query() method sends a SQL query to the database. If the query is an insert statement, the return value is the OID of the newly inserted row. If it is otherwise a query that does not return a result (i.e., is not a some kind of SELECT statement), it returns None. Otherwise, it returns a pgqueryobject that can be accessed via the getresult() or dictresult() methods or simply printed.
Read the upstream documentation. For corrections, first check the current manual.