Is it mandatory to call superclass init inside the class init?

[wavic]

It's not mandatory. But if you want the A.__init__ method to handle its own initialization and B.__init__() just something bit more, you wave to call it.

And the proper way is to call super():

class B(A):
    def __init__(self):
        super.__init__() # instead of A.__init(self)

When you use multiple inheritances don't calling super() could cause some issues.

In [1]: class A:
   ...:     def __init__(self):
   ...:         print('A')
   ...:

In [2]: class B(A):
   ...:     def __init__(self):
   ...:         A.__init__(self)
   ...:         print('B')
   ...:

In [3]: class C(A):
   ...:     def __init__(self):
   ...:         A.__init__(self)
   ...:         print('C')
   ...:

In [4]: class D(B, C):
   ...:     def __init__(self):
   ...:         B.__init__(self)
   ...:         C.__init__(self)
   ...:         print('D')
   ...:

In [5]: obj = D()
A
B
A
C

As you can see A is called twise.