Mypy: How to take a Type as function argument and invoke a class method on that type

balajeerc

The issue I'm facing is best explained with the following example:

from abc import ABC, abstractclassmethod
from typing import Type


class SomeInterface(ABC):
    """Interface definition"""

    @abstractclassmethod
    def say_something(cls, stuff_to_say: str) -> None:
        """Some class method to be implemented"""


class TheImplementation(SomeInterface):
    """Implementation of above interface"""

    @classmethod
    def say_something(cls, stuff_to_say: str) -> None:
        """Implementation of class method in interface"""
        print(stuff_to_say)


def do_something_with_type(input_class_type: Type[SomeInterface]) -> None:
    """Function that takes a class type as argument to call `say_something` on it"""
    input_class_type.say_something("hi")


do_something_with_type(TheImplementation)

Note that the above code is valid Python, which executes and prints the right string "hi".

However, mypy shows the following errors:

tests/annotation_test.py:28: error: Too few arguments for "say_something" of "SomeInterface"
tests/annotation_test.py:28: error: Argument 1 to "say_something" of "SomeInterface" has incompatible type "str"; expected "SomeInterface"

What am I doing wrong here? From reading the documentation I sense that input_class_type argument to do_something_with_type needs to be annotated differently but am not sure how exactly to go about this.

user2357112 supports Monica

It looks like mypy doesn't understand abstractclassmethod. Stacking classmethod and abstractmethod instead should work:

class SomeInterface(ABC):
    @classmethod
    @abstractmethod
    def say_something(cls, stuff_to_say: str) -> None:
        raise NotImplementedError

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