3021: Add comment about IPv4/IPv6 trap in websocket client.

[arvados.git] / sdk / python / arvados / events.py

diff --git a/sdk/python/arvados/events.py b/sdk/python/arvados/events.py
index 7e8cdc3282113ad7984c9e9d93cdb187a0c77b4f..94dd62b4ed1370cd239106c78abbbf8b249cb832 100644 (file)
--- a/sdk/python/arvados/events.py
+++ b/sdk/python/arvados/events.py
@@ -22,6 +22,11 @@ class EventClient(WebSocketClient):
             ssl_options['cert_reqs'] = ssl.CERT_NONE
         else:
             ssl_options['cert_reqs'] = ssl.CERT_REQUIRED
+
+        # Warning: If the host part of url resolves to both IPv6 and
+        # IPv4 addresses (common with "localhost"), only one of them
+        # will be attempted -- and it might not be the right one. See
+        # ws4py's WebSocketBaseClient.__init__.
         super(EventClient, self).__init__(url, ssl_options=ssl_options)
         self.filters = filters
         self.on_event = on_event
@@ -107,9 +112,9 @@ class PollClient(threading.Thread):
 
 def subscribe(api, filters, on_event, poll_fallback=15):
     '''
-    api: Must be a newly created from arvados.api(cache=False), not shared with the caller, as it may be used by a background thread.
+    api: a client object retrieved from arvados.api(). The caller should not use this client object for anything else after calling subscribe().
     filters: Initial subscription filters.
-    on_event: The callback when a message is received
+    on_event: The callback when a message is received.
     poll_fallback: If websockets are not available, fall back to polling every N seconds.  If poll_fallback=False, this will return None if websockets are not available.
     '''
     ws = None