all Python 3OOPfunction

__len__

magic methods

The method len(obj) calls. It must return a non-negative integer.

Syntax

def __len__(self) -> int: ...

Besides len(), __len__ defines truthiness: an object without __bool__ is false when __len__ returns 0, which is why if container: works for your own collections too. A negative or non-integer value raises ValueError or TypeError right inside len().

Examples

class Deck:
    def __init__(self, cards):
        self.cards = list(cards)
    def __len__(self):
        return len(self.cards)
d = Deck('AKQ')
print(len(d))  # → 3
print(bool(Deck('')))  # → False — пустая колода ложна

See also

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