composition and aggregation in python

Question

I want to know how to implement composition and aggregation in UML terms in python.

If I understood:

1. Aggregation:

class B:
    pass

class A(object):
    def __init__(self):
        self.B = B

2. Composition:

In other languages I saw this implemented as a pointer to B. I guess here self.B is a pointer in python.

class A(object):
    def __init__(self, B):
        self.B = B

Is it right?

Answer 1

Composition and aggregation are specialised form of Association. Whereas Association is a relationship between two classes without any rules.

Composition

In composition, one of the classes is composed of one or more instance of other classes. In other words, one class is container and other class is content and if you delete the container object then all of its contents objects are also deleted.

Now let's see an example of composition in Python 3.5. Class Employee is container and class Salary is content.

class Salary:
    def __init__(self,pay):
        self.pay=pay

    def get_total(self):
       return (self.pay*12)

class Employee:
    def __init__(self,pay,bonus):
        self.pay=pay
        self.bonus=bonus
        self.obj_salary=Salary(self.pay)

    def annual_salary(self):
        return "Total: "  +  str(self.obj_salary.get_total()+self.bonus)


obj_emp=Employee(100,10)
print (obj_emp.annual_salary())

Aggregation

Aggregation is a weak form of composition. If you delete the container object contents objects can live without container object.

Now let's see an example of aggregation in Python 3.5. Again Class Employee is container and class Salary is content.

class Salary:
    def __init__(self,pay):
        self.pay=pay

    def get_total(self):
       return (self.pay*12)

class Employee:
    def __init__(self,pay,bonus):
        self.pay=pay
        self.bonus=bonus

    def annual_salary(self):
        return "Total: "  +  str(self.pay.get_total()+self.bonus)


obj_sal=Salary(100)
obj_emp=Employee(obj_sal,10)
print (obj_emp.annual_salary())