Inheritance in Python

Question

I'm having a small amount of difficulty understanding inheritance in Python. I was under the impression that when Class B inherits from Class A it inherits all of the values that were initialized in A. I've made up an example to demonstrate what I mean:

Class A():
    def __init__(self,parameter):
        self.initialize_parameter=4*parameter

Class B(A):
    def __init__(self):
        pass

    def function(self,another_parameter):
        return self.initialize_parameter*another_parameter

But in this case, calling:

B_instance=B()
print B_instance.function(10)

Returns an AttributeError saying that class B doesn't not have self.initialized_parameter

So my question is, do I have to copy and paste all the initializations from A to B manually or is there a way that I can use them from within B without calling the A class itself? (I hope this is clear).

Answer 1

If you want to use the behaviour in the superclass' constructor you should either not override it:

Class B(A):
  def function(self,another_parameter):
    return self.initialize_parameter*another_parameter

Or if you want to extend the initialization behaviour you can use super()

Class B(A):
  def __init__(self, parameter)
    super(B, self).__init__(2*parameter)

  def function(self,another_parameter):
    return self.initialize_parameter*another_parameter

In Python 3 super() doesn't need explicit arguments (super().__init__()).

This general override behaviour and super() applies to all methods, not just constructors.

Answer 2

You have to use the super builtin in order to do the required initialisation for the parent class, like so:

class B(A):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

If you're not using Python 3, you might have to do it like this:

class B(A):
    def __init__(self, *args, **kwargs):
        super(A, self).__init__(*args, **kwargs)

Basically this calls A's __init__ function which is required if you want to reference A's methods and attributes.

Answer 3

The only way to initialize the base class is to use super().__init__() in B __init__() method. See: Understanding Python super() with __init__() methods

Answer 4

You are overriding the constructor from the super class here:

def __init__(self):
    pass

So, self.initialize_parameter is never being initialized.

Delete that method, and it should work.

Answer 5

The __init__ of the superclass is only called automatically when you do not specify an __init__ in the subclass. So you should not define __init__ in class B unless you need to. If you specify __init__ in class B you have to call __init__ in the superclass by hand. I prefer calling it this way: A.__init__(self,'provide_initialize_parameter_here'). Other people prefer using super.