Observer: defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

A very useful design pattern, often used in user interfaces and as part of the popular Model-View-Controller (MVC) pattern used, for instance, in Django. As I wrote out the code I found it very appealing that I did not need to change the subject at all to add new observers.

python weather.py