Why inheritance in python require the parent class to inherit object explicitly?

Question

Below is two versions of my code:

Non-working one

class A:
    def __init__(self):
        print "I am A "

class B:
    def __init__(self):
        print "I am B "

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

c = C()

This raises exception:

    super(C, self).__init__()  
TypeError: must be type, not classobj  

Working version

class A(object):
    def __init__(self):
         print "I am A "

class B:
    def __init__(self):
         print "I am B "

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

c = C()

If one of the parent class has inherited object explicitly , there is no exception and things works as desired. Any explanation, why?

With the above working one , it prints "I am A" but not "I am B" which means it initializes only Class A and not Class B. HOw to initialize multiple parent classes in children class?

Answer 1

That's because super only works with new-style classes (that inherit from object ). In Python 2, classes where object is nowhere in the inheritance hierarchy (like your first example) are called old-style classes, and should never be used anywhere.

It's an old historical artifact from when Python first got OO added to it, and the developers got it wrong. Among other things, new-style classes allow for the use of super , the descriptor protocol (properties), and makes several fixes to the way multiple inheritance is handled. super is one of them, but the most important is the way method resolution order is computed for diamond inheritance cases (A inherits from B and C which both inherit from D). Read more about it here: http://python-history.blogspot.fr/2010/06/method-resolution-order.html

Note that Python 3 ditched old-style classes entirely, and thus in Python 3 all classes are new-style classes and inherit from object, regardless of whether or not they do it explicitly.

Answer 2

super() only works for new-style classes(Any class which inherits from object).So You couldn't pass class object to super .

There are two typical use cases for super. In a class hierarchy with single inheritance, super can be used to refer to parent classes without naming them explicitly, thus making the code more maintainable. This use closely parallels the use of super in other programming languages.

The second use case is to support cooperative multiple inheritance in a dynamic execution environment. This use case is unique to Python and is not found in statically compiled languages or languages that only support single inheritance. This makes it possible to implement “diamond diagrams” where multiple base classes implement the same method. Good design dictates that this method have the same calling signature in every case (because the order of calls is determined at runtime, because that order adapts to changes in the class hierarchy, and because that order can include sibling classes that are unknown prior to runtime).

a typical superclass call looks like this:

class C(B):
    def method(self, arg):
        super(C, self).method(arg)