Hands-On Reactive Programming with Python

The sum operator

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

Figure 9.41: The sum 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 ...