Can I implement a generic descriptor in Python in a way it will support/respect/understand inheritance hierarchy of his owners?
It should be more clear in the code:
from typing import (
Generic, Optional, TYPE_CHECKING,
Type, TypeVar, Union, overload,
)
T = TypeVar("T", bound="A") # noqa
class Descr(Generic[T]):
@overload
def __get__(self: "Descr[T]", instance: None, owner: Type[T]) -> "Descr[T]": ...
@overload
def __get__(self: "Descr[T]", instance: T, owner: Type[T]) -> T: ...
def __get__(self: "Descr[T]", instance: Optional[T], owner: Type[T]) -> Union["Descr[T]", T]:
if instance is None:
return self
return instance
class A:
attr: int = 123
descr = Descr[T]() # I want to bind T here, but don't know how
class B(A):
new_attr: int = 123
qwerty: str = "qwe"
if __name__ == "__main__":
a = A()
if TYPE_CHECKING:
reveal_type(a.descr) # mypy guess it is T? but I want A*
print("a.attr =", a.descr.attr) # mypy error: T? has no attribute "attr"
# no runtime error
b = B()
if TYPE_CHECKING:
reveal_type(b.descr) # mypy said it's T? but I want B*
print("b.new_attr =", b.descr.new_attr) # mypy error: T? has no attribute "new_attr"
# no runtime error
print("b.qwerty =", b.descr.qwerty) # mypy error: T? has no attribute "qwerty"
# (no runtime error)
gist - almost the same code snippet on gist
from typing: How to bind owner class to generic descriptor?
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