Python Abstraction

What Abstraction Solves

Sometimes you want to guarantee that every subclass implements certain methods, without providing a default implementation yourself. Python's abc module (Abstract Base Classes) lets you define such a required interface.

from abc import ABC, abstractmethod

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

    @abstractmethod
    def perimeter(self):
        pass

A class that inherits from ABC and has at least one method decorated with @abstractmethod becomes an abstract class. You cannot create objects directly from it.

s = Shape()

TypeError: Can't instantiate abstract class Shape with abstract methods area, perimeter

Implementing the Interface

Any child class must override every abstract method, or it too remains abstract and cannot be instantiated. This guarantees a consistent set of methods across all shapes.

class Rectangle(Shape):
    def __init__(self, w, h):
        self.w = w
        self.h = h

    def area(self):
        return self.w * self.h

    def perimeter(self):
        return 2 * (self.w + self.h)

r = Rectangle(4, 5)
print(r.area(), r.perimeter())

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