code: def frequencies(self, sides=None):
  
          
          # use the attribute sides except if a valid sides argument is provided
          if sides is None:
              sides = self.sides
          if sides not in self._sides_choices:
              raise errors.SpectrumChoiceError(sides, self._sides_choices)
  
          if sides == 'onesided':
              return self._range.onesided()
          if sides == 'twosided':
              return self._range.twosided()
          if sides == 'centerdc':
              return self._range.centerdc()
docs: Return the frequency vector according to :attr:`sides`