Google App Engine - Querying for arrays containing a value

Google App Engine - Querying for arrays containing a value

I have a GAE Datastore table with an array field in it (containing a few
strings). I would like to filter this table, based on all array fields
that contain a specific string. How can i do that ? I didn't see a
'contains' operator in GQL, and the 'in' operator works the other way
around. Do I just need to loop over all entities and do the check myself ?
(P.S. I'm using Python in my work with GAE).