Manipulating read preferences

By default, all read operations (for example, database queries) are directed to the primary. On a busy website, however, this might not yield optimal performance, especially where the replica set is geographically dispersed. Accordingly, it's possible to specify a read preference when initiating a query.  

In the case of a pymongo.mongo_client.MongoClient instance, simple read preferences are expressed using the following values in conjunction with the readPreference **kwarg parameter:

Preference	Notes
primary	All read requests are directed to the primary. This is the default.
primaryPreferred	If this preference is set, requests are directed to the primary. If the primary is unavailable, however, the request falls back to one of the secondaries.
secondary	This preference causes requests to be directed to any secondary replica set member, but not to the primary.
secondaryPreferred...