subjects in rxpy observable if the subject was subscribed to the observable before the observers were subscribed to the subjectthen all the data could have been published before the observers were registered with the subject the output from this program isset up subject received function received ( datetime datetime( )lambda received ( datetime datetime( )received on_next ( datetime datetime( )subject received function received ( datetime datetime( )lambda received ( datetime datetime( )received on_next ( datetime datetime( )subject received function received ( datetime datetime( )lambda received ( datetime datetime( )received on_next ( datetime datetime( )subject received function received ( datetime datetime( )lambda received ( datetime datetime( )received on_next ( datetime datetime( )data stream completed received completed notification done as can be seen from this output the numbers and are received by all of the observers once the subject has added the timestamp observer concurrency by default rxpy uses single threaded modelthat is observables and observers execute in the same thread of execution howeverthis is only the default as it is the simplest approach it is possible to indicate that when observer subscribes to an observable that it should run in separate thread using the scheduler keyword parameter on the