Args:
            keys (list or str): the column name(s) to apply the `func` to
            func (callable): applied to each element in the specified columns

def map_column(self, keys, func):
        """
        Args:
            keys (list or str): the column name(s) to apply the `func` to
            func (callable): applied to each element in the specified columns
        """
        return [[func(v) for v in self[key]] for key in keys]