Operator Overloading

Another kind of built-in polymorphism in Python is the ability to override how an operator works. For example, the + operator works for built-in types like integers and strings.

print(3 + 4)
# 7

print("three " + "four")
# three four

Custom classes on the other hand don't have any built-in support for those operators:

class Point:
    def __init__(self, x: int, y: int) -> None:
        self.x = x
        self.y = y


p1 = Point(4, 5)
p2 = Point(2, 3)
p3 = p1 + p2
# TypeError: unsupported operand type(s) for +: 'Point' and 'Point'

But we can add our own support! Python uses special methods with double underscores, sometimes called "dunder methods", to hook into built-in behaviors. You've already used __init__ to customize how objects are created. If we create an __add__(self, other) method on our class, the Python interpreter will use it when instances of the class are being added with the + operator. The name of the second parameter (other in this example) is just a convention - you can use any valid parameter name. Here's an example:

class Point:
    def __init__(self, x: int, y: int) -> None:
        self.x = x
        self.y = y

    def __add__(self, other: "Point") -> "Point":
        x = self.x + other.x
        y = self.y + other.y
        return Point(x, y)


p1 = Point(4, 5)
p2 = Point(2, 3)
p3 = p1 + p2
# p3 is (6, 8)

Now, when p1 + p2 is executed, under the hood the Python interpreter just calls p1.__add__(p2).