How do you cast an instance to a derived class?

Question

I am trying to use a little inheritance in a Python program I am working on. I have a base class, User, which implements all of the functionality of a user. I am adding the co

Answer 1

class A:
 def __init__(self, variable):
    self.a = 10
    self.a_variable = variable

 def do_something(self):
    print("do something A")


class B(A):

 def __init__(self, variable=None):
    super().__init__(variable)
    self.b = 15

 @classmethod
 def from_A(cls, a: A):
    # Create new b_obj
    b_obj = cls()
    # Copy all values of A to B
    # It does not have any problem since they have common template
    for key, value in a.__dict__.items():
        b_obj.__dict__[key] = value
    return b_obj

if __name__ == "__main__":
 a = A(variable="something")
 b = B.from_A(a=a)
 print(a.__dict__)
 print(b.__dict__)
 b.do_something()
 print(type(b))

Result:

{'a': 10, 'a_variable': 'something'}
{'a': 10, 'a_variable': 'something', 'b': 15}
do something A