Python 3.3+collectionsterm

collections.abc.Iterator

collections.abc — protocols

The iterator protocol: has __next__ and __iter__.

Syntax

isinstance(obj, collections.abc.Iterator)

Every Iterator is also Iterable, but not the other way round: a list can be looped over yet is not an iterator; iter(list) is. The practical value of the check is spotting single-use input — once drained, an iterator yields nothing on a second loop, so a function that needs two passes should materialise its argument into a list first.

Examples

import collections.abc as abc
print(isinstance(iter([1]), abc.Iterator))   # → True
print(isinstance([1, 2], abc.Iterator))  # → False
print(isinstance([1, 2], abc.Iterable))  # → True
print(isinstance(map(str, [1, 2]), abc.Iterator))  # → True
it = iter([1])
print(next(it, 'stop'), next(it, 'stop'))  # → 1 stop

See also

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