all Python 3OOPfunction

__add__

magic methods

The method behind +: a + b calls a.__add__(b). Returning NotImplemented hands the turn to b.__radd__(a).

Syntax

def __add__(self, other): ...

When __add__ cannot handle the other operand's type it should return NotImplemented rather than raise TypeError: Python then tries the reflected __radd__ of the right operand and only after that reports an error. a += b looks for __iadd__ first and falls back to __add__ only without it — which is exactly how immutable types are meant to work.

Examples

class Money:
    def __init__(self, amount):
        self.amount = amount
    def __add__(self, other):
        if not isinstance(other, Money):
            return NotImplemented
        return Money(self.amount + other.amount)
print((Money(5) + Money(7)).amount)  # → 12

class Money:
    def __init__(self, amount):
        self.amount = amount
    def __add__(self, other):
        return NotImplemented
Money(1) + 2  # → TypeError: unsupported operand type(s) for +

See also

Common errors

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