What is the problem in this multiple inheritance?

What is the problem in this multiple inheritance? - python

class A:
def __init__(self,name):
self.name=name
class B(A):
def __init__(self,name,add):
super().__init__(name)
self.add = add
class C(A):
def __init__(self,name,tel):
super().__init__(name)
self.tel = tel
class D(B,C):
def __init__(self,name,add,tel,company):
super().__init__(name,add)
super().__init__(name,tel)
self.company = company
d = D('Hank','ctm',55514,'google')
enter image description here

A resolution to this multiple inheritance is to cooperatively design the classes, see Raymond's article Python’s super() considered super!:
class A:
def __init__(self, name, **kwargs):
self.name = name
class B(A):
def __init__(self, add, **kwargs):
super().__init__(**kwargs)
self.add = add
class C(A):
def __init__(self, tel, **kwargs):
super().__init__(**kwargs)
self.tel = tel
class D(B, C):
def __init__(self, company, **kwargs):
super().__init__(**kwargs)
self.company = company
d = D(name='Hank', add='ctm', tel=55514, company='google')
As pointed out by others, this will follow the MRO, e.g. D -> B -> C - A.

The error is caused by the call to super().__init__(name) in the class B. Unlike single-inheritance languages such as Java, super() in Python doesn't necessarily give you the superclass; it gives you the next class in the method resolution order. The MRO for class D is this:
>>> D.__mro__
(<class '__main__.D'>,
<class '__main__.B'>,
<class '__main__.C'>,
<class '__main__.A'>,
<class 'object'>)
As you can see, the next class in the MRO after B is C, so the call to super().__init__(name) invokes C.__init__ with a single argument. However, the C.__init__ method expects two arguments, so calling it with just one argument results in the error you've seen.

Related

Can Python subclasses return class of type(self) from method inherited from a parent class?

Suppose I have a parent class Parent and two child classes Child1 and Child2. Suppose Parent contains a method usable by both subclasses. Is it possible for this method to return a new instance of the same class of whatever class calls it, even if the particular subclass calling it is not known when the method is defined in the parent class?
class Parent:
def __init__(self, foo):
pass
def my_method(self):
return self.__class__(foo)
class Child1(Parent):
def __init__(self, foo):
super().__init__(foo)
class Child2(Parent):
def __init__(self, foo):
super().__init__(foo)
parent = Parent('hello')
type(p.my_method())
>>> class<Parent>
child1 = Child1('hey')
type(child1.my_method())
>>> class<Child1>
child2 = Child2('yo')
type(child2.my_method())
>>> class<Child2>
EDIT
I had made a mistake in passing foo without initializing it as an attribute first. The following code achieves the behavior I'm going for, but I'd be interested to know if this could lead to design problems down the road, so feedback on that would be appreciated.
class Parent:
def __init__(self, foo):
self.foo = foo
def my_method(self):
return self.__class__(self.foo)
class Child1(Parent):
def __init__(self, foo):
super().__init__(foo)
class Child2(Parent):
def __init__(self, foo):
super().__init__(foo)
p = Parent(foo='hi')
print(type(p.my_method()))
>>>__main__.Parent
c1 = Child1(foo='hey')
print(type(c1.my_method()))
>>>__main__.Child1
c2 = Child2(foo='hey')
print(type(c2.my_method()))
>>>__main__.Child2

What you have won't work, because you have my_method being an instance method, but you're trying to call it without an instance.
If you make that a class method, it can work, although I personally think this is a bad design choice:
class Parent:
def __init__(self, foo):
pass
#classmethod
def my_method(cls):
return cls(0)
class Child1(Parent):
def __init__(self, foo):
super().__init__(foo)
class Child2(Parent):
def __init__(self, foo):
super().__init__(foo)
print(type(Parent.my_method()))
print(type(Child1.my_method()))
print(type(Child2.my_method()))
Output:
<class '__main__.Parent'>
<class '__main__.Child1'>
<class '__main__.Child2'>

Why some init methods in class inheritance in Python are not called? [duplicate]

How does super() work with multiple inheritance? For example, given:
class First(object):
def __init__(self):
print "first"
class Second(object):
def __init__(self):
print "second"
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print "that's it"
Which parent method of Third does super().__init__ refer to? Can I choose which runs?
I know it has something to do with method resolution order (MRO).

This is detailed with a reasonable amount of detail by Guido himself in his blog post Method Resolution Order (including two earlier attempts).
In your example, Third() will call First.__init__. Python looks for each attribute in the class's parents as they are listed left to right. In this case, we are looking for __init__. So, if you define
class Third(First, Second):
...
Python will start by looking at First, and, if First doesn't have the attribute, then it will look at Second.
This situation becomes more complex when inheritance starts crossing paths (for example if First inherited from Second). Read the link above for more details, but, in a nutshell, Python will try to maintain the order in which each class appears on the inheritance list, starting with the child class itself.
So, for instance, if you had:
class First(object):
def __init__(self):
print "first"
class Second(First):
def __init__(self):
print "second"
class Third(First):
def __init__(self):
print "third"
class Fourth(Second, Third):
def __init__(self):
super(Fourth, self).__init__()
print "that's it"
the MRO would be [Fourth, Second, Third, First].
By the way: if Python cannot find a coherent method resolution order, it'll raise an exception, instead of falling back to behavior which might surprise the user.
Example of an ambiguous MRO:
class First(object):
def __init__(self):
print "first"
class Second(First):
def __init__(self):
print "second"
class Third(First, Second):
def __init__(self):
print "third"
Should Third's MRO be [First, Second] or [Second, First]? There's no obvious expectation, and Python will raise an error:
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution order (MRO) for bases Second, First
Why do the examples above lack super() calls? The point of the examples is to show how the MRO is constructed. They are not intended to print "first\nsecond\third" or whatever. You can – and should, of course, play around with the example, add super() calls, see what happens, and gain a deeper understanding of Python's inheritance model. But my goal here is to keep it simple and show how the MRO is built. And it is built as I explained:
>>> Fourth.__mro__
(<class '__main__.Fourth'>,
<class '__main__.Second'>, <class '__main__.Third'>,
<class '__main__.First'>,
<type 'object'>)

Your code, and the other answers, are all buggy. They are missing the super() calls in the first two classes that are required for co-operative subclassing to work. Better is:
class First(object):
def __init__(self):
super(First, self).__init__()
print("first")
class Second(object):
def __init__(self):
super(Second, self).__init__()
print("second")
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print("third")
Output:
>>> Third()
second
first
third
The super() call finds the next method in the MRO at each step, which is why First and Second have to have it too, otherwise execution stops at the end of Second.__init__().
Without the super() calls in First and Second, the output is missing second:
>>> Third()
first
third

I wanted to elaborate the answer by lifeless a bit because when I started reading about how to use super() in a multiple inheritance hierarchy in Python, I did't get it immediately.
What you need to understand is that super(MyClass, self).__init__() provides the next __init__ method according to the used Method Resolution Ordering (MRO) algorithm in the context of the complete inheritance hierarchy.
This last part is crucial to understand. Let's consider the example again:
#!/usr/bin/env python2
class First(object):
def __init__(self):
print "First(): entering"
super(First, self).__init__()
print "First(): exiting"
class Second(object):
def __init__(self):
print "Second(): entering"
super(Second, self).__init__()
print "Second(): exiting"
class Third(First, Second):
def __init__(self):
print "Third(): entering"
super(Third, self).__init__()
print "Third(): exiting"
According to this article about Method Resolution Order by Guido van Rossum, the order to resolve __init__ is calculated (before Python 2.3) using a "depth-first left-to-right traversal" :
Third --> First --> object --> Second --> object
After removing all duplicates, except for the last one, we get :
Third --> First --> Second --> object
So, lets follow what happens when we instantiate an instance of the Third class, e.g. x = Third().
According to MRO Third.__init__ executes.
prints Third(): entering
then super(Third, self).__init__() executes and MRO returns First.__init__ which is called.
First.__init__ executes.
prints First(): entering
then super(First, self).__init__() executes and MRO returns Second.__init__ which is called.
Second.__init__ executes.
prints Second(): entering
then super(Second, self).__init__() executes and MRO returns object.__init__ which is called.
object.__init__ executes (no print statements in the code there)
execution goes back to Second.__init__ which then prints Second(): exiting
execution goes back to First.__init__ which then prints First(): exiting
execution goes back to Third.__init__ which then prints Third(): exiting
This details out why instantiating Third() results in to :
Third(): entering
First(): entering
Second(): entering
Second(): exiting
First(): exiting
Third(): exiting
The MRO algorithm has been improved from Python 2.3 onwards to work well in complex cases, but I guess that using the "depth-first left-to-right traversal" + "removing duplicates expect for the last" still works in most cases (please comment if this is not the case). Be sure to read the blog post by Guido!

