Zystkmt

Suppose I have a base class with unimplemented methods as follows:

class Polygon():

    def __init__(self):

        pass



    def perimeter(self):

        pass



    def area(self):

        pass

Now, let's say one of my colleagues uses the Polygon class to create a subclass as follows:

import math



class Circle(Polygon):

    def __init__(self, radius):

        self.radius = radius



    def perimeter(self):

        return 2 * math.pi * self.radius

(H/Sh)e has forgotten to implement the area() method.

How can I force the subclass to implement the parent's area() method?

this could be your parent class:

class Polygon():

    def __init__(self):

        raise NotImplementedError



    def perimeter(self):

        raise NotImplementedError



    def area(self):

        raise NotImplementedError

although the problem will be spotted at runtime only, when one of the instances of the child classes tries to call one of these methods.

a different version is to use abc.abstractmethod.

from abc import ABCMeta, abstractmethod

import math



class Polygon(metaclass=ABCMeta):



    @abstractmethod

    def __init__(self):

        pass



    @abstractmethod

    def perimeter(self):

        pass



    @abstractmethod

    def area(self):

        pass



class Circle(Polygon):

    def __init__(self, radius):

        self.radius = radius



    def perimeter(self):

        return 2 * math.pi * self.radius



#    def area(self):

#        return math.pi * self.radius**2





c = Circle(9.0)

# TypeError: Can't instantiate abstract class Circle with abstract methods area

you will not be able to instantiate a Circle without it having all the methods implemented.

this is the python 3 syntax; in python 2 you'd need to

class Polygon(object):

    __metaclass__ = ABCMeta

also note that for the binary special functions __eq__(), __lt__(), __add__(), ... it is better to return NotImplemented instead of raising NotImplementedError.

That's exactly what NotImplementedError are used for :)

In your base class

def area(self):

     raise NotImplementedError("Hey, Don't forget to implement the area!"

You can raise NotImplementedError exception in base class method.

class Polygon:

    def area(self):

        raise NotImplementedError

Also you can use @abc.abstractmethod, but then you need to declare metaclass to be abc.ABCMeta, which would make your class abstract. More about abc module