code: def from_frequency(cls, freq):
          
          if 0 < freq <= 20000:
              return super(Tone, cls).__new__(cls, freq)
          raise ValueError('invalid frequency: %.2f' % freq)
docs: Construct a :class:`Tone` from a frequency specified in `Hz`_ which
          must be a positive floating-point value in the range 0 < freq <= 20000.
  
          .. _Hz: https://en.wikipedia.org/wiki/Hertz