This is known as the Diamond Problem, the page has an entry on Python, but in short, Python will call the superclass's methods from left to right.

Overall
Assuming everything descends from object (you are on your own if it doesn't), Python computes a method resolution order (MRO) based on your class inheritance tree. The MRO satisfies 3 properties:
Children of a class come before their parents
Left parents come before right parents
A class only appears once in the MRO
If no such ordering exists, Python errors. The inner workings of this is a C3 Linerization of the classes ancestry. Read all about it here: https://www.python.org/download/releases/2.3/mro/
When a method is called, the first occurrence of that method in the MRO is the one that is called. Any class that doesn't implement that method is skipped. Any call to super within that method will call the next occurrence of that method in the MRO. Consequently, it matters both what order you place classes in inheritance, and where you put the calls to super in the methods.
Note that you can see the MRO in python by using the __mro__ method.
Examples
All of the following examples have a diamond inheritance of classes like so:
Parent
/ \
/ \
Left Right
\ /
\ /
Child
The MRO is:
Child
Left
Right
Parent
You can test this by calling Child.__mro__, which returns:
(__main__.Child, __main__.Left, __main__.Right, __main__.Parent, object)
With super first in each method
class Parent(object):
def __init__(self):
super(Parent, self).__init__()
print("parent")
class Left(Parent):
def __init__(self):
super(Left, self).__init__()
print("left")
class Right(Parent):
def __init__(self):
super(Right, self).__init__()
print("right")
class Child(Left, Right):
def __init__(self):
super(Child, self).__init__()
print("child")
Child() outputs:
parent
right
left
child
With super last in each method
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
super(Left, self).__init__()
class Right(Parent):
def __init__(self):
print("right")
super(Right, self).__init__()
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Child() outputs:
child
left
right
parent
When not all classes call super
Inheritance order matters most if not all classes in the chain of inheritance call super. For example, if Left doesn't call super, then the method on Right and Parent are never called:
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
class Right(Parent):
def __init__(self):
print("right")
super(Right, self).__init__()
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Child() outputs:
child
left
Alternatively, if Right doesn't call super, Parent is still skipped:
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
super(Left, self).__init__()
class Right(Parent):
def __init__(self):
print("right")
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Here, Child() outputs:
child
left
right
Calling a method on a particular parent
If you want to access the method of a particular parent class, you should reference that class directly rather than using super. Super is about following the chain of inheritance, not getting to a specific class's method.
Here's how to reference a particular parent's method:
class Parent(object):
def __init__(self):
super(Parent, self).__init__()
print("parent")
class Left(Parent):
def __init__(self):
super(Left, self).__init__()
print("left")
class Right(Parent):
def __init__(self):
super(Right, self).__init__()
print("right")
class Child(Left, Right):
def __init__(self):
Parent.__init__(self)
print("child")
In this case, Child() outputs:
parent
child

I understand this doesn't directly answer the super() question, but I feel it's relevant enough to share.
There is also a way to directly call each inherited class:
class First(object):
def __init__(self):
print '1'
class Second(object):
def __init__(self):
print '2'
class Third(First, Second):
def __init__(self):
Second.__init__(self)
Just note that if you do it this way, you'll have to call each manually as I'm pretty sure First's __init__() won't be called.

This is to how I solved to issue of having multiple inheritance with different variables for initialization and having multiple MixIns with the same function call. I had to explicitly add variables to passed **kwargs and add a MixIn interface to be an endpoint for super calls.
Here A is an extendable base class and B and C are MixIn classes both who provide function f. A and B both expect parameter v in their __init__ and C expects w.
The function f takes one parameter y. Q inherits from all three classes. MixInF is the mixin interface for B and C.
IPython NoteBook Of This Code
Github Repo with code example
class A(object):
def __init__(self, v, *args, **kwargs):
print "A:init:v[{0}]".format(v)
kwargs['v']=v
super(A, self).__init__(*args, **kwargs)
self.v = v
class MixInF(object):
def __init__(self, *args, **kwargs):
print "IObject:init"
def f(self, y):
print "IObject:y[{0}]".format(y)
class B(MixInF):
def __init__(self, v, *args, **kwargs):
print "B:init:v[{0}]".format(v)
kwargs['v']=v
super(B, self).__init__(*args, **kwargs)
self.v = v
def f(self, y):
print "B:f:v[{0}]:y[{1}]".format(self.v, y)
super(B, self).f(y)
class C(MixInF):
def __init__(self, w, *args, **kwargs):
print "C:init:w[{0}]".format(w)
kwargs['w']=w
super(C, self).__init__(*args, **kwargs)
self.w = w
def f(self, y):
print "C:f:w[{0}]:y[{1}]".format(self.w, y)
super(C, self).f(y)
class Q(C,B,A):
def __init__(self, v, w):
super(Q, self).__init__(v=v, w=w)
def f(self, y):
print "Q:f:y[{0}]".format(y)
super(Q, self).f(y)

About #calfzhou's comment, you can use, as usually, **kwargs:
Online running example
class A(object):
def __init__(self, a, *args, **kwargs):
print("A", a)
class B(A):
def __init__(self, b, *args, **kwargs):
super(B, self).__init__(*args, **kwargs)
print("B", b)
class A1(A):
def __init__(self, a1, *args, **kwargs):
super(A1, self).__init__(*args, **kwargs)
print("A1", a1)
class B1(A1, B):
def __init__(self, b1, *args, **kwargs):
super(B1, self).__init__(*args, **kwargs)
print("B1", b1)
B1(a1=6, b1=5, b="hello", a=None)
Result:
A None
B hello
A1 6
B1 5
You can also use them positionally:
B1(5, 6, b="hello", a=None)
but you have to remember the MRO, it's really confusing. You can avoid this by using keyword-only parameters:
class A(object):
def __init__(self, *args, a, **kwargs):
print("A", a)
etcetera.
I can be a little annoying, but I noticed that people forgot every time to use *args and **kwargs when they override a method, while it's one of few really useful and sane use of these 'magic variables'.

Another not yet covered point is passing parameters for initialization of classes. Since the destination of super depends on the subclass the only good way to pass parameters is packing them all together. Then be careful to not have the same parameter name with different meanings.
Example:
class A(object):
def __init__(self, **kwargs):
print('A.__init__')
super().__init__()
class B(A):
def __init__(self, **kwargs):
print('B.__init__ {}'.format(kwargs['x']))
super().__init__(**kwargs)
class C(A):
def __init__(self, **kwargs):
print('C.__init__ with {}, {}'.format(kwargs['a'], kwargs['b']))
super().__init__(**kwargs)
class D(B, C): # MRO=D, B, C, A
def __init__(self):
print('D.__init__')
super().__init__(a=1, b=2, x=3)
print(D.mro())
D()
gives:
[<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>]
D.__init__
B.__init__ 3
C.__init__ with 1, 2
A.__init__
Calling the super class __init__ directly to more direct assignment of parameters is tempting but fails if there is any super call in a super class and/or the MRO is changed and class A may be called multiple times, depending on the implementation.
To conclude: cooperative inheritance and super and specific parameters for initialization aren't working together very well.

