Closed
Description
Mypy normally performs type narrowing for assignment statements when the LHS is a member access (e.g. a.b = x). However, it appears to avoid type narrowing in certain circumstances when b is a descriptor. This special casing leads to incorrect behaviors in some cases. (Look for "!!!" in the sample below.)
from typing import Any, Protocol, Union
class HasName(Protocol):
def __name__(self) -> str:
...
class Animal:
def __name__(self) -> str:
...
class Dog(Animal):
def __name__(self) -> str:
...
def run(self) -> None:
print("Run Spot Run!")
def bark(self) -> None:
print("Ruff!")
class Cat:
def __name__(self) -> str:
...
def run(self) -> None:
print("Run Kitty Run!")
def purr(self) -> None:
print("Purr!")
class AnimalMember:
def __get__(self, instance: Any, owner: Any) -> Animal:
...
def __set__(self, instance: Any, value: HasName) -> None:
...
class Struct:
member = AnimalMember()
def test_descriptor(cat: Cat, dog: Dog, cat_or_dog: Union[Cat, Dog]):
inst = Struct()
inst.member = cat_or_dog
reveal_type(inst.member) # mypy: Revealed type is 'Animal'
inst.member.run() # mypy: "Animal" has no attribute "run"
inst.member.bark() # mypy: "Animal" has no attribute "bark"
inst.member.purr() # mypy: "Animal" has no attribute "purr"
inst.member = cat
reveal_type(inst.member) # mypy: Revealed type is 'Animal'
inst.member.run() # mypy: "Animal" has no attribute "run"
inst.member.bark() # mypy: "Animal" has no attribute "bark"
inst.member.purr() # mypy: "Animal" has no attribute "purr"
inst.member = dog
reveal_type(inst.member) # mypy: Revealed type is 'Dog'
inst.member.run() # mypy: no error
inst.member.bark() # mypy: no error
inst.member.purr() # mypy: "Dog" has no attribute "purr"
inst.member = cat_or_dog
reveal_type(inst.member) # mypy: Revealed type is 'Dog' !!!!
inst.member.run() # mypy: no error
inst.member.bark() # mypy: no error
inst.member.purr() # mypy: "Dog" has no attribute "purr"
inst.member = cat
reveal_type(inst.member) # mypy: Revealed type is 'Dog' !!!!
inst.member.run() # mypy: no error
inst.member.bark() # mypy: no error !!!!
inst.member.purr() # mypy: "Dog" has no attribute "purr" !!!!Refer to this bug report in the pyright repo for details and a discussion of opitons. I'm interested in thoughts on a more robust heuristic that we can use across type checkers to handle this case properly.