class – Understanding Python super() with __init__() methods

class – Understanding Python super() with __init__() methods

super() lets you avoid referring to the base class explicitly, which can be nice. But the main advantage comes with multiple inheritance, where all sorts of fun stuff can happen. See the standard docs on super if you havent already.

Note that the syntax changed in Python 3.0: you can just say super().__init__() instead of super(ChildB, self).__init__() which IMO is quite a bit nicer. The standard docs also refer to a guide to using super() which is quite explanatory.

class – Understanding Python super() with __init__() methods

Im trying to understand super()

The reason we use super is so that child classes that may be using cooperative multiple inheritance will call the correct next parent class function in the Method Resolution Order (MRO).

In Python 3, we can call it like this:

class ChildB(Base):
    def __init__(self):
        super().__init__()

In Python 2, we were required to call super like this with the defining classs name and self, but well avoid this from now on because its redundant, slower (due to the name lookups), and more verbose (so update your Python if you havent already!):

        super(ChildB, self).__init__()

Without super, you are limited in your ability to use multiple inheritance because you hard-wire the next parents call:

        Base.__init__(self) # Avoid this.

I further explain below.

What difference is there actually in this code?:

class ChildA(Base):
    def __init__(self):
        Base.__init__(self)

class ChildB(Base):
    def __init__(self):
        super().__init__()

The primary difference in this code is that in ChildB you get a layer of indirection in the __init__ with super, which uses the class in which it is defined to determine the next classs __init__ to look up in the MRO.

I illustrate this difference in an answer at the canonical question, How to use super in Python?, which demonstrates dependency injection and cooperative multiple inheritance.

If Python didnt have super

Heres code thats actually closely equivalent to super (how its implemented in C, minus some checking and fallback behavior, and translated to Python):

class ChildB(Base):
    def __init__(self):
        mro = type(self).mro()
        check_next = mro.index(ChildB) + 1 # next after *this* class.
        while check_next < len(mro):
            next_class = mro[check_next]
            if __init__ in next_class.__dict__:
                next_class.__init__(self)
                break
            check_next += 1

Written a little more like native Python:

class ChildB(Base):
    def __init__(self):
        mro = type(self).mro()
        for next_class in mro[mro.index(ChildB) + 1:]: # slice to end
            if hasattr(next_class, __init__):
                next_class.__init__(self)
                break

If we didnt have the super object, wed have to write this manual code everywhere (or recreate it!) to ensure that we call the proper next method in the Method Resolution Order!

How does super do this in Python 3 without being told explicitly which class and instance from the method it was called from?

It gets the calling stack frame, and finds the class (implicitly stored as a local free variable, __class__, making the calling function a closure over the class) and the first argument to that function, which should be the instance or class that informs it which Method Resolution Order (MRO) to use.

Since it requires that first argument for the MRO, using super with static methods is impossible as they do not have access to the MRO of the class from which they are called.

Criticisms of other answers:

super() lets you avoid referring to the base class explicitly, which can be nice. . But the main advantage comes with multiple inheritance, where all sorts of fun stuff can happen. See the standard docs on super if you havent already.

Its rather hand-wavey and doesnt tell us much, but the point of super is not to avoid writing the parent class. The point is to ensure that the next method in line in the method resolution order (MRO) is called. This becomes important in multiple inheritance.

Ill explain here.

class Base(object):
    def __init__(self):
        print(Base inited)

class ChildA(Base):
    def __init__(self):
        print(ChildA inited)
        Base.__init__(self)

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

And lets create a dependency that we want to be called after the Child:

class UserDependency(Base):
    def __init__(self):
        print(UserDependency inited)
        super().__init__()

Now remember, ChildB uses super, ChildA does not:

class UserA(ChildA, UserDependency):
    def __init__(self):
        print(UserA inited)
        super().__init__()

class UserB(ChildB, UserDependency):
    def __init__(self):
        print(UserB inited)
        super().__init__()

And UserA does not call the UserDependency method:

>>> UserA()
UserA inited
ChildA inited
Base inited
<__main__.UserA object at 0x0000000003403BA8>

But UserB does in-fact call UserDependency because ChildB invokes super:

>>> UserB()
UserB inited
ChildB inited
UserDependency inited
Base inited
<__main__.UserB object at 0x0000000003403438>

Criticism for another answer

In no circumstance should you do the following, which another answer suggests, as youll definitely get errors when you subclass ChildB:

super(self.__class__, self).__init__()  # DONT DO THIS! EVER.

_{(That answer is not clever or particularly interesting, but in spite of direct criticism in the comments and over 17 downvotes, the answerer persisted in suggesting it until a kind editor fixed his problem.)}

Explanation: Using self.__class__ as a substitute for the class name in super() will lead to recursion. super lets us look up the next parent in the MRO (see the first section of this answer) for child classes. If you tell super were in the child instances method, it will then lookup the next method in line (probably this one) resulting in recursion, probably causing a logical failure (in the answerers example, it does) or a RuntimeError when the recursion depth is exceeded.

>>> class Polygon(object):
...     def __init__(self, id):
...         self.id = id
...
>>> class Rectangle(Polygon):
...     def __init__(self, id, width, height):
...         super(self.__class__, self).__init__(id)
...         self.shape = (width, height)
...
>>> class Square(Rectangle):
...     pass
...
>>> Square(a, 10, 10)
Traceback (most recent call last):
  File <stdin>, line 1, in <module>
  File <stdin>, line 3, in __init__
TypeError: __init__() missing 2 required positional arguments: width and height

Python 3s new super() calling method with no arguments fortunately allows us to sidestep this issue.

class – Understanding Python super() with __init__() methods

Its been noted that in Python 3.0+ you can use

super().__init__()

to make your call, which is concise and does not require you to reference the parent OR class names explicitly, which can be handy. I just want to add that for Python 2.7 or under, some people implement a name-insensitive behaviour by writing self.__class__ instead of the class name, i.e.

super(self.__class__, self).__init__()  # DONT DO THIS!

HOWEVER, this breaks calls to super for any classes that inherit from your class, where self.__class__ could return a child class. For example:

class Polygon(object):
    def __init__(self, id):
        self.id = id

class Rectangle(Polygon):
    def __init__(self, id, width, height):
        super(self.__class__, self).__init__(id)
        self.shape = (width, height)

class Square(Rectangle):
    pass

Here I have a class Square, which is a sub-class of Rectangle. Say I dont want to write a separate constructor for Square because the constructor for Rectangle is good enough, but for whatever reason I want to implement a Square so I can reimplement some other method.

When I create a Square using mSquare = Square(a, 10,10), Python calls the constructor for Rectangle because I havent given Square its own constructor. However, in the constructor for Rectangle, the call super(self.__class__,self) is going to return the superclass of mSquare, so it calls the constructor for Rectangle again. This is how the infinite loop happens, as was mentioned by @S_C. In this case, when I run super(...).__init__() I am calling the constructor for Rectangle but since I give it no arguments, I will get an error.