Python object as an optional argument of a class instructor

Question

When creating a new instance (say A) of a class, I want an instance (say B) of another class to be an optional argument of the constructor. If B is given as an argument, then I want the constructor of A to specify that B belongs to A. If not given as an argument, I want my constructor to create a NEW instance (say C) of the same class as where B belong to and specify that this new instance belongs to A.

More specifically, I'm working on an arrival process for a hospital. Arrivals happen according to a stochastic process. I want it to be possible to specify whether an arrival event is regarding an already existing customer (in this case, I should give it as an argument) or that it is going to be a NEW customer, in which case my arrival constructor should create a new instance of customer.

Answer 1

class A:
    def __init__(self, b=None):
        if b is None:
            self.b = B()
        else:
            self.b = b