The sum operator computes the sum of all items emitted by the source observable. The following figure shows the marble diagram of this operator:

Its prototype is the following:

Observable.sum(self, key_selector=None)

Here, the key_selector argument is a transform function that returns the value to sum from an item. If no key_selector is provided, then the item itself is used.

Here is an example of the sum operator:

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

numbers1.sum().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 10
on_completed