The client hangs when you call conn.database_names() whilst a replica set member is doing initialSyncOplogApplication

>>> import pymongo
>>> conn = pymongo.connection.Connection('localhost', 27018, slave_okay=True)
>>> print conn.database_names()

rs.status()
...

{ "_id" : 2, "name" : "rs3c:27018", "health" : 1, "state" : 3, "stateStr" : "RECOVERING", "self" : true }

,

Log output:
Fri Jan 28 14:58:00 [rs_sync] replSet initialSyncOplogApplication 5100000
Fri Jan 28 14:58:23 [rs_sync] replSet initialSyncOplogApplication 5200000