pubsub: improve error reporting if caller forgets to subscribe

This is an easy mistake to make -- at least, I keep making it. It
formerly resulted in a confusing crash, "AttributeError: 'NoneType'
object has no attribute 'can_read'", from parse_response(). I have had
to dig into the redis-py source code more than once to figure out what
went wrong.

With this patch, it still crashes, but with a clearer error that
clarifies what the calling code forgot to do.

Fixes issue #716.

1 files changed, 4 insertions, 0 deletions

diff --git a/redis/client.py b/redis/client.py
index d6bd5c3..d84c8b3 100755
--- a/redis/client.py
+++ b/redis/client.py
@@ -2226,6 +2226,10 @@ class PubSub(object):
     def parse_response(self, block=True, timeout=0):
         "Parse the response from a publish/subscribe command"
         connection = self.connection
+        if connection is None:
+            raise RuntimeError(
+                'pubsub connection not set: '
+                'did you forget to call subscribe() or psubscribe()?')
         if not block and not connection.can_read(timeout=timeout):
             return None
         return self._execute(connection, connection.read_response)