Understanding Python super with init methods

Question

I m trying to understand the use of super    From the looks of it  both child classes can be created  just fine    I m curious to know about the actual difference between the following 2 child classes   class Base object       def   init   self           print  Base created   class ChildA Base       def   init   self           Base   init   self   class ChildB Base       def   init   self           super ChildB  self    init      ChildA    ChildB

User · Answer

Super has no side effects  Base   ChildB  Base     works as expected  Base   ChildA  Base     gets into infinite recursion

User · Answer

I m 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 use it like this  but we ll avoid this here          super ChildB  self    init      Without super  you are limited in your ability to use multiple inheritance because you hard-wire the next parent s call          Base   init   self    Avoid this   I further explain below    quot What difference is there actually in this code   quot   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 class s   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 didn t have super Here s code that s actually closely equivalent to super  how it s 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  lt  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 didn t have the super object  we d 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 haven t already   It s rather hand-wavey and doesn t 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  I ll explain here  class Base object       def   init   self           print  quot Base init ed quot    class ChildA Base       def   init   self           print  quot ChildA init ed quot           Base   init   self   class ChildB Base       def   init   self           print  quot ChildB init ed quot           super     init      And let s create a dependency that we want to be called after the Child  class UserDependency Base       def   init   self           print  quot UserDependency init ed quot           super     init      Now remember  ChildB uses super  ChildA does not  class UserA ChildA  UserDependency       def   init   self           print  quot UserA init ed quot           super     init      class UserB ChildB  UserDependency       def   init   self           print  quot UserB init ed quot           super     init      And UserA does not call the UserDependency method   gt  gt  gt  UserA   UserA init ed ChildA init ed Base init ed  lt   main   UserA object at 0x0000000003403BA8 gt   But UserB does in-fact call UserDependency because ChildB invokes super   gt  gt  gt  UserB   UserB init ed ChildB init ed UserDependency init ed Base init ed  lt   main   UserB object at 0x0000000003403438 gt   Criticism for another answer In no circumstance should you do the following  which another answer suggests  as you ll definitely get errors when you subclass ChildB  super self   class    self    init        DON T 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 we re in the child instance s method  it will then lookup the next method in line  probably this one  resulting in recursion  probably causing a logical failure  in the answerer s example  it does  or a RuntimeError when the recursion depth is exceeded   gt  gt  gt  class Polygon object           def   init   self  id               self id   id      gt  gt  gt  class Rectangle Polygon           def   init   self  id  width  height               super self   class    self    init   id              self shape    width  height       gt  gt  gt  class Square Rectangle           pass      gt  gt  gt  Square  a   10  10  Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt    File  quot  lt stdin gt  quot   line 3  in   init   TypeError    init     missing 2 required positional arguments   width  and  height   Python 3 s new super   calling method with no arguments fortunately allows us to sidestep this issue

User · Answer

It s 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        DON T 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 don t 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 haven t 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

User · Answer

There isn t  really  super   looks at the next class in the MRO  method resolution order  accessed with cls   mro    to call the methods  Just calling the base   init   calls the base   init    As it happens  the MRO has exactly one item-- the base  So you re really doing the exact same thing  but in a nicer way with super    particularly if you get into multiple inheritance later

User · Answer

Just a heads up    with Python 2 7  and I believe ever since super   was introduced in version 2 2  you can only call super   if one of the parents inherit from a class that eventually inherits object  new-style classes    Personally  as for python 2 7 code  I m going to continue using BaseClassName   init   self  args  until I actually get the advantage of using super

User · Answer

The main difference is that ChildA   init   will unconditionally call Base   init   whereas ChildB   init   will call   init   in whatever class happens to be ChildB ancestor in self s line of ancestors  which may differ from what you expect   If you add a ClassC that uses multiple inheritance  class Mixin Base     def   init   self       print  quot Mixin stuff quot      super Mixin  self    init      class ChildC ChildB  Mixin      Mixin is now between ChildB and Base   pass  ChildC   help ChildC    shows that the Method Resolution Order is ChildC- gt ChildB- gt Mixin- gt Base  then Base is no longer the parent of ChildB for ChildC instances  Now super ChildB  self  will point to Mixin if self is a ChildC instance  You have inserted Mixin in between ChildB and Base  And you can take advantage of it with super   So if you are designed your classes so that they can be used in a Cooperative Multiple Inheritance scenario  you use super because you don t really know who is going to be the ancestor at runtime  The super considered super post and pycon 2015 accompanying video explain this pretty well

User · Answer

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 haven t 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

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