Observer pattern

The observer pattern is useful for state monitoring and event handling situations. This pattern ensures a single core object can be monitored by an unknown, possibly expanding, array of "observer" objects. Whenever a value on the core object changes, it lets all the observer objects know that a change has occurred, by calling an update() method. Each observer may be responsible for different tasks whenever the core object changes; the core object doesn't know or care what those tasks are, and the observers don't typically know or care what other observers are doing. Here it is in UML:

Observer example

The observer pattern could be useful in a redundant backup system. We can write a core object that maintains certain values, and then have one or more observers create serialized copies of that object. These copies might be stored in a database, on a remote host, or in a local file, for example. Let's implement the core object using properties:

class Inventory:
    def __init__...