Cast Python class to parent class

I have class A. I need to store an additional value on class A so I extend it, creating class B which includes a my_variable attribute. I have a load of code that uses class B, but at the end of that code, I need to return a class A object.
How can I do this?

>Solution :

For all intents and purposes, a class B object is a class A object:

class A:
    pass
class B(A):
    pass
b = B()
print(isinstance(b, A)) # True

It will even pass type annotations this way. There is no need to cast the object, because it already is an instance of the parent class.