The just operator returns an observable that emits only one item, and then it completes immediately. The marble diagram of this operator is shown in the following figure:

Its prototype is as follows:

Observable.just(value, scheduler=None)

The following is an example of how to use it:

number = Observable.just(1)
number.subscribe(
        on_next=lambda i: print("item: {}".format(i)),
        on_error=lambda e: print("error: {}".format(e)),
        on_completed=lambda: print("completed")
    )

Upon subscription, the number 1, provided in the just operator, is emitted as the only item of the observable before the observable completes:

item: 1
completed