def batch(reader, batch_size, drop_last=False):
    """
    This operator creates a batched reader which combines the data from the
    input reader to batched data.

    Args:
        reader(generator): the data reader to read from.
        batch_size(int): size of each mini-batch.
        drop_last(bool, optional): If set to True, the last batch is dropped when
            the size of last batch is not equal to batch_size, if set to False,
            it will not. Default: False.
    Returns:
        The batched reader.

    Return Type:
        generator

    Examples:
        .. code-block:: python

            import paddle.fluid as fluid
            def reader():
                for i in range(10):
                    yield i
            batch_reader = fluid.io.batch(reader, batch_size=2)

            for data in batch_reader():
                print(data)

            # Output is
            # [0, 1]
            # [2, 3]
            # [4, 5]
            # [6, 7]
            # [8, 9]
    """

    def batch_reader():
        r = reader()
        b = []
        for instance in r:
            b.append(instance)
            if len(b) == batch_size:
                yield b
                b = []
        if drop_last == False and len(b) != 0:
            yield b

    # Batch size check
    batch_size = int(batch_size)
    if batch_size <= 0:
        raise ValueError("batch_size should be a positive integeral value, "
                         "but got batch_size={}".format(batch_size))

    return batch_reader
