python - How to make Mypy deal with subclasses in functions as expected

Question

I have the following code:

from typing import Callable

MyCallable = Callable[[object], int]
MyCallableSubclass = Callable[['MyObject'], int]

def get_id(obj: object) -> int:
    return id(obj)

def get_id_subclass(obj: 'MyObject') -> int:
    return id(obj)

def run_mycallable_function_on_object(obj: object, func: MyCallable) -> int:
    return func(obj)

class MyObject(object):
    '''Object that is a direct subclass of `object`'''
    pass

my_object = MyObject()

# works just fine
run_mycallable_function_on_object(my_object, get_id)

# Does not work (it runs, but Mypy raises the following error:)
# Argument 2 to "run_mycallable_function_on_object" has incompatible type "Callable[[MyObject], int]"; expected "Callable[[object], int]"
run_mycallable_function_on_object(my_object, get_id_subclass)

Since MyObject inherits from object, why doesn't MyCallableSubclass work in every place that MyCallable does?

I've read a bit about the Liskov substitution principle, and also consulted the Mypy docs about covariance and contravariance. However, even in the docs themselves, they give a very similar example where they say

Callable is an example of type that behaves contravariant in types of arguments, namely Callable[[Employee], int] is a subtype of Callable[[Manager], int].

So then why is using Callable[[MyObject], int] instead of Callable[[object], int] throwing an error in Mypy?

Overall I have two questions:

1. Why is this happening?
2. How do I fix it?

Answer 1

As I was writing this question, I realized the answer to my problem, so I figured I'd still ask the question and answer it to save people some time with similar questions later.

What's going on?

Notice that last example from the Mypy docs:

Callable is an example of type that behaves contravariant in types of arguments, namely Callable[[Employee], int] is a subtype of Callable[[Manager], int].

Here, Manager subclasses from Employee. That is, if something is expecting a function that can take in managers, it's alright if the function it gets overgeneralizes and can take in any employee, because it will definitely take in managers.

However, in our case, MyObject subclasses from object. So, if something is expecting a function that can take in objects, then it's not okay if the function it gets overspecifies and can only take in MyObjects.

Why? Imagine a class called NotMyObject that inherits from object, but doesn't inherit from MyObject. If a function should be able to take any object, it should be able to take in both NotMyObjects and MyObjects. However, the specific function can only take in MyObjects, so it won't work for this case.

How can I fix it?

Mypy is correct. You need to have the more specific function (MyCallableSubclass) as the type, otherwise either your code could have bugs, or you are typing incorrectly.