import collections
import typing

T = typing.TypeVar('T')


class ChainableIterator(typing.Iterator[T], collections.Iterator):
    def __init__(self, *iterators: typing.Iterator[T]):
        self._index = -1
        self._iterators = list((a for a in b) for b in iterators)

    def __next__(self):
        try:
            if self._index < 0:
                raise StopIteration
            return next(self._iterators[self._index])
        except StopIteration as e:
            if self._index < len(self._iterators) - 1:
                self._index += 1
                return next(self)
            raise e

    def chain(self, *iterators: typing.Iterator[T]) -> 'ChainableIterator[T]':
        self._iterators.extend((a for a in b) for b in iterators)
        return self


if __name__ == '__main__':
    iterator = ChainableIterator((1, 2, 3), (4, 5, 6))
    iterator.chain((7, 8, 9), (10, 11, 12))

    for i in range(1, 13):
        assert i == next(iterator)

    try:
        next(iterator)
        assert False
    except StopIteration:
        pass