automatically pick the best async_mode

example/app.py

@@ -1,9 +1,31 @@
 #!/usr/bin/env python
 
-# set this variable to "threading", "eventlet" or "gevent" to test the
-# different async modes
-async_mode = 'threading'
-
+# Set this variable to "threading", "eventlet" or "gevent" to test the
+# different async modes, or leave it set to None for the application to choose
+# the best option based on available packages.
+async_mode = 'gevent'
+
+if async_mode is None:
+    try:
+        import eventlet
+        async_mode = 'eventlet'
+    except ImportError:
+        pass
+
+    if async_mode is None:
+        try:
+            from gevent import monkey
+            async_mode = 'gevent'
+        except ImportError:
+            pass
+
+    if async_mode is None:
+        async_mode = 'threading'
+
+    print('async_mode is ' + async_mode)
+
+# monkey patching is necessary because this application uses a background
+# thread
 if async_mode == 'eventlet':
     import eventlet
     eventlet.monkey_patch()