Implementation

There are many ways to implement Chain of Responsibility in Python, but my favorite implementation is the one by Vespe Savikko [j.mp/savviko]. Vespe's implementation uses dynamic dispatching in a Pythonic style to handle requests [j.mp/ddispatch].

Let's implement a simple event-based system using Vespe's implementation as a guide. The following is the UML class diagram of the system:

The Event class describes an event. We'll keep it simple, so in our case an event has only name:

class Event: 
    def __init__(self, name): 
        self.name = name 

    def __str__(self): 
        return self.name 

The Widget class is the core class of the application. The parent aggregation shown in the UML diagram indicates that each widget can have a reference to a parent object, which by convention, we assume is a Widget instance. Note, however, that according to the rules of inheritance, an instance of any of the subclasses of Widget (for example, an instance of MsgText) is also an instance...