It's a pretty nice best-of-both-worlds arrangement. The type information is there, but the program still runs without it (unless one is doing something really fancy, since it does actually make a runtime construct that can be introspected; some ORMs use the static type data to figure out database-to-object bindings). So you can go without types for prototyping, and then when you're happy with your prototype you can let mypy beat you up until the types are sound. There is a small nonzero cost to using the types at runtime (since they do create metadata that doesn't get dropped like in most languages with a static compilation step, like C++ or TypeScript).

I can name an absolute handful of languages I've used that have that flexibility. Common LISP comes to mind. But in general you get one or the other option.