'Args:
df_lists (list): a list of DataFlow.
When these dataflows have different sizes, JoinData will stop when any
of them is exhausted.'

def __init__(self, df_lists):

    self.df_lists = df_lists
    try:
        self._size = self.df_lists[0].size()
        for d in self.df_lists:
            assert (d.size() == self._size), 'All    DataFlow    must    have    the    same    size!    {}    !=    {}'.format(d.size(), self._size)
    except Exception:
        logger.info('[JoinData]    Size    check    failed    for    the    list    of    dataflow    to    be    joined!')
