The contains operator emits a True item if the source observable contains a specific item. Otherwise, it returns a False item. The following figure shows the marble diagram of this operator:

Its prototype is the following:

numbers = Observable.from_([1, 2, 3, 4])

numbers.contains(3).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")
)

This example gives the following result:

on_next True
on_completed