CODEWITHSUNDEEP
pythonpython-typing
Jonathan Herrera
Jonathan Herrera

Reputation: 6144

Enforcing Type of Attribute of Argument of Custom Type

Working with Python's typing module, I would like to create a custom Type that checks for the type of an attribute of an argument. To be more precise, in my case the argument's type is supposed to be an instance of a certain class, and the attribute is supposed to be of a certain Type.

What I want to achieve might be clearer with a minimal example:

class Wrapper:
    value: typing.Any


def f(obj: WrapperType[int]) -> None:
    assert isinstance(obj, Wrapper) and type(obj.value) is int

How can I create a WrapperType that reflects the behaviour of this assert (in terms of typing)? I was unable to find hints for this in the documentation of the typing module. But I am not sure whether I misunderstood something in there, or whether what I want is just not possible.

Upvotes: 1

Views: 105

Answers (1)

chepner
chepner

Reputation: 531235

Wrapper should inherit from Typing.Generic.

from typing import Generic, TypeVar

T = TypeVar('T')

class Wrapper(Generic[T]):
    value: T


def f(obj: Wrapper[int]) -> None:
    ...

Upvotes: 3

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