[mic47]

> Statically-typed languages can guarantee to get this right 100% of the time.

No they don't. They usually provide escape hatches for things typesystem does not cover, so there are cases where typechecker will just trust that you know what you are doing (even if you don't).

But more importantly, you don't need 100% to be useful. For aid in IDE, high precision (with somewhat lacking recall) is good enough. Of course, for refactoring, higher recall, the better (but you could substitute lacking recall with tests, which is suboptimal, but viable).

But it's interesting question on what python/mypy (python typechecker) can actually do. The answer here is it depends on configuration. Mypy with default configuration typechecks only typed code (i.e. functions which have type annotations) so you get guarantees only there. But you can configure it to be more and more strict (checking untyped defs, not allowing untyped code, and more), which increases guarantees you get (and it also increases the number of valid programs that it rejects). You can get in python into really strictly typed code, but you can also hit the wall if you need libraries that does not provide proper type hints (unless you write type hints by yourself).