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Now, in the context of a GUI, we can register instances of this class as event handlersfor buttons, even though the GUI expects to be able to invoke event handlers assimple functions with no arguments:cb1 = Callback('blue')cb2 = Callback('green')# 'Remember' blueB1 = Button(command=cb1)B2 = Button(command=cb2)# Register handlers# Register handlersWhen the button is later pressed, the instance object is called as a simple function,exactly like in the following calls. Because it retains state as instance attributes,though, it remembers what to do:cb1( )cb2( )# On events: prints 'blue'# Prints 'green'In fact, this is probably the best way to retain state information in the Python language—betterthan the techniques discussed earlier for functions (global variables,enclosing-function scope references, and default mutable arguments). With OOP,the state remembered is made explicit with attribute assignments.Before we move on, there are two other ways that Python programmers sometimestie information to a callback function like this. One option is to use default argumentsin lambda functions:cb3 = (lambda color='red': 'turn ' + color)print cb3( )# Or: defaultsThe other is to use bound methods of a class—a kind of object that remembers theself instance and the referenced function, such that it may be called as a simplefunction without an instance later:class Callback:def _ _init_ _(self, color):self.color = colordef changeColor(self):print 'turn', self.colorcb1 = Callback('blue')cb2 = Callback('yellow')B1 = Button(command=cb1.changeColor)B2 = Button(command=cb2.changeColor)# Class with state information# A normal named method# Reference, but don't call# Remembers function+selfWhen this button is later pressed, it’s as if the GUI does this, which invokes thechangeColor method to process the object’s state information:object = Callback('blue')cb = object.changeColorcb( )# Registered event handler# On event prints 'blue'This technique is simpler, but less general than overloading calls with _ _call_ _;again, watch for more about bound methods in the next chapter.504 | Chapter 24: Class Coding Details