The first operator returns the first item emitted on the source observable. Its marble diagram is shown in the following figure:

Its prototype is the following:

Observable.first(self, predicate=None)

Here, predicate is an optional function used to filter items in the source observable. If no predicate is provided, the first item of the observable is emitted.

Here is an example of this operator:

numbers = Observable.from_([1, 2, 3, 4, 5, 6])
numbers.first().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 1
on_completed