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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 get_default_currency(self) -> Commodity: """ returns the book default currency """

        result = None

        if self.default_currency:
            result = self.default_currency
        else:
            def_currency = self.__get_default_currency()
            self.default_currency = def_currency
            result = def_currency

        return result