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.

---

a different version is to use abc.abstractmethod.

from abc import ABC, abstractmethod
import math


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

---

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

Answer 2

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

Answer 3

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!")