correctly autodetect asgi async mode (Fixes #122)

engineio/__init__.py

@@ -6,8 +6,11 @@
 if sys.version_info >= (3, 5):  # pragma: no cover
     from .asyncio_server import AsyncServer
     from .asyncio_client import AsyncClient
-    from .async_drivers.tornado import get_tornado_handler
     from .async_drivers.asgi import ASGIApp
+    try:
+        from .async_drivers.tornado import get_tornado_handler
+    except ImportError:
+        get_tornado_handler = None
 else:  # pragma: no cover
     AsyncServer = None
     AsyncClient = None

engineio/async_drivers/tornado.py

@@ -3,11 +3,8 @@
 from urllib.parse import urlsplit
 from .. import exceptions
 
-try:
-    import tornado.web
-    import tornado.websocket
-except ImportError:  # pragma: no cover
-    pass
+import tornado.web
+import tornado.websocket
 import six
 
 

engineio/asyncio_server.py

@@ -20,8 +20,10 @@ class AsyncServer(server.Server):
                        section in the documentation for a description of the
                        available options. Valid async modes are "aiohttp",
                        "sanic", "tornado" and "asgi". If this argument is not
-                       given, an async mode is chosen based on the installed
-                       packages.
+                       given, "aiohttp" is tried first, followed by "sanic",
+                       "tornado", and finally "asgi". The first async mode that
+                       has all its dependencies installed is the one that is
+                       chosen.
     :param ping_timeout: The time in seconds that the client waits for the
                          server to respond before disconnecting.
     :param ping_interval: The interval in seconds at which the client pings