title: what is the (ev=none) argument?
body: <p>What is the <code>(ev=none)</code> argument in the below code?   </p>
  
  <pre><code>// This program detects shock and responds by turning an LED on. 
  
  import RPi.GPIO as GPIO
  
  #!/usr/bin/env python
  import RPi.GPIO as GPIO
  
  ShockPin = 11
  LedPin   = 12
  
  Led_status = 1
  
  def setup():
      GPIO.setmode(GPIO.BOARD)       
      GPIO.setup(LedPin, GPIO.OUT)   
      GPIO.setup(ShockPin, GPIO.IN, pull_up_down=GPIO.PUD_UP)
      GPIO.output(LedPin, GPIO.HIGH) 
  def swLed(ev=None):
      global Led_status
      Led_status = not Led_status
      GPIO.output(LedPin, Led_status)  
      print "led: " + ("on" if Led_status else "off")
  
  def loop():
      GPIO.add_event_detect(ShockPin, GPIO.FALLING, callback=swLed, bouncetime=200)
      while True:
          pass   
  
  def destroy():
      GPIO.output(LedPin, GPIO.LOW)      # led off
      GPIO.cleanup()                     # Release resource
  
  if __name__ == '__main__':     # Program start from here
      setup()
      try:
          loop()
      except KeyboardInterrupt:  
          destroy()
  </code></pre>