume that code is wrong). It's that even though you write type hints and they pass mypy, you still have to assume that the code has type errors. That is, mypy doesn't rigorously check that the program's types match its annotations. It only approximately checks that. In (some) other languages, the type checker is 100% sound, so if your program passes type checking, you can be completely sure that the types all match. Opportunities for the code to be wrong are thus limited to mismatches between the types and the desired behaviour.