mypy cannot see through iterator dereference?

[thejohnfreeman]

Let me start with the example (Python 3.6, mypy 0.590):

set().union(*(x for x in [{1}, {2}, {3}]))

error: Argument 1 to "union" of "set" has incompatible type "*Iterator[Set[int]]"; expected "Iterable[<nothing>]"

In this case, I think the Set[int] should suffice for the Iterable[<nothing>], but mypy cannot see through the iterator dereference. I'm not entirely sure if this is a bug in mypy, or my understanding of its type system, which is why the subject is written as a question. I don't have immediate access to a more recent version for testing, so I don't know if this has already been fixed; I tried searching for it.

[refi64]

I believe this is because mypy can't tell the type of the set() result, and it confuses it?

[ethanhs]

Yes, it seems mypy is confused by the empty set constructor. It is strange to me that we don't require an annotation, but I suppose that is due to the failed inference. As a work around, you can do this:

from typing import Set
a: Set[Set[int]] = set()
a = a.union(x for x in [{1}, {2}, {3}])

which is a bit weird, but works.

[JelleZijlstra]

set.union(*(x for x in ({1}, {2}, {3}))) works too.

[thejohnfreeman]

Interestingly, set.union(*iterable_of_sets) works, but set.union(*iterable_of_lists) does not, but set().union(*iterable_of_lists) does. In my case, set.union(...) works because my sets come from dict.keys(), and so I'll happily switch to it as a workaround, but this may still be a bug.

[AndrewUshakov]

set().union(*iterable_of_lists)

Few days ago I discovered discussed mypy error for such code. In this case, as you already mentioned, set().union can't be directly replaced by set.union but variant below works (without mypy warnings):

set.union(*(set(x) for x in iterable_of_lists))

I remember, that stopped using it after code profiling. Variant with set().union was more than 2 times faster.

[tmr232]

The issue with set.union(*iterable) is that it fails if the iterable is empty, while set().union(*iterable) works. So it is not a drop-in replacement.