Python · OOP: inheritance and dunder methods

Inheritance: class Child(Parent) reuses everything the parent has. The child can override methods and call the parent’s version through super().

Dunder (“double underscore”) methods are how objects hook into Python’s own syntax:

you write	Python calls
repr(v) / in the REPL	__repr__
str(v) / print(v)	__str__
a == b	__eq__
a + b	__add__
len(v)	__len__

import math

class Shape:
    def __init__(self, name):
        self.name = name
    def area(self):
        raise NotImplementedError
    def __repr__(self):
        return f"{type(self).__name__}({self.name!r}, area={self.area():.2f})"

class Circle(Shape):
    def __init__(self, r):
        super().__init__("circle")
        self.r = r
    def area(self):
        return math.pi * self.r ** 2

class Vec:
    def __init__(self, *xs): self.xs = xs
    def __add__(self, o):    return Vec(*(a + b for a, b in zip(self.xs, o.xs)))
    def __eq__(self, o):     return self.xs == o.xs
    def __len__(self):       return len(self.xs)
    def __repr__(self):      return f"Vec{self.xs}"

print(Circle(1))
print(isinstance(Circle(2), Shape))
v = Vec(1, 2, 3) + Vec(10, 20, 30)
print(v, len(v), v == Vec(11, 22, 33))

Output:

Circle('circle', area=3.14)
True
Vec(11, 22, 33) 3 True