Class inheritance

Inheritance lets one class specialize another.

Use it when there is a real "is a" relationship between the two types. The derived class inherits behavior from the base class and can add or override methods of its own.

Use inheritance sparingly. It couples representation, lifecycle, and method lookup, so a base class becomes part of the derived type's public contract. When the shared need is only behavior, a protocol is often a better fit. When the shared need is only state reuse, composition is often clearer.

class Shape(object):
    def area(self) -> float:
        raise NotImplementedError("subclasses must implement area()")


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

    def area(self):
        return 3.14 * self.radius ** 2


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

    def area(self):
        return self.side ** 2


actor main(env):
    circle = Circle(3.14)
    square = Square(3.14)
    print(circle.area() + square.area())
    env.exit(0)

In this example:

Shape defines a common interface
Circle and Square inherit from Shape
each subclass provides its own area() implementation
code written against Shape can still call area() on either value

Read class Circle(Shape): as "Circle is a kind of Shape". If that sentence feels wrong, inheritance is usually the wrong tool.

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Class inheritance