Polymorphism and Type Hints in Python

Consider the following case:

class Base:
    ...

class Sub(Base):
    ...

def get_base_instance(*args) -> Base:
    ...

def do_something_with_sub(instance: Sub):
    ...

Let’s say I’m calling get_base_instance in a context where I kow it will return a Sub instance – maybe based on what args I’m passing. Now I want to pass the returned instance to do_something_with_sub:

sub_instance = get_base_instance(*args)
do_something_with_sub(sub_instance)

The problem is that my IDE complains about passing a Base instance to a method that only accepts a Sub instance.

I think I remember from other programming languages that I would just cast the returned instance to Sub. How do I solve the problem in Python? Conditionally throw an exception based on the return type, or is there a better way?

Answer

I think you were on the right track when you thought about it in terms of casting. We could use cast from typing to stop the IDE complaining. For example:

from typing import cast


class Base:
    pass


class Sub(Base):
    pass


def get_base_instance(*args) -> Base:
    return Sub()


def do_something_with_sub(instance: Sub):
    print(instance)


sub_instance = cast(Sub, get_base_instance())
do_something_with_sub(sub_instance)