Consider calling super().Foo() called from a sub-class. The Method Resolution Order (MRO) method is the order in which method calls are resolved.
Case 1: Single Inheritance
In this, super().Foo() will be searched up in the hierarchy and will consider the closest implementation, if found, else raise an Exception. The "is a" relationship will always be True in between any visited sub-class and its super class up in the hierarchy. But this story isn't the same always in Multiple Inheritance.
Case 2: Multiple Inheritance
Here, while searching for super().Foo() implementation, every visited class in the hierarchy may or may not have is a relation. Consider following examples:
class A(object): pass
class B(object): pass
class C(A): pass
class D(A): pass
class E(C, D): pass
class F(B): pass
class G(B): pass
class H(F, G): pass
class I(E, H): pass
Here, I is the lowest class in the hierarchy. Hierarchy diagram and MRO for I will be
(Red numbers showing the MRO)
MRO is I E C D A H F G B object
Note that a class X will be visited only if all its sub-classes, which inherit from it, have been visited(i.e., you should never visit a class that has an arrow coming into it from a class below that you have not yet visited).
Here, note that after visiting class C , D is visited although C and D DO NOT have is a relationship between them(but both have with A). This is where super() differs from single inheritance.
Consider a slightly more complicated example:
(Red numbers showing the MRO)
MRO is I E C H D A F G B object
In this case we proceed from I to E to C. The next step up would be A, but we have yet to visit D, a subclass of A. We cannot visit D, however, because we have yet to visit H, a subclass of D. The leaves H as the next class to visit. Remember, we attempt to go up in hierarchy, if possible, so we visit its leftmost superclass, D. After D we visit A, but we cannot go up to object because we have yet to visit F, G, and B. These classes, in order, round out the MRO for I.
Note that no class can appear more than once in MRO.
This is how super() looks up in the hierarchy of inheritance.
Credits for resources: Richard L Halterman Fundamentals of Python Programming

In the case where each class you're trying to inherit from has it's own positional arguments for it's init, simply call each class's own init method, and don't use super if trying to inherit from multiple objects.
class A():
def __init__(self, x):
self.x = x
class B():
def __init__(self, y, z):
self.y = y
self.z = z
class C(A, B):
def __init__(self, x, y, z):
A.__init__(self, x)
B.__init__(self, y, z)
>>> c = C(1,2,3)
>>>c.x, c.y, c.z
(1, 2, 3)

In python 3.5+ inheritance looks predictable and very nice for me.
Please looks at this code:
class Base(object):
def foo(self):
print(" Base(): entering")
print(" Base(): exiting")
class First(Base):
def foo(self):
print(" First(): entering Will call Second now")
super().foo()
print(" First(): exiting")
class Second(Base):
def foo(self):
print(" Second(): entering")
super().foo()
print(" Second(): exiting")
class Third(First, Second):
def foo(self):
print(" Third(): entering")
super().foo()
print(" Third(): exiting")
class Fourth(Third):
def foo(self):
print("Fourth(): entering")
super().foo()
print("Fourth(): exiting")
Fourth().foo()
print(Fourth.__mro__)
Outputs:
Fourth(): entering
Third(): entering
First(): entering Will call Second now
Second(): entering
Base(): entering
Base(): exiting
Second(): exiting
First(): exiting
Third(): exiting
Fourth(): exiting
(<class '__main__.Fourth'>, <class '__main__.Third'>, <class '__main__.First'>, <class '__main__.Second'>, <class '__main__.Base'>, <class 'object'>)
As you can see, it calls foo exactly ONE time for each inherited chain in the same order as it was inherited. You can get that order by calling .mro :
Fourth -> Third -> First -> Second -> Base -> object

class First(object):
def __init__(self, a):
print "first", a
super(First, self).__init__(20)
class Second(object):
def __init__(self, a):
print "second", a
super(Second, self).__init__()
class Third(First, Second):
def __init__(self):
super(Third, self).__init__(10)
print "that's it"
t = Third()
Output is
first 10
second 20
that's it
Call to Third() locates the init defined in Third. And call to super in that routine invokes init defined in First. MRO=[First, Second].
Now call to super in init defined in First will continue searching MRO and find init defined in Second, and any call to super will hit the default object init. I hope this example clarifies the concept.
If you don't call super from First. The chain stops and you will get the following output.
first 10
that's it

Consider child AB, where parents A and B have keyword arguments in their constructors.
A B
\ /
AB
To init AB, you need to call the parent class constructors explicitly instead of using super().
Example:
class A():
def __init__(self, a="a"):
self.a = a
print(f"a={a}")
def A_method(self):
print(f"A_method: {self.a}")
class B():
def __init__(self, b="b"):
self.b = b
print(f"b={b}")
def B_method(self):
print(f"B_method: {self.b}")
def magical_AB_method(self):
print(f"magical_AB_method: {self.a}, {self.b}")
class AB(A,B):
def __init__(self, a="A", b="B"):
# super().__init__(a=a, b=b) # fails!
A.__init__(self, a=a)
B.__init__(self, b=b)
self.A_method()
self.B_method()
self.magical_AB_method()
A()
>>> a=a
B()
>>> b=b
AB()
>>> a=A
>>> b=B
>>> A_method: A
>>> B_method: B
To demonstrate that the two parents are combined into the child, consider magical_AB_method defined inside class B. When called from an instance of B, the method fails since it does not have access to member variables inside A. However, when called from an instance of child AB, this method works since it has inherited the required member variable from A.
B().magical_AB_method()
>>> AttributeError: 'B' object has no attribute 'a'
AB().magical_AB_method()
>>> magical_AB_method: A, B

I would like to add to what #Visionscaper says at the top:
Third --> First --> object --> Second --> object
In this case the interpreter doesnt filter out the object class because its duplicated, rather its because Second appears in a head position and doesnt appear in the tail position in a hierarchy subset. While object only appears in tail positions and is not considered a strong position in C3 algorithm to determine priority.
The linearisation(mro) of a class C, L(C), is the
the Class C
plus the merge of
linearisation of its parents P1, P2, .. = L(P1, P2, ...) and
the list of its parents P1, P2, ..
Linearised Merge is done by selecting the common classes that appears as the head of lists and not the tail since order matters(will become clear below)
The linearisation of Third can be computed as follows:
L(O) := [O] // the linearization(mro) of O(object), because O has no parents
L(First) := [First] + merge(L(O), [O])
= [First] + merge([O], [O])
= [First, O]
// Similarly,
L(Second) := [Second, O]
L(Third) := [Third] + merge(L(First), L(Second), [First, Second])
= [Third] + merge([First, O], [Second, O], [First, Second])
// class First is a good candidate for the first merge step, because it only appears as the head of the first and last lists
// class O is not a good candidate for the next merge step, because it also appears in the tails of list 1 and 2,
= [Third, First] + merge([O], [Second, O], [Second])
// class Second is a good candidate for the second merge step, because it appears as the head of the list 2 and 3
= [Third, First, Second] + merge([O], [O])
= [Third, First, Second, O]
Thus for a super() implementation in the following code:
class First(object):
def __init__(self):
super(First, self).__init__()
print "first"
class Second(object):
def __init__(self):
super(Second, self).__init__()
print "second"
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print "that's it"
it becomes obvious how this method will be resolved
Third.__init__() ---> First.__init__() ---> Second.__init__() --->
Object.__init__() ---> returns ---> Second.__init__() -
prints "second" - returns ---> First.__init__() -
prints "first" - returns ---> Third.__init__() - prints "that's it"

