Python's PEP 484 type annotation for Generator Expression

Question

What is the correct type annotation for a function that returns a generator expression?

e.g.:

def foo():
    return (x*x for x in range(10))

I can't figure out if this is -> Iterator[int], -> Iterable[int], -> Generator[int, None, None], or something else.

If there should be one-- and preferably only one --obvious way to do it, then what is the obvious way here?

Answer 1

All three forms mentioned by you in question are listed as valid alternatives in documentation, Generator expression simply creates a generator that only yields.

Quote 1:

A generator can be annotated by the generic type Generator[YieldType, SendType, ReturnType].

Quote 2:

If your generator will only yield values, set the SendType and ReturnType to None

Quote 3:

Alternatively, annotate your generator as having a return type of either Iterable[YieldType] or Iterator[YieldType]: