Operator Overloading

Operator methods let objects define arithmetic and comparison syntax.

__add__ defines how the + operator combines two objects. Checking the operand type and returning NotImplemented for foreign types lets Python try the other operand's reflected method instead of crashing inside yours.

Source

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        if not isinstance(other, Vector):
            return NotImplemented
        return Vector(self.x + other.x, self.y + other.y)

    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

print(Vector(2, 3) + Vector(4, 5))

Output

Vector(6, 8)

a + bdispatchesa.__add__(b)Defining __add__ on a class lets + dispatch into the class's own behavior.

__eq__ defines value equality for ==. Without it, user-defined objects compare by identity. Returning NotImplemented for foreign types matters most here: equality against an unrelated value should answer False, never raise Python falls back to identity when both sides decline.

Source

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __eq__(self, other):
        if not isinstance(other, Vector):
            return NotImplemented
        return (self.x, self.y) == (other.x, other.y)

print(Vector(1, 1) == Vector(1, 1))
print(Vector(1, 1) == 5)

Output

True
False

A useful __repr__ makes operator results inspectable while debugging.

Source

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        if not isinstance(other, Vector):
            return NotImplemented
        return Vector(self.x + other.x, self.y + other.y)

    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

print(repr(Vector(2, 3) + Vector(4, 5)))

Output

Vector(6, 8)

Notes

Overload operators only when the operation is unsurprising.
Return NotImplemented when an operand type is unsupported.
Implement equality deliberately when value comparison matters.

Run the complete example

Expected output

Vector(6, 8)
True
False

Execution time appears here after you run the example.

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Operator Overloading

See also

Run the complete example

Expected output