In learningpythonthehardway I learn something called super() an in-built function if not mistaken. Calling super() function can help the inheritance to pass through the parent and 'siblings' and help you to see clearer. I am still a beginner but I love to share my experience on using this super() in python2.7.
If you have read through the comments in this page, you will hear of Method Resolution Order (MRO), the method being the function you wrote, MRO will be using Depth-First-Left-to-Right scheme to search and run. You can do more research on that.
By adding super() function
super(First, self).__init__() #example for class First.
You can connect multiple instances and 'families' with super(), by adding in each and everyone in them. And it will execute the methods, go through them and make sure you didn't miss out! However, adding them before or after does make a difference you will know if you have done the learningpythonthehardway exercise 44. Let the fun begins!!
Taking example below, you can copy & paste and try run it:
class First(object):
def __init__(self):
print("first")
class Second(First):
def __init__(self):
print("second (before)")
super(Second, self).__init__()
print("second (after)")
class Third(First):
def __init__(self):
print("third (before)")
super(Third, self).__init__()
print("third (after)")
class Fourth(First):
def __init__(self):
print("fourth (before)")
super(Fourth, self).__init__()
print("fourth (after)")
class Fifth(Second, Third, Fourth):
def __init__(self):
print("fifth (before)")
super(Fifth, self).__init__()
print("fifth (after)")
Fifth()
How does it run? The instance of fifth() will goes like this. Each step goes from class to class where the super function added.
1.) print("fifth (before)")
2.) super()>[Second, Third, Fourth] (Left to right)
3.) print("second (before)")
4.) super()> First (First is the Parent which inherit from object)
The parent was found and it will go continue to Third and Fourth!!
5.) print("third (before)")
6.) super()> First (Parent class)
7.) print ("Fourth (before)")
8.) super()> First (Parent class)
Now all the classes with super() have been accessed! The parent class has been found and executed and now it continues to unbox the function in the inheritances to finished the codes.
9.) print("first") (Parent)
10.) print ("Fourth (after)") (Class Fourth un-box)
11.) print("third (after)") (Class Third un-box)
12.) print("second (after)") (Class Second un-box)
13.) print("fifth (after)") (Class Fifth un-box)
14.) Fifth() executed
The outcome of the program above:
fifth (before)
second (before
third (before)
fourth (before)
first
fourth (after)
third (after)
second (after)
fifth (after)
For me by adding super() allows me to see clearer on how python would execute my coding and make sure the inheritance can access the method I intended.

Maybe there's still something that can be added, a small example with Django rest_framework, and decorators. This provides an answer to the implicit question: "why would I want this anyway?"
As said: we're with Django rest_framework, and we're using generic views, and for each type of objects in our database we find ourselves with one view class providing GET and POST for lists of objects, and an other view class providing GET, PUT, and DELETE for individual objects.
Now the POST, PUT, and DELETE we want to decorate with Django's login_required. Notice how this touches both classes, but not all methods in either class.
A solution could go through multiple inheritance.
from django.utils.decorators import method_decorator
from django.contrib.auth.decorators import login_required
class LoginToPost:
#method_decorator(login_required)
def post(self, arg, *args, **kwargs):
super().post(arg, *args, **kwargs)
Likewise for the other methods.
In the inheritance list of my concrete classes, I would add my LoginToPost before ListCreateAPIView and LoginToPutOrDelete before RetrieveUpdateDestroyAPIView. My concrete classes' get would stay undecorated.

Posting this answer for my future referance.
Python Multiple Inheritance should use a diamond model and the function signature shouldn't change in the model.
A
/ \
B C
\ /
D
The sample code snippet would be ;-
class A:
def __init__(self, name=None):
# this is the head of the diamond, no need to call super() here
self.name = name
class B(A):
def __init__(self, param1='hello', **kwargs):
super().__init__(**kwargs)
self.param1 = param1
class C(A):
def __init__(self, param2='bye', **kwargs):
super().__init__(**kwargs)
self.param2 = param2
class D(B, C):
def __init__(self, works='fine', **kwargs):
super().__init__(**kwargs)
print(f"{works=}, {self.param1=}, {self.param2=}, {self.name=}")
d = D(name='Testing')
Here class A is object

Multiple inheritance in Python: how does super().init affect? [duplicate]

How does super() work with multiple inheritance? For example, given:
class First(object):
def __init__(self):
print "first"
class Second(object):
def __init__(self):
print "second"
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print "that's it"
Which parent method of Third does super().__init__ refer to? Can I choose which runs?
I know it has something to do with method resolution order (MRO).

This is detailed with a reasonable amount of detail by Guido himself in his blog post Method Resolution Order (including two earlier attempts).
In your example, Third() will call First.__init__. Python looks for each attribute in the class's parents as they are listed left to right. In this case, we are looking for __init__. So, if you define
class Third(First, Second):
...
Python will start by looking at First, and, if First doesn't have the attribute, then it will look at Second.
This situation becomes more complex when inheritance starts crossing paths (for example if First inherited from Second). Read the link above for more details, but, in a nutshell, Python will try to maintain the order in which each class appears on the inheritance list, starting with the child class itself.
So, for instance, if you had:
class First(object):
def __init__(self):
print "first"
class Second(First):
def __init__(self):
print "second"
class Third(First):
def __init__(self):
print "third"
class Fourth(Second, Third):
def __init__(self):
super(Fourth, self).__init__()
print "that's it"
the MRO would be [Fourth, Second, Third, First].
By the way: if Python cannot find a coherent method resolution order, it'll raise an exception, instead of falling back to behavior which might surprise the user.
Example of an ambiguous MRO:
class First(object):
def __init__(self):
print "first"
class Second(First):
def __init__(self):
print "second"
class Third(First, Second):
def __init__(self):
print "third"
Should Third's MRO be [First, Second] or [Second, First]? There's no obvious expectation, and Python will raise an error:
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution order (MRO) for bases Second, First
Why do the examples above lack super() calls? The point of the examples is to show how the MRO is constructed. They are not intended to print "first\nsecond\third" or whatever. You can – and should, of course, play around with the example, add super() calls, see what happens, and gain a deeper understanding of Python's inheritance model. But my goal here is to keep it simple and show how the MRO is built. And it is built as I explained:
>>> Fourth.__mro__
(<class '__main__.Fourth'>,
<class '__main__.Second'>, <class '__main__.Third'>,
<class '__main__.First'>,
<type 'object'>)

Your code, and the other answers, are all buggy. They are missing the super() calls in the first two classes that are required for co-operative subclassing to work. Better is:
class First(object):
def __init__(self):
super(First, self).__init__()
print("first")
class Second(object):
def __init__(self):
super(Second, self).__init__()
print("second")
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print("third")
Output:
>>> Third()
second
first
third
The super() call finds the next method in the MRO at each step, which is why First and Second have to have it too, otherwise execution stops at the end of Second.__init__().
Without the super() calls in First and Second, the output is missing second:
>>> Third()
first
third

