Initializing superclasses Python3

Question

I am trying to understand when to initialize a superclass when using inheritance in python. Initially I thought that just by declaring a class inheriting from a super class, ex. class my_class(superclass):, would make available all the superclass's attributes and methods to the subclass. Which makes sense for somebody coming from Java. Then I read that Python forces us to initialize superclasses before we can implement them in our subclass, either by using the superclass.init() or super().init(). Then I came across this piece of code where I am not initializing the parent's class, however Python gave me access to the self.queue attribute from superclass without having initialized the parent class. I read the Python documentation and sometimes I think I know what they mean and some other I dont. Can anyone please explain to me when do we have to initialize superclasses in our subclasses?

class QueueError(IndexError):
pass

class Queue:
    def __init__(self):
       self.queue = []
    def put(self,elem):
       self.queue.insert(0,elem)
    def get(self):
        if len(self.queue) > 0:
            elem = self.queue[-1]
            del self.queue[-1]
            return elem
        else:
            raise QueueError

    class SuperQueue(Queue):
    def isempty(self):
        if not self.queue:
            return True
        else:
            return False

que = SuperQueue()
que.put(1)
que.put("dog")
que.put(False)
for i in range(4):
    if not que.isempty():
        print(que.get())
    else:
        print("Queue empty")

Sven Eberth · Accepted Answer · 2021-05-22 21:22:32Z

2

In general, if you override __init__ in your subclass, you should call the super __init__ method only. This is necessary if you extend the superclass. But if you want to overwrite the whole __init__, you can also omit the super call.

Example:

You have class A that has one attribute value1. And now you need a second attribute, so you subclass A with B and overwrite the __init__ where you call the super class (A), so A can set value1 and in B you can not set value2.
But now you need some other attributes in C, but need the same methods as in A. So you can entirely overwrite __init__ and omit the super call to A.

class A:
    def __init__(self, value1):
        print("Initialize A")
        self.value1 = value1


class B(A):
    def __init__(self, value1, value2):
        super().__init__(value1)
        print("Initialize B")
        self.value2 = value2


class C(A):
    def __init__(self, value3):
        print("Initialize C")
        self.value3 = value3


a = A("foo")
b = B("foo", "bar")
c = C("baz")

print(a.value1)
print(b.value1, b.value2)
print(c.value3)
print(c.value1)

Output

$ python main.py
Initialize A
Initialize A
Initialize B
Initialize C
foo
foo bar
baz
Traceback (most recent call last):
  File "main.py", line 27, in <module>
    print(c.value1)
AttributeError: 'C' object has no attribute 'value1'

You can see C wasn't initialized with value1, because C didn't call A's __init__.

edited May 22, 2021 at 21:22

answered May 22, 2021 at 15:41

Sven Eberth

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1 Comment

BifBully Over a year ago

In other words, we can says that a subclass automatically inherits all of the superclass's traits. But if a subclass tries to access the superclass traits, it will generate an AttributeError exception, only if we override the superclass init. Now I see what super().__init__() is doing, it is basically giving a subclass access to the superclass' traits defined in the superclass init when this one has been overrided in the subclass. Thank you.

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