--- a/viewer.py	Wed Jul 27 23:55:31 2016 +0800
+++ b/viewer.py	Wed Jul 27 23:56:49 2016 +0800
@@ -309,7 +309,7 @@
 
     def listen(self, port, address='', **kwargs):
         name = self.__class__.__name__
-        logging.info('{} is serving on {}:{}'.format(name, address, port))
+        logging.info('%s is serving on %s:%d', name, address, port)
         super(Viewer, self).listen(port, address, **kwargs)