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).

score 5 · Accepted Answer · 2015-09-06 11:54:41Z

5

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.

edited Sep 6, 2015 at 11:54

answered Sep 6, 2015 at 11:02

user3978329

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1 Comment

bsa Over a year ago

Nice answer nilem. Inheritance in Python in general is quite confusing, but you've summed it up nicely.

cyphar · Accepted Answer · 2015-09-06 11:01:26Z

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.

answered Sep 6, 2015 at 11:01

cyphar

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Comments

Community · Accepted Answer · 2017-05-23 12:25:33Z

1

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

edited May 23, 2017 at 12:25

CommunityBot

11 silver badge

answered Sep 6, 2015 at 11:01

jendas

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Comments

bsa · Accepted Answer · 2015-09-06 11:02:05Z

1

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.

answered Sep 6, 2015 at 11:02

bsa

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Comments

mkiever · Accepted Answer · 2015-09-06 11:06:23Z

1

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.

answered Sep 6, 2015 at 11:06

mkiever

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