def to_type(self, dtype: type, *cols, **kwargs):
        """
        Convert colums values to a given type in the 
        main dataframe

        :param dtype: a type to convert to: ex: ``str``
        :type dtype: type
        :param \*cols: names of the colums
        :type \*cols: str, at least one
        :param \*\*kwargs: keyword arguments for ``df.astype``
        :type \*\*kwargs: optional

        :example: ``ds.to_type(str, "mycol")``
        """
        try:
            allcols = self.df.columns.values
            for col in cols:
                if col not in allcols:
                    self.err("Column " + col + " not found")
                    return
                self.df[col] = self.df[col].astype(dtype, **kwargs)
        except Exception as e:
            self.err(e, "Can not convert to type")
Convert colums values to a given type in the 
        main dataframe

        :param dtype: a type to convert to: ex: ``str``
        :type dtype: type
        :param \*cols: names of the colums
        :type \*cols: str, at least one
        :param \*\*kwargs: keyword arguments for ``df.astype``
        :type \*\*kwargs: optional

        :example: ``ds.to_type(str, "mycol")``