applying sum() to List[Decimal] does not result in Decimal, but Union[Decimal, int]

[varunabgupta]

Using mypy 0.700 (no additional flags) on the following method:

def add_array(array):
    # type: (List[Decimal]) -> Decimal
    return sum(array)

results in:

error: Incompatible return value type (got "Union[Decimal, int]", expected "Decimal")

Even if I add a check for an empty list:

def add_array(array):
    # type: (List[Decimal]) -> Decimal
    if len(array) == 0:
        return Decimal('0')
    return sum(array)

the same exact error is raised.

Is this intended?

[emmatyping]

Technically, if you pass an empty iterable like [], then sum will return 0 (as an int!). So the Union is correct.

In addition, mypy currently doesn't (nor likely will it) have support for checking the length of a list like in your second example. See also #7626 for a related issue.

Also, mypy 0.740 was just released, so you may want to update :)