All of the iControl calls work on the "active" partition you have selected with the API. You can switch the active partition you are working on with
1. Call Management.UserManagement.get_my_permission() which returns a list of partitions that the given user has access to along with the role associated with that partition.
2. For each partition returned from get_my_permissions(), call the Management.Partition.set_active_partition() with the specified partition name.
3. Call LocalLB.Class.get_external_class_list() which will return a list of external classes for the partition you previously called set_active_partition() with.
Keep in mind that the "active" partition is stored as a server variable for the given user and it will persist across iControl sessions, so if you are switching partitions, you may need to switch back to the one you want to work with on your next task.
I wrote this tech tip a while back on the use of Partitions from iControl, it might help out...