Mypy infers types of NamedTuple subclass as 'nothing' when using isinstance.

[toshihikoyanase]

I encountered an issue that mypy failed to infer a type of a variable. This issue occurs when isinstance() is applied to a subclass of NamedTuple.

Environment:

• Python 3.6.5
• mypy: 0.630

Code

from typing import NamedTuple


class Base:
    pass


class BaseTuple(NamedTuple):
    value: float


class MyTuple(BaseTuple, Base):
    pass


def f(o: Base) -> float:
    if isinstance(o, MyTuple):
        return o.value + 10
    return 0


t = MyTuple(1)
print(f(t))

Python 3.6.5 successfully executes this code and returns 11 as expected. But mypy 0.630 fails to infer the type of o after checking its type using isinstance as follows:

sample.py:18: error: <nothing> has no attribute "value"

Observations:

• Current master head (commit 5751f42) also has this issue.
• mypy 0.620 successfully infer the type and does not show this error message.
• This issue seems to occur from 3fb16a2.
• mypy 0.630 successfully infer the type if BaseTuple is not a subclass of NamedTuple as follows:

class BaseTuple:

    def __init__(self, value: float) -> None:
        self.value = value

[ilevkivskyi]

Thanks for reporting! We are sorry for this regression. I will try fixing this shortly.

[toshihikoyanase]

This fix solved actual problems in my project. Thank you for your swift action!