Mypy: NamedTuple subclass with default arguments doesn't type check

[graingert]

from typing import NamedTuple

Unit = str
Factor = str

_Spec = NamedTuple(
        '_Spec',
    [('unit', Unit), ('factor', Factor), ('source', str)],
)


class Spec(_Spec):
    def __new__(cls, unit, factor, source='derived'):
        return super().__new__(cls, unit, factor, source)


def foo() -> Spec:
    return Spec('ham', 'spam')

but this gets:
spec.py: note: In function "foo":
spec.py:18: error: Too few arguments for "Spec"

[elazarg]

I will see if this can be changed. At first glance it seems to be nontrivial.

(Personally I would prefer defining Spec as a namedtuple and use a make_spec function).

[graingert]

@elazarg still fails in the same way

[elazarg]

Perhaps we can improve the lookup in checkmember,py#L443 - look up for __new__ first or otherwise mimic the runtime lookup.

[ilevkivskyi]

(Raising priority to high since this affects some important internal code)

[ilevkivskyi]

This now works correctly on master.