Should init call the parent class s init

Question

I m used that in Objective-C I ve got this construct   -  void init       if  self    super init                init class           return self      Should Python also call the parent class s implementation for   init     class NewClass SomeOtherClass       def   init   self           SomeOtherClass   init   self            init class   Is this also true false for   new     and   del       Edit  There s a very similar question  Inheritance and Overriding   init   in Python

User · Answer

In Anon s answer    If you need something from super s   init   to be done in addition to what is being done in the current class s   init     you must call it yourself  since that will not happen automatically     It s incredible  he is wording exactly the contrary of the principle of inheritance     It is not that  something from super s   init           will not happen automatically    it is that it WOULD happen automatically  but it doesn t happen because the base-class    init   is overriden by the definition of the derived-clas   init        So then  WHY defining a derived class    init     since it overrides what is aimed at when someone resorts to inheritance         It s because one needs to define something that is NOT done in the base-class    init     and the only possibility to obtain that is to put its execution in a derived-class    init   function  In other words  one needs something in base-class    init   in addition to what would be automatically done in the base-classe    init   if this latter wasn t overriden   NOT the contrary        Then  the problem is that the desired instructions present in the base-class    init   are no more activated at the moment of instantiation  In order to offset this inactivation  something special is required  calling explicitly the base-class    init     in order to KEEP   NOT TO ADD  the initialization performed by the base-class    init     That s exactly what is said in the official doc      An overriding method in a derived class may in fact want to extend   rather than simply replace the base class method of the same name    There is a simple way to call the base class method directly  just   call BaseClassName methodname self  arguments     http   docs python org tutorial classes html inheritance   That s all the story    when the aim is to KEEP the initialization performed by the base-class  that is pure inheritance  nothing special is needed  one must just avoid to define an   init   function in the derived class when the aim is to REPLACE the initialization performed by the base-class    init   must be defined in the derived-class when the aim is to ADD processes to the initialization performed by the base-class  a derived-class    init    must be defined   comprising an explicit call to the base-class   init          What I feel astonishing in the post of Anon is not only that he expresses the contrary of the inheritance theory  but that there have been 5 guys passing by that upvoted without turning a hair  and moreover there have been nobody to react in 2 years in a thread whose interesting subject must be read relatively often

User · Answer

If you need something from super s   init   to be done in addition to what is being done in the current class s   init    you must call it yourself  since that will not happen automatically   But if you don t need anything from super s   init    no need to call it   Example    gt  gt  gt  class C object           def   init   self               self b   1    gt  gt  gt  class D C           def   init   self               super     init       in Python 2 use super D  self    init                 self a   1    gt  gt  gt  class E C           def   init   self               self a   1    gt  gt  gt  d   D    gt  gt  gt  d a 1  gt  gt  gt  d b    This works because of the call to super s init 1  gt  gt  gt  e   E    gt  gt  gt  e a 1  gt  gt  gt  e b    This is going to fail since nothing in E initializes b    Traceback  most recent call last     File   lt pyshell 70 gt    line 1  in  lt module gt      e b    This is going to fail since nothing in E initializes b    AttributeError   E  object has no attribute  b      del   is the same way   but be wary of relying on   del   for finalization - consider doing it via the with statement instead    I rarely use   new     I do all the initialization in   init

User · Answer

In Python  calling the super-class    init   is optional  If you call it  it is then also optional whether to use the super identifier  or whether to explicitly name the super class   object   init   self    In case of object  calling the super method is not strictly necessary  since the super method is empty  Same for   del     On the other hand  for   new    you should indeed call the super method  and use its return as the newly-created object - unless you explicitly want to return something different

User · Answer

Edit   after the code change  There is no way for us to tell you whether you need or not to call your parent s   init    or any other function   Inheritance obviously would work without such call  It all depends on the logic of your code  for example  if all your   init   is done in parent class  you can just skip child-class   init   altogether   consider the following example    gt  gt  gt  class A      def   init   self  val           self a   val    gt  gt  gt  class B A       pass   gt  gt  gt  class C A       def   init   self  val           A   init   self  val          self a    val    gt  gt  gt  A 4  a 4  gt  gt  gt  B 5  a 5  gt  gt  gt  C 6  a 12

User · Answer

There s no hard and fast rule  The documentation for a class should indicate whether subclasses should call the superclass method  Sometimes you want to completely replace superclass behaviour  and at other times augment it - i e  call your own code before and or after a superclass call   Update  The same basic logic applies to any method call  Constructors sometimes need special consideration  as they often set up state which determines behaviour  and destructors because they parallel constructors  e g  in the allocation of resources  e g  database connections   But the same might apply  say  to the render   method of a widget   Further update  What s the OPP  Do you mean OOP  No - a subclass often needs to know something about the design of the superclass  Not the internal implementation details - but the basic contract that the superclass has with its clients  using classes   This does not violate OOP principles in any way  That s why protected is a valid concept in OOP in general  though not  of course  in Python

User · Answer

IMO  you should call it  If your superclass is object  you should not  but in other cases I think it is exceptional not to call it  As already answered by others  it is very convenient if your class doesn t even have to override   init   itself  for example when it has no  additional  internal state to initialize

User · Answer

Yes  you should always call base class   init   explicitly as a good coding practice  Forgetting to do this can cause subtle issues or run time errors  This is true even if   init   doesn t take any parameters  This is unlike other languages where compiler would implicitly call base class constructor for you  Python doesn t do that   The main reason for always calling base class  init   is that base class may typically create member variable and initialize them to defaults  So if you don t call base class init  none of that code would be executed and you would end up with base class that has no member variables   Example   class Base    def   init   self       print  base init    class Derived1 Base     def   init   self       print  derived1 init    class Derived2 Base     def   init   self       super Derived2  self    init         print  derived2 init    print  Creating Derived1      d1   Derived1   print  Creating Derived2      d2   Derived2     This prints    Creating Derived1    derived1 init Creating Derived2    base init derived2 init   Run this code

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