'"ClassName" expects no type arguments, but 1 given' while inheriting from Iterable

[sametmax]

I have a class looking like this:

from collection import Iterable, Iterator
class IterableWrapper(Iterator, Iterable, BaseWrapper):
    ...

And a function type definition later going:

# type: (...) -> IterableWrapper[tuple[T, T2]]

Mypy complains:

src/ww/wrappers/iterables.py:600: error: "IterableWrapper" expects no type arguments, but 1 given

While:

# type: (...) -> IterableWrapper and # type: (...) -> Iterable[tuple[T, T2]] don't get any warning.

I was expecting that, inheriting from Iterable, I could do IterableWrapper[tuple[T, T2]] like I would do Iterable[tuple[T, T2]] and benefit from code checking on yielded elements.

[gvanrossum]

It looks like you want IterableWrapper to be generic. The way to do that is

T = TypeVar('T')
class IterableWrapper(Iterator[T], Iterable[T], BaseWrapper, Generic[T]):
    ...

Also you shouldn't need Iterable[T] because Iterator already inherits from Iterable.

I am presuming your BaseWrapper is not generic.

[JukkaL]

Note that Iterable means Iterator[Any] instead of Iterator[T].

[sametmax]

Righ. Thanks for this explanation. I think I'm going to write a big tutorial on mypy in the coming months.

[sametmax]

Ooops. Forgot to ask: why do I have to inherit from Iterator[T] AND Generic[T]? Iterator[T] is not a generic already ?

[gvanrossum]

Oh, that's a separate issue. According to PEP 484 you shouldn't need the Generic[T] unless you have multiple type variables and you want to specify their order, but mypy currently always requires the Generic[T]. It's issue #302.