I wanted to elaborate the answer by lifeless a bit because when I started reading about how to use super() in a multiple inheritance hierarchy in Python, I did't get it immediately.
What you need to understand is that super(MyClass, self).__init__() provides the next __init__ method according to the used Method Resolution Ordering (MRO) algorithm in the context of the complete inheritance hierarchy.
This last part is crucial to understand. Let's consider the example again:
#!/usr/bin/env python2
class First(object):
def __init__(self):
print "First(): entering"
super(First, self).__init__()
print "First(): exiting"
class Second(object):
def __init__(self):
print "Second(): entering"
super(Second, self).__init__()
print "Second(): exiting"
class Third(First, Second):
def __init__(self):
print "Third(): entering"
super(Third, self).__init__()
print "Third(): exiting"
According to this article about Method Resolution Order by Guido van Rossum, the order to resolve __init__ is calculated (before Python 2.3) using a "depth-first left-to-right traversal" :
Third --> First --> object --> Second --> object
After removing all duplicates, except for the last one, we get :
Third --> First --> Second --> object
So, lets follow what happens when we instantiate an instance of the Third class, e.g. x = Third().
According to MRO Third.__init__ executes.
prints Third(): entering
then super(Third, self).__init__() executes and MRO returns First.__init__ which is called.
First.__init__ executes.
prints First(): entering
then super(First, self).__init__() executes and MRO returns Second.__init__ which is called.
Second.__init__ executes.
prints Second(): entering
then super(Second, self).__init__() executes and MRO returns object.__init__ which is called.
object.__init__ executes (no print statements in the code there)
execution goes back to Second.__init__ which then prints Second(): exiting
execution goes back to First.__init__ which then prints First(): exiting
execution goes back to Third.__init__ which then prints Third(): exiting
This details out why instantiating Third() results in to :
Third(): entering
First(): entering
Second(): entering
Second(): exiting
First(): exiting
Third(): exiting
The MRO algorithm has been improved from Python 2.3 onwards to work well in complex cases, but I guess that using the "depth-first left-to-right traversal" + "removing duplicates expect for the last" still works in most cases (please comment if this is not the case). Be sure to read the blog post by Guido!

This is known as the Diamond Problem, the page has an entry on Python, but in short, Python will call the superclass's methods from left to right.

Overall
Assuming everything descends from object (you are on your own if it doesn't), Python computes a method resolution order (MRO) based on your class inheritance tree. The MRO satisfies 3 properties:
Children of a class come before their parents
Left parents come before right parents
A class only appears once in the MRO
If no such ordering exists, Python errors. The inner workings of this is a C3 Linerization of the classes ancestry. Read all about it here: https://www.python.org/download/releases/2.3/mro/
When a method is called, the first occurrence of that method in the MRO is the one that is called. Any class that doesn't implement that method is skipped. Any call to super within that method will call the next occurrence of that method in the MRO. Consequently, it matters both what order you place classes in inheritance, and where you put the calls to super in the methods.
Note that you can see the MRO in python by using the __mro__ method.
Examples
All of the following examples have a diamond inheritance of classes like so:
Parent
/ \
/ \
Left Right
\ /
\ /
Child
The MRO is:
Child
Left
Right
Parent
You can test this by calling Child.__mro__, which returns:
(__main__.Child, __main__.Left, __main__.Right, __main__.Parent, object)
With super first in each method
class Parent(object):
def __init__(self):
super(Parent, self).__init__()
print("parent")
class Left(Parent):
def __init__(self):
super(Left, self).__init__()
print("left")
class Right(Parent):
def __init__(self):
super(Right, self).__init__()
print("right")
class Child(Left, Right):
def __init__(self):
super(Child, self).__init__()
print("child")
Child() outputs:
parent
right
left
child
With super last in each method
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
super(Left, self).__init__()
class Right(Parent):
def __init__(self):
print("right")
super(Right, self).__init__()
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Child() outputs:
child
left
right
parent
When not all classes call super
Inheritance order matters most if not all classes in the chain of inheritance call super. For example, if Left doesn't call super, then the method on Right and Parent are never called:
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
class Right(Parent):
def __init__(self):
print("right")
super(Right, self).__init__()
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Child() outputs:
child
left
Alternatively, if Right doesn't call super, Parent is still skipped:
class Parent(object):
def __init__(self):
print("parent")
super(Parent, self).__init__()
class Left(Parent):
def __init__(self):
print("left")
super(Left, self).__init__()
class Right(Parent):
def __init__(self):
print("right")
class Child(Left, Right):
def __init__(self):
print("child")
super(Child, self).__init__()
Here, Child() outputs:
child
left
right
Calling a method on a particular parent
If you want to access the method of a particular parent class, you should reference that class directly rather than using super. Super is about following the chain of inheritance, not getting to a specific class's method.
Here's how to reference a particular parent's method:
class Parent(object):
def __init__(self):
super(Parent, self).__init__()
print("parent")
class Left(Parent):
def __init__(self):
super(Left, self).__init__()
print("left")
class Right(Parent):
def __init__(self):
super(Right, self).__init__()
print("right")
class Child(Left, Right):
def __init__(self):
Parent.__init__(self)
print("child")
In this case, Child() outputs:
parent
child

I understand this doesn't directly answer the super() question, but I feel it's relevant enough to share.
There is also a way to directly call each inherited class:
class First(object):
def __init__(self):
print '1'
class Second(object):
def __init__(self):
print '2'
class Third(First, Second):
def __init__(self):
Second.__init__(self)
Just note that if you do it this way, you'll have to call each manually as I'm pretty sure First's __init__() won't be called.

This is to how I solved to issue of having multiple inheritance with different variables for initialization and having multiple MixIns with the same function call. I had to explicitly add variables to passed **kwargs and add a MixIn interface to be an endpoint for super calls.
Here A is an extendable base class and B and C are MixIn classes both who provide function f. A and B both expect parameter v in their __init__ and C expects w.
The function f takes one parameter y. Q inherits from all three classes. MixInF is the mixin interface for B and C.
IPython NoteBook Of This Code
Github Repo with code example
class A(object):
def __init__(self, v, *args, **kwargs):
print "A:init:v[{0}]".format(v)
kwargs['v']=v
super(A, self).__init__(*args, **kwargs)
self.v = v
class MixInF(object):
def __init__(self, *args, **kwargs):
print "IObject:init"
def f(self, y):
print "IObject:y[{0}]".format(y)
class B(MixInF):
def __init__(self, v, *args, **kwargs):
print "B:init:v[{0}]".format(v)
kwargs['v']=v
super(B, self).__init__(*args, **kwargs)
self.v = v
def f(self, y):
print "B:f:v[{0}]:y[{1}]".format(self.v, y)
super(B, self).f(y)
class C(MixInF):
def __init__(self, w, *args, **kwargs):
print "C:init:w[{0}]".format(w)
kwargs['w']=w
super(C, self).__init__(*args, **kwargs)
self.w = w
def f(self, y):
print "C:f:w[{0}]:y[{1}]".format(self.w, y)
super(C, self).f(y)
class Q(C,B,A):
def __init__(self, v, w):
super(Q, self).__init__(v=v, w=w)
def f(self, y):
print "Q:f:y[{0}]".format(y)
super(Q, self).f(y)

About #calfzhou's comment, you can use, as usually, **kwargs:
Online running example
class A(object):
def __init__(self, a, *args, **kwargs):
print("A", a)
class B(A):
def __init__(self, b, *args, **kwargs):
super(B, self).__init__(*args, **kwargs)
print("B", b)
class A1(A):
def __init__(self, a1, *args, **kwargs):
super(A1, self).__init__(*args, **kwargs)
print("A1", a1)
class B1(A1, B):
def __init__(self, b1, *args, **kwargs):
super(B1, self).__init__(*args, **kwargs)
print("B1", b1)
B1(a1=6, b1=5, b="hello", a=None)
Result:
A None
B hello
A1 6
B1 5
You can also use them positionally:
B1(5, 6, b="hello", a=None)
but you have to remember the MRO, it's really confusing. You can avoid this by using keyword-only parameters:
class A(object):
def __init__(self, *args, a, **kwargs):
print("A", a)
etcetera.
I can be a little annoying, but I noticed that people forgot every time to use *args and **kwargs when they override a method, while it's one of few really useful and sane use of these 'magic variables'.

