Why are parent constructors not called when instantiating a class? [duplicate]

Question

class A:
    def __init__(self):
        print 'A'


class B(A):
    def __init__(self):
        print 'B'


b = B()
B

In C++, I would have expected to see A B output, but in Python I am getting only B. I know that I can do super(B, self).__init__() to achieve the same in Python, but as this is apparently not the default (or is it - I am new to the syntax as well), I am worried that the paradigms for instatinating objects are completely different.

So what are objects in Python, what is their relation with classes and what is the standard way to initialize all data in all parent classes in Python?

Answer 1

Python rarely does anything automatically. As you say, if you want to invoke the superclass __init__, then you need to do it yourself, usually by calling super:

class B(A):
    def __init__(self):
        print 'B'
        super(B, self).__init__()

The point to note is that instance attributes, like everything else in Python, are dynamic. __init__ is not the constructor, that's __new__ which you rarely need to meddle with. The object is fully constructed by the time __init__ is called, but since instance attributes are dynamic they are usually added by that method, which is only special in that it's called first once the object is created.

You can of course create instance attributes in any other method, or even from outside the class itself by simply doing something like myBobj.foo = 'bar'.