On the other hand, this could be used for some nice "duck type checking". Rather than the classic interface pattern, where the passed object must be a subclass

On the other hand, this could be used for some nice "duck type checking". Rather than the classic interface pattern, where the passed object must be a subclass of a certain type, these annotations could be used inversely to that. Something like: duckclass foo: def x(): pass def y(a: int) -> str: pass n: int def bar(it: foo) -> boolean: ... Where other classes don't need to explicitly inherit from foo, but rather, a checker can raise a warning where you are passing something that doesn't have an integer attribute (or getter function) called n, and the x and y functions. This allows some nice analysis, while still allowing good duck typing. There are ways to do this without special keyworks too... that was just a quick first example. (maybe something like:) class foo: def x(): pass def y(a: int) -> str: pass n: int def bar(it: duck_as(foo)) ->str: pass and so on.