Another not yet covered point is passing parameters for initialization of classes. Since the destination of super depends on the subclass the only good way to pass parameters is packing them all together. Then be careful to not have the same parameter name with different meanings.
Example:
class A(object):
def __init__(self, **kwargs):
print('A.__init__')
super().__init__()
class B(A):
def __init__(self, **kwargs):
print('B.__init__ {}'.format(kwargs['x']))
super().__init__(**kwargs)
class C(A):
def __init__(self, **kwargs):
print('C.__init__ with {}, {}'.format(kwargs['a'], kwargs['b']))
super().__init__(**kwargs)
class D(B, C): # MRO=D, B, C, A
def __init__(self):
print('D.__init__')
super().__init__(a=1, b=2, x=3)
print(D.mro())
D()
gives:
[<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>]
D.__init__
B.__init__ 3
C.__init__ with 1, 2
A.__init__
Calling the super class __init__ directly to more direct assignment of parameters is tempting but fails if there is any super call in a super class and/or the MRO is changed and class A may be called multiple times, depending on the implementation.
To conclude: cooperative inheritance and super and specific parameters for initialization aren't working together very well.

Consider calling super().Foo() called from a sub-class. The Method Resolution Order (MRO) method is the order in which method calls are resolved.
Case 1: Single Inheritance
In this, super().Foo() will be searched up in the hierarchy and will consider the closest implementation, if found, else raise an Exception. The "is a" relationship will always be True in between any visited sub-class and its super class up in the hierarchy. But this story isn't the same always in Multiple Inheritance.
Case 2: Multiple Inheritance
Here, while searching for super().Foo() implementation, every visited class in the hierarchy may or may not have is a relation. Consider following examples:
class A(object): pass
class B(object): pass
class C(A): pass
class D(A): pass
class E(C, D): pass
class F(B): pass
class G(B): pass
class H(F, G): pass
class I(E, H): pass
Here, I is the lowest class in the hierarchy. Hierarchy diagram and MRO for I will be
(Red numbers showing the MRO)
MRO is I E C D A H F G B object
Note that a class X will be visited only if all its sub-classes, which inherit from it, have been visited(i.e., you should never visit a class that has an arrow coming into it from a class below that you have not yet visited).
Here, note that after visiting class C , D is visited although C and D DO NOT have is a relationship between them(but both have with A). This is where super() differs from single inheritance.
Consider a slightly more complicated example:
(Red numbers showing the MRO)
MRO is I E C H D A F G B object
In this case we proceed from I to E to C. The next step up would be A, but we have yet to visit D, a subclass of A. We cannot visit D, however, because we have yet to visit H, a subclass of D. The leaves H as the next class to visit. Remember, we attempt to go up in hierarchy, if possible, so we visit its leftmost superclass, D. After D we visit A, but we cannot go up to object because we have yet to visit F, G, and B. These classes, in order, round out the MRO for I.
Note that no class can appear more than once in MRO.
This is how super() looks up in the hierarchy of inheritance.
Credits for resources: Richard L Halterman Fundamentals of Python Programming

In the case where each class you're trying to inherit from has it's own positional arguments for it's init, simply call each class's own init method, and don't use super if trying to inherit from multiple objects.
class A():
def __init__(self, x):
self.x = x
class B():
def __init__(self, y, z):
self.y = y
self.z = z
class C(A, B):
def __init__(self, x, y, z):
A.__init__(self, x)
B.__init__(self, y, z)
>>> c = C(1,2,3)
>>>c.x, c.y, c.z
(1, 2, 3)

In python 3.5+ inheritance looks predictable and very nice for me.
Please looks at this code:
class Base(object):
def foo(self):
print(" Base(): entering")
print(" Base(): exiting")
class First(Base):
def foo(self):
print(" First(): entering Will call Second now")
super().foo()
print(" First(): exiting")
class Second(Base):
def foo(self):
print(" Second(): entering")
super().foo()
print(" Second(): exiting")
class Third(First, Second):
def foo(self):
print(" Third(): entering")
super().foo()
print(" Third(): exiting")
class Fourth(Third):
def foo(self):
print("Fourth(): entering")
super().foo()
print("Fourth(): exiting")
Fourth().foo()
print(Fourth.__mro__)
Outputs:
Fourth(): entering
Third(): entering
First(): entering Will call Second now
Second(): entering
Base(): entering
Base(): exiting
Second(): exiting
First(): exiting
Third(): exiting
Fourth(): exiting
(<class '__main__.Fourth'>, <class '__main__.Third'>, <class '__main__.First'>, <class '__main__.Second'>, <class '__main__.Base'>, <class 'object'>)
As you can see, it calls foo exactly ONE time for each inherited chain in the same order as it was inherited. You can get that order by calling .mro :
Fourth -> Third -> First -> Second -> Base -> object

class First(object):
def __init__(self, a):
print "first", a
super(First, self).__init__(20)
class Second(object):
def __init__(self, a):
print "second", a
super(Second, self).__init__()
class Third(First, Second):
def __init__(self):
super(Third, self).__init__(10)
print "that's it"
t = Third()
Output is
first 10
second 20
that's it
Call to Third() locates the init defined in Third. And call to super in that routine invokes init defined in First. MRO=[First, Second].
Now call to super in init defined in First will continue searching MRO and find init defined in Second, and any call to super will hit the default object init. I hope this example clarifies the concept.
If you don't call super from First. The chain stops and you will get the following output.
first 10
that's it

Consider child AB, where parents A and B have keyword arguments in their constructors.
A B
\ /
AB
To init AB, you need to call the parent class constructors explicitly instead of using super().
Example:
class A():
def __init__(self, a="a"):
self.a = a
print(f"a={a}")
def A_method(self):
print(f"A_method: {self.a}")
class B():
def __init__(self, b="b"):
self.b = b
print(f"b={b}")
def B_method(self):
print(f"B_method: {self.b}")
def magical_AB_method(self):
print(f"magical_AB_method: {self.a}, {self.b}")
class AB(A,B):
def __init__(self, a="A", b="B"):
# super().__init__(a=a, b=b) # fails!
A.__init__(self, a=a)
B.__init__(self, b=b)
self.A_method()
self.B_method()
self.magical_AB_method()
A()
>>> a=a
B()
>>> b=b
AB()
>>> a=A
>>> b=B
>>> A_method: A
>>> B_method: B
To demonstrate that the two parents are combined into the child, consider magical_AB_method defined inside class B. When called from an instance of B, the method fails since it does not have access to member variables inside A. However, when called from an instance of child AB, this method works since it has inherited the required member variable from A.
B().magical_AB_method()
>>> AttributeError: 'B' object has no attribute 'a'
AB().magical_AB_method()
>>> magical_AB_method: A, B

I would like to add to what #Visionscaper says at the top:
Third --> First --> object --> Second --> object
In this case the interpreter doesnt filter out the object class because its duplicated, rather its because Second appears in a head position and doesnt appear in the tail position in a hierarchy subset. While object only appears in tail positions and is not considered a strong position in C3 algorithm to determine priority.
The linearisation(mro) of a class C, L(C), is the
the Class C
plus the merge of
linearisation of its parents P1, P2, .. = L(P1, P2, ...) and
the list of its parents P1, P2, ..
Linearised Merge is done by selecting the common classes that appears as the head of lists and not the tail since order matters(will become clear below)
The linearisation of Third can be computed as follows:
L(O) := [O] // the linearization(mro) of O(object), because O has no parents
L(First) := [First] + merge(L(O), [O])
= [First] + merge([O], [O])
= [First, O]
// Similarly,
L(Second) := [Second, O]
L(Third) := [Third] + merge(L(First), L(Second), [First, Second])
= [Third] + merge([First, O], [Second, O], [First, Second])
// class First is a good candidate for the first merge step, because it only appears as the head of the first and last lists
// class O is not a good candidate for the next merge step, because it also appears in the tails of list 1 and 2,
= [Third, First] + merge([O], [Second, O], [Second])
// class Second is a good candidate for the second merge step, because it appears as the head of the list 2 and 3
= [Third, First, Second] + merge([O], [O])
= [Third, First, Second, O]
Thus for a super() implementation in the following code:
class First(object):
def __init__(self):
super(First, self).__init__()
print "first"
class Second(object):
def __init__(self):
super(Second, self).__init__()
print "second"
class Third(First, Second):
def __init__(self):
super(Third, self).__init__()
print "that's it"
it becomes obvious how this method will be resolved
Third.__init__() ---> First.__init__() ---> Second.__init__() --->
Object.__init__() ---> returns ---> Second.__init__() -
prints "second" - returns ---> First.__init__() -
prints "first" - returns ---> Third.__init__() - prints "that's it"

