Private method to return the variables in a SAS Data set that are of type char

        :param data: SAS Data object to process
        :return: list of character variables
        :rtype: list

def _charlist(self, data) -> list:
        """
        Private method to return the variables in a SAS Data set that are of type char

        :param data: SAS Data object to process
        :return: list of character variables
        :rtype: list
        """
        # Get list of character variables to add to nominal list
        char_string = """
        data _null_; file LOG;
          d = open('{0}.{1}');
          nvars = attrn(d, 'NVARS');
          put 'VARLIST=';
          do i = 1 to nvars;
             vart = vartype(d, i);
             var  = varname(d, i);
             if vart eq 'C' then
                put var; end;
          put 'VARLISTend=';
        run;
        """
        # ignore teach_me_SAS mode to run contents
        nosub = self.sas.nosub
        self.sas.nosub = False
        ll = self.sas.submit(char_string.format(data.libref, data.table + data._dsopts()))
        self.sas.nosub = nosub
        l2 = ll['LOG'].partition("VARLIST=\n")
        l2 = l2[2].rpartition("VARLISTend=\n")
        charlist1 = l2[0].split("\n")
        del charlist1[len(charlist1) - 1]
        charlist1 = [x.casefold() for x in charlist1]
        return charlist1