The count operator counts the number of items in the source observable that meet some criteria. The following figure shows the marble diagram of this operator:

Its prototype is the following:

Observable.count(self, predicate=None)

Here, the predicate argument is a function that returns True if the item must be counted, or False otherwise.

Here is an example of the count operator:

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

numbers.count(lambda i: i > 2).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 2
on_completed