<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def zero_nan(self, *cols):
 """ Converts zero values to nan values in selected columns :param \*cols: names of the colums :type \*cols: str, at least one :example: ``ds.zero_nan("mycol1", "mycol2")`` """

        if len(cols) == 0:
            self.warning("Can not nan zero values if a column name "
                         "is not provided")
        df = self._zero_nan(*cols)
        if df is None:
            self.err("Can not fill zero values with nan")
            return
        self.df = df