`sum(List[Decimal]) + sum(List[Decimal])` incompatible type error

[myartsev]

Using mypy-0.770 with no additional flags

from decimal import Decimal

foo = [Decimal("1"), Decimal("2")]
sum(foo)

sum(foo) + sum(foo)

The first sum statement is okay, but the second is an error: Argument 1 to "sum" has incompatible type "List[Decimal]"; expected "Iterable[int]" [arg-type]

[hauntsaninja]

Interesting.

For some context, sum is typed as the following in typeshed.

@overload
def sum(__iterable: Iterable[_T]) -> Union[_T, int]: ...
@overload
def sum(__iterable: Iterable[_T], start: _S) -> Union[_T, _S]: ...

This is how the int gets in there; i.e. how reveal_type(sum(foo)) is Union[decimal.Decimal*, builtins.int]. This is correct, since you could run:

foo.clear()
assert isinstance(sum(foo), int)

It seems that mypy is using the context that the return value of the first sum could be an int to infer the argument type to the second sum.

If you convince it that it's dealing with a Decimal by the time it gets to the second sum(foo) you no longer see an error, e.g. by:

cast(Decimal, sum(foo)) + sum(foo)  # or
Decimal(0) + sum(foo) + sum(foo)  # or
sum(foo) + Decimal(0) + sum(foo)  # but not
sum(foo) + sum(foo) + Decimal(0)

My guess is that mypy is forgetting about __radd__ somewhere.

[hynekcer]

This is currently fixed, probably between versions 0.950 and 0.960

[hauntsaninja]

Hmm, the lack of error here bisects to #12631
@huguesb / @JelleZijlstra is it expected that that PR changes what errors get reported?

[JelleZijlstra]

No, that's not expected. Maybe something changed to overload resolution.

[hauntsaninja]

Are you looking into it?

[JelleZijlstra]

No

[hauntsaninja]

Okay, false alarm. I bisected this wrong. Looks like it's a typeshed change, which is believable https://github.com/python/typeshed/pull/8000/files

Edit: I don't remembr where I got that^ from. Seems to clearly repro without a3abd36