The OpenDXL Python Client does not currently provide a method to directly determine the identifier of the client. For "open" clients it is actually the DXL broker that assigns the identifier.
With that said, it is pretty simple to determine what the identifier is. This information is included in the response message received for any request that is sent by the client. The client identifier is located in the client_ids property of the response message.
The following code attempts to invoke a service that does not exist. The response from the broker will include the client's identifier (as it is set to route directly back to the client).
Hope this helps,