code: def set_pwm_frequency(self, frequency, pin=None):
          
          if pin is None:
              self._set_pwm_frequency(frequency, None)
          else:
              pin_id = self._pin_mapping.get(pin, None)
              if pin_id:
                  self._set_pwm_frequency(frequency, pin_id)
              else:
                  raise KeyError('Requested pin is not mapped: %s' % pin)
docs: Sets PWM frequency, if supported by hardware
  
          If the driver supports per pin frequency setting, set pin to the
          desired frequency. If not, passing None means set to all. If only per
          pin frequency is supported and pin is None, raise RuntimeError.
  
          If you're developing a driver, implement
          _set_pwm_frequency(self, frequency, pin). Raise RuntimeError if pin
          was set but is not supported by the platform.
  
          @arg frequency pwm frequency to be set, in Hz
          @arg pin if the the driver supports it, the pin that will use
              `frequency` as pwm frequency. None for all/global.
  
          @throw RuntimeError if pin is None on a per pin only hardware, or if
              it's a valid pin on a global only hardware.
          @throw KeyError if pin isn't mapped.