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.

why should the class B care if the customer is new or existing? if it represents a customer that should be sufficient, and B can have an attribute indicating whether or not it is a new customer — gold_cy
– gold_cy, Commented Jun 8, 2019 at 12:14
so have some attribute on B that indicates if it's new and you can have A check for that with a method, I think you're making this more complicated than it needs to be — gold_cy
– gold_cy, Commented Jun 8, 2019 at 12:18
I still believe my solution is viable. let me post the code snippet and let me know what you think. If it's not what you meant, i'll just delete it — DSC
– DSC, Commented Jun 8, 2019 at 12:33

DSC · Accepted Answer · 2019-06-08 12:33:52Z

1

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

answered Jun 8, 2019 at 12:33

DSC

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3 Comments

user8426627 Over a year ago

doesn't he want also check if its known or not?

DSC Over a year ago

If no argument is passed to A, it's not known. Otherwise it is.

Rutger Mauritz Over a year ago

@user8426627, no this is perfectly fine.

user8426627 · Accepted Answer · 2019-06-08 12:41:20Z

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.

it is just usage of dict and setdefault:

customers = {}

for co in all_customers:

known_customer = customers.setdefault(hash(co), customer(co))

or if you do want to have it as constructor:

class arrival:

known_cos = {}# quasi static

def __init__(self, co)

  self.customer = known_cos.setdefault(hash(co) , customer(co))

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