title: BLE subscribe to notification using gatttool or bluepy
body: <p>I am writing a program using bluepy that listen for a characteristic sent by a bluetooth device. I can also use any library or language, the only constraint is to run on Linux and not in mobile environment (it seems is widely used only in mobile devices, no one use BLE with desktop). 
  Using bluepy I register the delegate and after trying to register for notification calling <code>write('\x01\x00')</code> as described in the bluetooth rfc.
  But it doesn't work, any notification for the characteristic is received.
  Maybe I am wrong in writing the message for subscribing.
  Is there an error in the small snippet I wrote? Thank you so much.</p>
  
  <pre><code>class MyDelegate(btle.DefaultDelegate):
  
      def __init__(self, hndl):
          btle.DefaultDelegate.__init__(self)
     self.hndl=hndl;
  
     def handleNotification(self, cHandle, data):
     if (cHandle==self.hndl):
              val = binascii.b2a_hex(data)
              val = binascii.unhexlify(val)
              val = struct.unpack('f', val)[0]
              print str(val) + " deg C"
  
  
  p = btle.Peripheral("xx:xx:xx:xx", "random")
  
  try:
     srvs = (p.getServices());
     chs=srvs[2].getCharacteristics();
     ch=chs[1];
     print(str(ch)+str(ch.propertiesToString()));
     p.setDelegate(MyDelegate(ch.getHandle()));
     # Setup to turn notifications on, e.g.
     ch.write("\x01\x00");
  
     # Main loop --------
     while True:
        if p.waitForNotifications(1.0):
        continue
  
        print "Waiting..."
  finally:
      p.disconnect();
  </code></pre>