In learningpythonthehardway I learn something called super() an in-built function if not mistaken. Calling super() function can help the inheritance to pass through the parent and 'siblings' and help you to see clearer. I am still a beginner but I love to share my experience on using this super() in python2.7.
If you have read through the comments in this page, you will hear of Method Resolution Order (MRO), the method being the function you wrote, MRO will be using Depth-First-Left-to-Right scheme to search and run. You can do more research on that.
By adding super() function
super(First, self).__init__() #example for class First.
You can connect multiple instances and 'families' with super(), by adding in each and everyone in them. And it will execute the methods, go through them and make sure you didn't miss out! However, adding them before or after does make a difference you will know if you have done the learningpythonthehardway exercise 44. Let the fun begins!!
Taking example below, you can copy & paste and try run it:
class First(object):
def __init__(self):
print("first")
class Second(First):
def __init__(self):
print("second (before)")
super(Second, self).__init__()
print("second (after)")
class Third(First):
def __init__(self):
print("third (before)")
super(Third, self).__init__()
print("third (after)")
class Fourth(First):
def __init__(self):
print("fourth (before)")
super(Fourth, self).__init__()
print("fourth (after)")
class Fifth(Second, Third, Fourth):
def __init__(self):
print("fifth (before)")
super(Fifth, self).__init__()
print("fifth (after)")
Fifth()
How does it run? The instance of fifth() will goes like this. Each step goes from class to class where the super function added.
1.) print("fifth (before)")
2.) super()>[Second, Third, Fourth] (Left to right)
3.) print("second (before)")
4.) super()> First (First is the Parent which inherit from object)
The parent was found and it will go continue to Third and Fourth!!
5.) print("third (before)")
6.) super()> First (Parent class)
7.) print ("Fourth (before)")
8.) super()> First (Parent class)
Now all the classes with super() have been accessed! The parent class has been found and executed and now it continues to unbox the function in the inheritances to finished the codes.
9.) print("first") (Parent)
10.) print ("Fourth (after)") (Class Fourth un-box)
11.) print("third (after)") (Class Third un-box)
12.) print("second (after)") (Class Second un-box)
13.) print("fifth (after)") (Class Fifth un-box)
14.) Fifth() executed
The outcome of the program above:
fifth (before)
second (before
third (before)
fourth (before)
first
fourth (after)
third (after)
second (after)
fifth (after)
For me by adding super() allows me to see clearer on how python would execute my coding and make sure the inheritance can access the method I intended.

Maybe there's still something that can be added, a small example with Django rest_framework, and decorators. This provides an answer to the implicit question: "why would I want this anyway?"
As said: we're with Django rest_framework, and we're using generic views, and for each type of objects in our database we find ourselves with one view class providing GET and POST for lists of objects, and an other view class providing GET, PUT, and DELETE for individual objects.
Now the POST, PUT, and DELETE we want to decorate with Django's login_required. Notice how this touches both classes, but not all methods in either class.
A solution could go through multiple inheritance.
from django.utils.decorators import method_decorator
from django.contrib.auth.decorators import login_required
class LoginToPost:
#method_decorator(login_required)
def post(self, arg, *args, **kwargs):
super().post(arg, *args, **kwargs)
Likewise for the other methods.
In the inheritance list of my concrete classes, I would add my LoginToPost before ListCreateAPIView and LoginToPutOrDelete before RetrieveUpdateDestroyAPIView. My concrete classes' get would stay undecorated.

Posting this answer for my future referance.
Python Multiple Inheritance should use a diamond model and the function signature shouldn't change in the model.
A
/ \
B C
\ /
D
The sample code snippet would be ;-
class A:
def __init__(self, name=None):
# this is the head of the diamond, no need to call super() here
self.name = name
class B(A):
def __init__(self, param1='hello', **kwargs):
super().__init__(**kwargs)
self.param1 = param1
class C(A):
def __init__(self, param2='bye', **kwargs):
super().__init__(**kwargs)
self.param2 = param2
class D(B, C):
def __init__(self, works='fine', **kwargs):
super().__init__(**kwargs)
print(f"{works=}, {self.param1=}, {self.param2=}, {self.name=}")
d = D(name='Testing')
Here class A is object

Python multi-inheritance, init

Regarding multiple parent inheritance, when I call the super.__init__, why doesn't parent2's __init__ function get called? Thanks.
class parent(object):
var1=1
var2=2
def __init__(self,x=1,y=2):
self.var1=x
self.var2=y
class parent2(object):
var4=11
var5=12
def __init__(self,x=3,y=4):
self.var4=x
self.var5=y
def parprint(self):
print self.var4
print self.var5
class child(parent, parent2):
var3=5
def __init__(self,x,y):
super(child, self).__init__(x,y)
childobject = child(9,10)
print childobject.var1
print childobject.var2
print childobject.var3
childobject.parprint()
Output is
9
10
5
11
12

If you want to use super in child to call parent.__init__ and parent2._init__, then both parent __init__s must also call super:
class parent(Base):
def __init__(self,x=1,y=2):
super(parent,self).__init__(x,y)
class parent2(Base):
def __init__(self,x=3,y=4):
super(parent2,self).__init__(x,y)
See "Python super method and calling alternatives" for more details on the sequence of calls to __init__ caused by using super.
class Base(object):
def __init__(self,*args):
pass
class parent(Base):
var1=1
var2=2
def __init__(self,x=1,y=2):
super(parent,self).__init__(x,y)
self.var1=x
self.var2=y
class parent2(Base):
var4=11
var5=12
def __init__(self,x=3,y=4):
super(parent2,self).__init__(x,y)
self.var4=x
self.var5=y
def parprint(self):
print self.var4
print self.var5
class child(parent, parent2):
var3=5
def __init__(self,x,y):
super(child, self).__init__(x,y)
childobject = child(9,10)
print childobject.var1
print childobject.var2
print childobject.var3
childobject.parprint()
You might be wondering, "Why use Base?". If parent and parent2 had inherited directly from object, then
super(parent2,self).__init__(x,y) would call object.__init__(x,y). That raises a TypeError since object.__init__() takes no parameters.
To workaround this issue, you can make a class Base which accepts arguments to __init__ but does not pass them on to object.__init__. With parent and parent2 inheriting from Base, you avoid the TypeError.

Because parent is next in method resolution order (MRO), and it never uses super() to call into parent2.

