import itertools


def chain(*readers):
    """
    Use the input data readers to create a chained data reader. The new created reader
    chains the outputs of input readers together as its output, and it do not change
    the format of the outputs.

    **Note**:
        ``paddle.reader.chain`` is the alias of ``paddle.fluid.io.chain``, and
        ``paddle.fluid.io.chain`` is recommended to use.

    For example, if three input readers' outputs are as follows:
    [0, 0, 0],
    [10, 10, 10],
    [20, 20, 20].
    The chained reader will output:
    [0, 0, 0], [10, 10, 10], [20, 20, 20].

    Args:
        readers(list): input data readers.

    Returns:
        callable: the new chained data reader.

    Examples:
        ..  code-block:: python

            import paddle

            def reader_creator_3(start):
                def reader():
                    for i in range(start, start + 3):
                        yield [i, i, i]
                return reader

            c = paddle.reader.chain(reader_creator_3(0), reader_creator_3(10), reader_creator_3(20))
            for e in c():
                print(e)
            # Output:
            # [0, 0, 0]
            # [1, 1, 1]
            # [2, 2, 2]
            # [10, 10, 10]
            # [11, 11, 11]
            # [12, 12, 12]
            # [20, 20, 20]
            # [21, 21, 21]
            # [22, 22, 22]

    """

    def reader():
        rs = []
        for r in readers:
            rs.append(r())

        for e in itertools.chain(*rs):
            yield e

    return reader