I have a somewhat tricky python typing question. I have a function (generator) that iterates over a list of objects, and then over a particular attribute of each of these objects like this:

T = TypeVar('T')
S = TypeVar('S')

def iter_attribute(outer: Iterable[T], attribute: str) -> Iterable[S]:
    for item in outer:
        yield from getattr(item, attribute)

How would I properly convey the fact that T should have an iterable attribute called attribute that yields S objects? By the way, mypy is totally happy with this and doesn’t catch this:

class A:
    a: str
    def __init__(self, a: str):
        self.a = a

a: str
for a in iter_attribute([A('a'), A('b'), A('c')], 'b'):
    print(a)

How would I correctly type this?