How to invoke the super constructor in Python?

Question

class A:
    def __init__(self):
        print(\"world\")

class B(A):
    def __init__(self):
       print(\"hello\")

B()  # output: hello

In all

Answer 1

Short Answer

super(DerivedClass, self).__init__()

Long Answer

What does super() do?

It takes specified class name, finds its base classes (Python allows multiple inheritance) and looks for the method (__init__ in this case) in each of them from left to right. As soon as it finds method available, it will call it and end the search.

How do I call init of all base classes?

Above works if you have only one base class. But Python does allow multiple inheritance and you might want to make sure all base classes are initialized properly. To do that, you should have each base class call init:

class Base1:
  def __init__():
    super(Base1, self).__init__()

class Base2:
  def __init__():
    super(Base2, self).__init__()

class Derived(Base1, Base2):
  def __init__():
    super(Derived, self).__init__()

What if I forget to call init for super?

The constructor (__new__) gets invoked in a chain (like in C++ and Java). Once the instance is created, only that instance's initialiser (__init__) is called, without any implicit chain to its superclass.