code: def animate_police_lights(self, color1, color2, group1='LEFT', group2='RIGHT', sleeptime=0.5, duration=5, block=True):
          
  
          def _animate_police_lights():
              self.all_off()
              even = True
              start_time = dt.datetime.now()
  
              while True:
                  if even:
                      self.set_color(group1, color1)
                      self.set_color(group2, color2)
                  else:
                      self.set_color(group1, color2)
                      self.set_color(group2, color1)
  
                  if self.animate_thread_stop or duration_expired(start_time, duration):
                      break
  
                  even = not even
                  sleep(sleeptime)
  
              self.animate_thread_stop = False
              self.animate_thread_id = None
  
          self.animate_stop()
  
          if block:
              _animate_police_lights()
          else:
              self.animate_thread_id = _thread.start_new_thread(_animate_police_lights, ())
docs: Cycle the ``group1`` and ``group2`` LEDs between ``color1`` and ``color2``
          to give the effect of police lights.  Alternate the ``group1`` and ``group2``
          LEDs every ``sleeptime`` seconds.
  
          Animate for ``duration`` seconds.  If ``duration`` is None animate for forever.
  
          Example:
  
          .. code-block:: python
  
              from ev3dev2.led import Leds
              leds = Leds()
              leds.animate_police_lights('RED', 'GREEN', sleeptime=0.75, duration=10)