By default PyMongo's Connection and ReplicaSetConnection objects do fire-and-forget write operations. This means they don't receive (or even request) a response from the server for any write operation (insert, save, update, remove). To enable write acknowledgement the developer has to set safe=True on the connection, database, or collection instance, or pass safe=True as a keyword parameter to the write method.

PyMongo will support new MongoClient and MongoReplicaSetClient objects that, by default, request acknowledgement that a write was successful, raising an appropriate exception otherwise. The old fire-and-forget behavior can still be configured with these new objects using w=0.

Other changes:

• auto_start_request will default to False, since this is more efficient (fewer sockets, less time spent creating connections) and requests are not required for read-your-writes consistency if writes are acknowledged.
• The network_timeout parameter is not supported by these classes. Use socketTimeoutMS instead.

Connection and ReplicaSetConnection will be deprecated but continue to exist, with their current behavior, until at least PyMongo 3.0.