CODEWITHSUNDEEP
pythonclassinheritancederived
M Wade
M Wade

Reputation: 187

Sharing a base class between two derived classes in Python

In this situation class A is a parent of classes B and C. The goal is to share state between the instances b and c. It's not about adding or overriding functions, but about the state value a.

 class A():
     def __init__(self):
         self.a ='a'
         print('@a')
 class B(A):
     def __init__(self):
         A.__init__(self)
         self.x = 'b'
 class C(A):
     def __init__(self):
         A.__init__(self)
         self.x = 'c'

 b = B()
 c = C()
 print(b.a, b.x)
 print(c.a, c.x)

The output is

 @a
 @a
 a b
 a c

I understand why this is happening; __init__() in A is being called twice. I could put a test in so it only sets the initial value once, but that does not seem to be a very elegant solution. Additionally it only allows one instance of A to be shared. If I want another instance of A to be shared between two other instance of B and C it can't be done. The same goes for making a global inside the module and getting rid of class A entirely. Is there a good Pythonic way to solve this provlem?

Upvotes: 1

Views: 2036

Answers (1)

user2357112
user2357112

Reputation: 281958

Agreeing with abarnert: this doesn't make any sense. This isn't what inheritance means.

Inheritance means that every B instance is an A instance, and every C instance is an A instance. A B instance doesn't have a separate A instance, and it certainly can't share with a C instance.

If you want B and C objects to have an A instance, and to have the same A instance, then you need composition:

class A(object):
    def __init__(self):
        self.state = whatever()
        self.other_state = more_stuff()

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

class C(object):
    def __init__(self, a):
        self.a = a

a = A()
b = B(a)
c = C(a)

Now b and c are both sharing a single instance of A, and changes to that shared instance will be visible through both objects.

Upvotes: 1

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