MongoDB 3.0 introduced commands to retrieve metadata, in place of queries on system collections. SERVER-15994 then changed the new commands to require the slaveOk bit be set when run against a secondary/slave. PyMongo 2.8's MongoClient calls these commands with read preference PRIMARY, ignoring the read preference configured at collection, command, or client level, and doesn't set the slaveOk bit. This leaves PyMongo 2.x users with no way to query metadata over a direct connection to a MongoDB 3.0+ secondary or slave. PyMongo 3.x already does the right thing.