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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def change_sheet(self, sheet_name_or_num):
 """ Calling this method changes the sheet in anticipation for the next time you create an iterator. If you change the active sheet while iterating on a UnicodeReader instance, it will continue to iterate correctly until completion. The next time you iterate through reader, it will begin all over again at whichever sheet you most recently changed to. """

        if isinstance(sheet_name_or_num, int):
            self._sheet = self.__wb[self.__wb.sheetnames[sheet_name_or_num]]
        elif isinstance(sheet_name_or_num, basestring):
            self._sheet = self.__wb[sheet_name_or_num]
        else:
            reason = "Must enter either sheet name or sheet number."
            raise Exception(reason)