Put screenshot.py back to work

build/android/screenshot.py

 #!/usr/bin/env python
 
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Takes a screenshot or a screen video capture from an Android device."""
 
 import logging
 import optparse
@@ skipping 51 matching lines @@
   video_options.add_option('-s', '--size', metavar='WIDTHxHEIGHT',
                            help='Frame size to use instead of the device '
                            'screen size.', default=None)
   parser.add_option_group(video_options)
 
   (options, args) = parser.parse_args()
 
   if options.verbose:
     logging.getLogger().setLevel(logging.DEBUG)
 
-  if not options.device and len(android_commands.GetAttachedDevices()) > 1:
+  devices = android_commands.GetAttachedDevices()
+
+  if not options.device and len(devices) > 1:
     parser.error('Multiple devices are attached. '
                  'Please specify device serial number with --device.')
+  elif not options.device and len(devices) == 1:
+    options.device = devices[0]
 
   if len(args) > 1:
     parser.error('Too many positional arguments.')
   host_file = args[0] if args else options.file
   device = device_utils.DeviceUtils(options.device)
 
   if options.video:
     _CaptureVideo(device, host_file, options)
   else:
     _CaptureScreenshot(device, host_file)
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())