Iterate on ItemsView and KeysView

[kaste]

• IMO all dunder operators herein construct proper Sets
• __sub__ aka - aka difference return the Left not a union (this also matches the implementation of AbstractSet
• __and__ aka & aka intersection also returns Left. This is also consistent with AbstractSet and Set

Related python/mypy#7236

[srittau]

Sorry for the late reply and thank you for the PR. Remarks below.

[srittau]

In reference to #3184: Let's make this consistent for the annotation of set for now. When we decide to make set() more strict, we can change it later.

[kaste]

I just applied the suggestions you made. But just wait a minute: I used Iterable[T] probably because lazy, but I would have used Iterable[Any] (or maybe just Iterable) in my own code. Is there a difference? I never used Something[object], I sometimes use an unbound T, mypy doesn't complain here, and I thought it will substitute something like Any in such cases. But here in this codebase, set always references AbstractSet[object] in such cases, but Iterable[Any] if Iterable is used. 😕

[srittau]

I think an unbound type var is basically equivalent to Any, but less obvious. The difference between object and Any for argument types is largely academic: object implies that literally anything can used here, while Any says that the expected type can't be expressed adequately with the current type system. For example, str() could be annotated as str(o: object) -> str, since anything can be converted to a string. (It's a class in reality, though.)

[kaste]

Thanks for explaining.