The subscribe method of the Observable class returns a disposable object. When a subscription is not used anymore, it must be disposed to free all of the resources that the observable may have allocated:

numbers = Observable.from_([1, 2, 3])
subscription = numbers.subscribe(
    on_next = lambda i: print("on_next {}".format(i)),
    on_error = lambda e: print("on_error: {}".format(e)),
    on_completed = lambda: print("on_completed")
)
subscription.dispose()