Force child class to override parent's methods

Question

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?

Answer 1

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.

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a different version is to use abc.abstractmethod.

from abc import ABCMeta, abstractmethod
# simpler alternative: from abc import ABC, abstractmethod
import math

class Polygon(metaclass=ABCMeta):
# simpler alternative: class Polygon(ABC)

    @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

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