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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 book(self) -> Book: """ GnuCash Book. Opens the book or creates an database, based on settings. """

        if not self.__book:
            # Create/open the book.
            book_uri = self.settings.database_path
            self.__book = Database(book_uri).open_book(
                for_writing=self.__for_writing)

        return self.__book