Python 3.0+Iterators & generatorsfunction

__next__

iterator protocol

The iterator method next() calls: it returns the next item or raises StopIteration.

Syntax

def __next__(self): ...

StopIteration signals the end rather than an error: the for loop catches it itself. Once an iterator has raised StopIteration it must keep raising it. Python 2 called the method next; in Python 3 it is __next__, and next(it) became a built-in with an optional default.

Examples

class Counter:
    def __init__(self, limit):
        self.i, self.limit = 0, limit
    def __iter__(self):
        return self
    def __next__(self):
        if self.i >= self.limit:
            raise StopIteration
        self.i += 1
        return self.i
it = Counter(2)
print(next(it), next(it))  # → 1 2
print(next(it, 'конец'))  # → конец — default вместо StopIteration

See also

Common errors

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