PYTHON-3021 Send primaryPreferred when connected to standalone servers (#799)

Author: ShaneHarvey

pymongo/mongo_client.py

@@ -1170,8 +1170,8 @@ def _socket_for_reads(self, read_preference, session):
         # for topology type Single."
         # Thread safe: if the type is single it cannot change.
         topology = self._get_topology()
-        single = topology.description.topology_type == TOPOLOGY_TYPE.Single
         server = self._select_server(read_preference, session)
+        single = topology.description.topology_type == TOPOLOGY_TYPE.Single
 
         with self._get_socket(server, session) as sock_info:
             secondary_ok = (single and not sock_info.is_mongos) or (

test/mockupdb/test_op_msg_read_preference.py

@@ -158,15 +158,12 @@ def test(self):
         else:
             self.fail('unrecognized op_type %r' % operation.op_type)
         # For single mongod we send primaryPreferred instead of primary.
-        if (expected_pref == ReadPreference.PRIMARY and self.single_mongod
-                and operation.name != "command"):
+        if expected_pref == ReadPreference.PRIMARY and self.single_mongod:
             expected_pref = ReadPreference.PRIMARY_PREFERRED
-        with going(operation.function, client) as future:
+        with going(operation.function, client):
             request = expected_server.receive()
             request.reply(operation.reply)
 
-        future()  # No error.
-
         self.assertEqual(expected_pref.document,
                          request.doc.get('$readPreference'))
         self.assertNotIn('$query', request.doc)