See this example:
class Base(object):
def __init__(self, c):
print('Base called by {0}'.format(c))
super().__init__()
class ParentA(Base):
def __init__(self, c):
print('ParentA called by {0}'.format(c))
super().__init__('ParentA')
class ParentB(Base):
def __init__(self, c):
print('ParentB called by {0}'.format(c))
super().__init__('ParentB')
class Child(ParentA, ParentB):
def __init__(self, c):
print('Child called by {0}'.format(c))
super().__init__('Child')
Child('Construct')
print(Child.mro())
This will output:
Child called by Construct
ParentA called by Child
ParentB called by ParentA
Base called by ParentB
[<class '__main__.Child'>, <class '__main__.ParentA'>, <class '__main__.ParentB'>, <class '__main__.Base'>, <class 'object'>]
Python multiple inheritance is like a chain, in Child class mro, the super class of ParentA is ParentB, so you need call super().__init__() in ParentA to init ParentB.
If you change super().__init__('ParentA') to Base.__init__(self, 'ParentA'), this will break the inheritance chain, output:
Child called by Construct
ParentA called by Child
Base called by ParentA
[<class '__main__.Child'>, <class '__main__.ParentA'>, <class '__main__.ParentB'>, <class '__main__.Base'>, <class 'object'>]
More info about MRO

Implicitly invoking parent class initializer

class A(object):
def __init__(self, a, b, c):
#super(A, self).__init__()
super(self.__class__, self).__init__()
class B(A):
def __init__(self, b, c):
print super(B, self)
print super(self.__class__, self)
#super(B, self).__init__(1, b, c)
super(self.__class__, self).__init__(1, b, c)
class C(B):
def __init__(self, c):
#super(C, self).__init__(2, c)
super(self.__class__, self).__init__(2, c)
C(3)
In the above code, the commented out __init__ calls appear to the be the commonly accepted "smart" way to do super class initialization. However in the event that the class hierarchy is likely to change, I have been using the uncommented form, until recently.
It appears that in the call to the super constructor for B in the above hierarchy, that B.__init__ is called again, self.__class__ is actually C, not B as I had always assumed.
Is there some way in Python-2.x that I can maintain proper MRO (with respect to initializing all parent classes in the correct order) when calling super constructors while not naming the current class (the B in in super(B, self).__init__(1, b, c))?

Short answer: no, there's no way to implicitly invoke the right __init__ with the right arguments of the right parent class in Python 2.x.
Incidentally, the code as shown here is incorrect: if you use super().__init__, then all classes in your hierarchy must have the same signature in their __init__ methods. Otherwise your code can stop working if you introduce a new subclass that uses multiple inheritance.
See http://fuhm.net/super-harmful/ for a longer description of the issue (with pictures).

Your code has nothing to do with method resolution order. Method resolution comes in the case of multiple inheritance which is not the case of your example. Your code is simply wrong because you assume that self.__class__ is actually the same class of the one where the method is defined and this is wrong:
>>> class A(object):
... def __init__(self):
... print self.__class__
...
>>>
>>> class B(A):
... def __init__(self):
... A.__init__(self)
...
>>> B()
<class '__main__.B'>
<__main__.B object at 0x1bcfed0>
>>> A()
<class '__main__.A'>
<__main__.A object at 0x1bcff90>
>>>
so when you should call:
super(B, self).__init__(1, b, c)
you are indeed calling:
# super(self.__class__, self).__init__(1, b, c)
super(C, self).__init__(1, b, c)
EDIT: trying to better answer the question.
class A(object):
def __init__(self, a):
for cls in self.__class__.mro():
if cls is not object:
cls._init(self, a)
def _init(self, a):
print 'A._init'
self.a = a
class B(A):
def _init(self, a):
print 'B._init'
class C(A):
def _init(self, a):
print 'C._init'
class D(B, C):
def _init(self, a):
print 'D._init'
d = D(3)
print d.a
prints:
D._init
B._init
C._init
A._init
3
(A modified version of template pattern).
Now parents' methods are really called implicitly, but i have to agree with python zen where explicit is better than implicit because the code is lesser readable and the gain is poor. But beware that all _init methods have the same parameters, you cannot completely forget about parents and I don't suggest to do so.
For single inheritance, a better approach is explicitly calling parent's method, without invoking super. Doing so you don't have to name the current class, but still you must care about who is the parent's class.
Good reads are: how-does-pythons-super-do-the-right-thing and the links suggested in that question and in particularity Python's Super is nifty, but you can't use it
If hierarchy is likely to change is symptoms of bad design and has consequences in all the parts who are using that code and should not be encouraged.
EDIT 2
Another example comes me in mind, but which uses metaclasses. Urwid library uses metaclass to store an attribute, __super, in class so that you need just to access to that attribute.
Ex:
>>> class MetaSuper(type):
... """adding .__super"""
... def __init__(cls, name, bases, d):
... super(MetaSuper, cls).__init__(name, bases, d)
... if hasattr(cls, "_%s__super" % name):
... raise AttributeError, "Class has same name as one of its super classes"
... setattr(cls, "_%s__super" % name, super(cls))
...
>>> class A:
... __metaclass__ = MetaSuper
... def __init__(self, a):
... self.a = a
... print 'A.__init__'
...
>>> class B(A):
... def __init__(self, a):
... print 'B.__init__'
... self.__super.__init__(a)
...
>>> b = B(42)
B.__init__
A.__init__
>>> b.a
42
>>>

Perhaps what you are looking for is metaclasses?
class metawrap(type):
def __new__(mcs,name, bases, dict):
dict['bases'] = bases
return type.__new__(mcs,name,bases,dict)
class A(object):
def __init__(self):
pass
def test(self):
print "I am class A"
class B(A):
__metaclass__ = metawrap
def __init__(self):
pass
def test(self):
par = super(self.bases[0],self)
par.__thisclass__.test(self)
foo = B()
foo.test()
Prints "I am class A"
What the metaclass does is overriding the initial creation of the B class (not the object) and makes sure that the builtin dictionary for each B object now contains a bases array where you can find all the baseclasses for B

To my knowledge, the following isn't commonly done. But it does seem to work.
Methods in a given class definition always mangle double-underscore attributes to include the name of the class they're defined in. So, if you stash a reference to the class in name-mangled form where the instances can see it, you can use that in the call to super.
An example stashing the references on the object itself, by implementing __new__ on the baseclass:
def mangle(cls, name):
if not name.startswith('__'):
raise ValueError('name must start with double underscore')
return '_%s%s' % (cls.__name__, name)
class ClassStasher(object):
def __new__(cls, *args, **kwargs):
obj = object.__new__(cls)
for c in cls.mro():
setattr(obj, mangle(c, '__class'), c)
return obj
class A(ClassStasher):
def __init__(self):
print 'init in A', self.__class
super(self.__class, self).__init__()
class B(A):
def __init__(self):
print 'init in B', self.__class
super(self.__class, self).__init__()
class C(A):
def __init__(self):
print 'init in C', self.__class
super(self.__class, self).__init__()
class D(B, C):
def __init__(self):
print 'init in D', self.__class
super(self.__class, self).__init__()
d = D()
print d
And, doing a similar thing, but using a meta-class and stashing the __class references on the class objects themselves:
class ClassStasherType(type):
def __init__(cls, name, bases, attributes):
setattr(cls, mangle(cls, '__class'), cls)
class ClassStasher(object):
__metaclass__ = ClassStasherType
class A_meta(ClassStasher):
def __init__(self):
print 'init in A_meta', self.__class
super(self.__class, self).__init__()
class B_meta(A_meta):
def __init__(self):
print 'init in B_meta', self.__class
super(self.__class, self).__init__()
class C_meta(A_meta):
def __init__(self):
print 'init in C_meta', self.__class
super(self.__class, self).__init__()
class D_meta(B_meta, C_meta):
def __init__(self):
print 'init in D_meta', self.__class
super(self.__class, self).__init__()
d = D_meta()
print d
Running this all together, as one source file:
% python /tmp/junk.py
init in D <class '__main__.D'>
init in B <class '__main__.B'>
init in C <class '__main__.C'>
init in A <class '__main__.A'>
<__main__.D object at 0x1004a4a50>
init in D_meta <class '__main__.D_meta'>
init in B_meta <class '__main__.B_meta'>
init in C_meta <class '__main__.C_meta'>
init in A_meta <class '__main__.A_meta'>
<__main__.D_meta object at 0x1004a4bd0>

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