Python typing issue for child classes

Question

This question is to clarify my doubts related to python typing

from typing import Union

class ParentClass:
    parent_prop = 1

class ChildA(ParentClass):
    child_a_prop = 2

class ChildB(ParentClass):
    child_b_prop = 3

def method_body(val) -> ParentClass:
    if val:
        return ChildA()
    else:
        return ChildB()

def another_method() -> ChildA:
    return method_body(True)

print(another_method().child_a_prop)

In the above piece of code, the linting tool I used is printing error as below

error: Incompatible return value type (got "ParentClass", expected "ChildA")

(where I do method_body(True))

I have also set the method_body return type as Union[ChildA, ChildB]. This will result error: Incompatible return value type (got "Union[ChildA, ChildB]", expected "ChildA")

I am looking for a better way to do this. If anyone knows the solution, your help will be very much appreciated.

Answer 1

mypy does not do runtime analysis so it cannot guess that calling method_body with argument True will always result in a ChildA object. So the error it produces does make sense.

You have to guide mypy in some way to tell him that you know what you are doing and that another_method indeed produces a ChildA object when called with argument True. One is to use a cast:

from typing import cast
def another_method() -> ChildA:
    return cast(ChildA, method_body(True))

Another one is to add an assertion:

def another_method() -> ChildA:
    result = method_body(True)
    assert isinstance(result, ChildA)
    return result

The difference between the two is that the cast does not have any runtime implication. You can think of it as a comment put here to guide mypy in its checks, but the cast function only returns its second parameter, ie, here is the body of cast:

def cast(typ, val):
    return val

Whereas the assert can naturally raise an AssertionError error (not in that case obviously, but in general).