title: python timer mystery
body: <p>Well, at least a mystery to me.  Consider the following:</p>
  
  <pre><code>import time
  import signal
  
  def catcher(signum, _):
      print "beat!"
  
  signal.signal(signal.SIGALRM, catcher)
  signal.setitimer(signal.ITIMER_REAL, 2, 2)
  
  while True:
      time.sleep(5)
  </code></pre>
  
  <p>Works as expected i.e. delivers a "beat!" message every 2 seconds. Next, no output is produced:</p>
  
  <pre><code>import time
  import signal
  
  def catcher(signum, _):
      print "beat!"
  
  signal.signal(signal.SIGVTALRM, catcher)
  signal.setitimer(signal.ITIMER_VIRTUAL, 2, 2)
  
  while True:
      time.sleep(5)
  </code></pre>
  
  <p>Where is the issue?</p>