Abstract class in Python (abc, @abstractmethod)

An abstract base class is a parent class that only describes what its children must have. You cannot create an object from it, and a child class that forgets one of the required methods cannot be created either.

The program

python

from abc import ABC, abstractmethod
import math


class Shape(ABC):
    @abstractmethod
    def area(self):
        ...

    @abstractmethod
    def perimeter(self):
        ...

    def describe(self):
        return f"{type(self).__name__}: area {self.area():.2f}, perimeter {self.perimeter():.2f}"


class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

    def perimeter(self):
        return 2 * (self.width + self.height)


class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return math.pi * self.radius ** 2

    def perimeter(self):
        return 2 * math.pi * self.radius


for shape in [Rectangle(4, 6), Circle(1)]:
    print(shape.describe())

output

Rectangle: area 24.00, perimeter 20.00
Circle: area 3.14, perimeter 6.28

How it works

class Shape(ABC) makes Shape abstract. @abstractmethod marks area and perimeter as methods that every child class must define.
... (the Ellipsis) is an empty body. It does the same here as pass.
describe is a normal method. The children inherit it, and it can call area() and perimeter() because every child is guaranteed to have them.

A child that forgets a method

Python checks when you create the object, not when you call the missing method:

python

from abc import ABC, abstractmethod


class Shape(ABC):
    @abstractmethod
    def area(self):
        ...

    @abstractmethod
    def perimeter(self):
        ...


class Square(Shape):
    def __init__(self, side):
        self.side = side

    def area(self):
        return self.side ** 2
    # perimeter is missing


try:
    Shape()
except TypeError as e:
    print("Error:", e)

try:
    Square(3)
except TypeError as e:
    print("Error:", e)

output

Error: Can't instantiate abstract class Shape without an implementation for abstract methods 'area', 'perimeter'
Error: Can't instantiate abstract class Square without an implementation for abstract method 'perimeter'

The error shows up as soon as someone writes Square(3), with the name of the missing method. Without the abstract base class, the program would run until something called perimeter(), and only then fail.

Abstract base class in Python

The program

How it works

A child that forgets a method