What does 'super' do in Python? - difference between super().__init__() and explicit superclass __init__()

The benefits of super() in single-inheritance are minimal -- mostly, you don't have to hard-code the name of the base class into every method that uses its parent methods.

However, it's almost impossible to use multiple-inheritance without super(). This includes common idioms like mixins, interfaces, abstract classes, etc. This extends to code that later extends yours. If somebody later wanted to write a class that extended Child and a mixin, their code would not work properly.

Doesn't all of this assume that the base class is a new-style class?

class A:
    def __init__(self):
        print("A.__init__()")

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

Will not work in Python 2. class A must be new-style, i.e: class A(object)

Super() in a nutshell

• Every Python instance has a class that created it.
• Every class in Python has a chain of ancestor classes.
• A method using super() delegates work to the next ancestor in the chain for the instance's class.

Example

This small example covers all the interesting cases:

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

class B(A):
    def m(self):
        print('B start')
        super().m()
        print('B end')
        
class C(A):
    def m(self):
        print('C start')
        super().m()
        print('C end')

class D(B, C):
    def m(self):
        print('D start')
        super().m()
        print('D end')

The exact order of calls is determined by the instance the method is called from:

>>> a = A()
>>> b = B()
>>> c = C()
>>> d = D()

For instance a, there is no super call:

>>> a.m()
A

For instance b, the ancestor chain is B -> A -> object:

>>> type(b).__mro__   
(<class '__main__.B'>, <class '__main__.A'>, <class 'object'>)

>>> b.m()
B start
A
B end

For instance c, the ancestor chain is C -> A -> object:

>>> type(c).__mro__   
(<class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

>>> b.m()
C start
A
C end

For instance d, the ancestor chain is more interesting D -> B -> C -> A -> object (mro stands for method resolution order) :

>>> type(d).__mro__
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

>>> d.m()
D start
B start
C start
A
C end
B end
D end

More information

Having answered the question of "What does super do in Python?", the next question is how to use it effectively. See this step-by-step tutorial or this 45 minute video.

Many great answers, but for visual learners: Firstly lets explore with arguments to super, and then without.

Imagine theres an instance jack created from the class Jack, who has the inheritance chain as shown in green in the picture. Calling:

super(Jack, jack).method(...)

will use the MRO (Method Resolution Order) of jack (its inheritance tree in a certain order), and will start searching from Jack. Why can one provide a parent class? Well if we start searching from the instance jack, it would find the instance method, the whole point is to find its parents method.

If one does not supply arguments to super, its like the first argument passed in is the class of self, and the second argument passed in is self. These are auto-calculated for you in Python3.

However say we dont want to use Jack's method, instead of passing in Jack, we could of passed in Jen to start searching upwards for the method from Jen.

It searches one layer at a time (width not depth), e.g. if Adam and Sue both have the required method, the one from Sue will be found first.

If Cain and Sue both had the required method, Cain's method would be called first. This corresponds in code to:

Class Jen(Cain, Sue):

MRO is from left to right.

In the case of multiple inheritance, you normally want to call the initializers of both parents, not just the first. Instead of always using the base class, super() finds the class that is next in Method Resolution Order (MRO), and returns the current object as an instance of that class. For example:

class Base(object):
    def __init__(self):
        print("initializing Base")

class ChildA(Base):
    def __init__(self):
        print("initializing ChildA")
        Base.__init__(self)

class ChildB(Base):
    def __init__(self):
        print("initializing ChildB")
        super().__init__()

class Grandchild(ChildA, ChildB):
    def __init__(self):
        print("initializing Grandchild")
        super().__init__()
        
Grandchild()

results in

initializing Grandchild
initializing ChildA
initializing Base

Replacing Base.__init__(self) with super().__init__() results in

initializing Grandchild
initializing ChildA
initializing ChildB
initializing Base

as desired.

some great answers here, but they do not tackle how to use super() in the case where different classes in the hierarchy have different signatures ... especially in the case of __init__

to answer that part and to be able to effectively use super() i'd suggest reading my answer super() and changing the signature of cooperative methods.

here's just the solution to this scenario:

1. the top-level classes in your hierarchy must inherit from a custom class like SuperObject:
2. if classes can take differing arguments, always pass all arguments you received on to the super function as keyword arguments, and, always accept **kwargs.

class SuperObject:        
    def __init__(self, **kwargs):
        print('SuperObject')
        mro = type(self).__mro__
        assert mro[-1] is object
        if mro[-2] is not SuperObject:
            raise TypeError(
                'all top-level classes in this hierarchy must inherit from SuperObject',
                'the last class in the MRO should be SuperObject',
                f'mro={[cls.__name__ for cls in mro]}'
            )

        # super().__init__ is guaranteed to be object.__init__        
        init = super().__init__
        init()

example usage:

class A(SuperObject):
    def __init__(self, **kwargs):
        print("A")
        super(A, self).__init__(**kwargs)

class B(SuperObject):
    def __init__(self, **kwargs):
        print("B")
        super(B, self).__init__(**kwargs)

class C(A):
    def __init__(self, age, **kwargs):
        print("C",f"age={age}")
        super(C, self).__init__(age=age, **kwargs)

class D(B):
    def __init__(self, name, **kwargs):
        print("D", f"name={name}")
        super(D, self).__init__(name=name, **kwargs)

class E(C,D):
    def __init__(self, name, age, *args, **kwargs):
        print( "E", f"name={name}", f"age={age}")
        super(E, self).__init__(name=name, age=age, *args, **kwargs)

E(name='python', age=28)

output:

E name=python age=28
C age=28
A
D name=python
B
SuperObject

Consider the following code:

class X():
    def __init__(self):
        print("X")

class Y(X):
    def __init__(self):
        # X.__init__(self)
        super(Y, self).__init__()
        print("Y")

class P(X):
    def __init__(self):
        super(P, self).__init__()
        print("P")

class Q(Y, P):
    def __init__(self):
        super(Q, self).__init__()
        print("Q")

Q()

If change constructor of Y to X.__init__, you will get:

X
Y
Q

But using super(Y, self).__init__(), you will get:

X
P
Y
Q

And P or Q may even be involved from another file which you don't know when you writing X and Y. So, basically, you won't know what super(Child, self) will reference to when you are writing class Y(X), even the signature of Y is as simple as Y(X). That's why super could be a better choice.

class Child(SomeBaseClass):
    def __init__(self):
        SomeBaseClass.__init__(self)

This is fairly easy to understand.

class Child(SomeBaseClass):
    def __init__(self):
        super(Child, self).__init__()

Ok, what happens now if you use super(Child,self)?

When a Child instance is created, its MRO(Method Resolution Order) is in the order of (Child, SomeBaseClass, object) based on the inheritance. (assume SomeBaseClass doesn't have other parents except for the default object)

By passing Child, self, super searches in the MRO of the self instance, and return the proxy object next of Child, in this case it's SomeBaseClass, this object then invokes the __init__ method of SomeBaseClass. In other word, if it's super(SomeBaseClass,self), the proxy object that super returns would be object

For multi inheritance, the MRO could contain many classes, so basically super lets you decide where you want to start searching in the MRO.