def __init__(self, df_lists):
        """
        Args:
            df_lists (list): a list of DataFlow.
                All DataFlow must implement ``__len__()``.
        """
        super(RandomMixData, self).__init__()
        self.df_lists = df_lists
        self.sizes = [len(k) for k in self.df_lists]

        Args:
            df_lists (list): a list of DataFlow.
                All DataFlow must implement ``__len__()``.
        
tensorpack/dataflow/common.py