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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 xform(self, number, base):
 """ Get a number as a string. :param number: a number :type number: list of int :param int base: the base in which this number is being represented :raises BasesValueError: if config is unsuitable for number """

        if self.CONFIG.use_letters:
            digits = \
               self._UPPER_DIGITS if self.CONFIG.use_caps else \
               self._LOWER_DIGITS
            return ''.join(digits[x] for x in number)
        separator = '' if base <= 10 else self.CONFIG.separator
        return separator.join(str(x) for x in number)