""" * buttonHandler is a function object, and can be used as a callback binding. * buttonHandler() (note the parenthesis) on the other hand is an actual call to the "buttonHandler" function. At the time that the statement self.button1 = Button(self.myContainer1, command = self.buttonHandler(button_name, 1, "Good stuff!")) is executed, it is actually making a call to the "buttonHandler" routine. The buttonHandler routine executes, printing its message, and returns the results of the call (in this case, the None object). Then the button's "command" option is bound to the results of the call. In short, the command is bound to the "None" object. And that is why, when you click on either of the buttons, nothing happens. IS THERE A SOLUTION? So... what's the solution? Is there any way to parameterize, and reuse, an event-handler function? Yes. There are a couple of generally recognized techniques for doing this. One uses the Python built-in "lambda" function, and the other uses a technique called "currying". In this program we will discuss how to work with lambda, and in the next program we will look at currying. I'm not going to try to explain how lambda and currying work -- it is too complicated and too far off-track from our main goal, which is to get Tkinter programs working. So I'm going to simply treat them as black boxes. I won't talk about how they work -- I'll only talk about how to work with them. So let's look at lambda. COMMAND BINDING Originally, we thought the following statement might work: self.button1 = Button(self.myContainer1, command = self.buttonHandler(button_name, 1, "Good stuff!") ) ... but we found out that it didn't work the way we thought it would. The way to do what we want is to re-write this statement this way: self.button1 = Button(self.myContainer1, command = lambda arg1=button_name, arg2=1, arg3="Good stuff!" : self.buttonHandler(arg1, arg2, arg3) ) EVENT BINDING Happily, lambda also gives us a way to parameterize event binding. Instead of: self.button1.bind("", self.buttonHandler_a(event, button_name, 1, "Good stuff!")) (which wouldn't work, because there was no way to include the event argument in the argument list), we can use lambda, this way: # event binding -- passing the event as an argument self.button1.bind("", lambda event, arg1=button_name, arg2=1, arg3="Good stuff!" : self.buttonHandler_a(event, arg1, arg2, arg3) ) [Note that "event" here is a variable name -- it is not a Python keyword or anything like that. This example uses the name "event" for the event argument, but some discussions of this technique use the name "e" for the event argument, and we could just as easily have called it "event_arg" if we had wanted to.] One of the nice features of using lambda is that we can (if we wish), simply not pass the event argument. If we don't pass the event argument, then we can call the self.buttonHandler function directly, instead of indirectly through the self.buttonHandler_a function. To illustrate this technique, we will code the event binding for button2 differently than we did for button1. This is what we do with button2: # event binding -- without passing the event as an argument self.button2.bind("", lambda event, arg1=button_name, arg2=2, arg3="Bad stuff!" : self.buttonHandler(arg1, arg2, arg3) ) PROGRAM BEHAVIOR If you run this program, it will behave just as we wish. Note that you can change the keyboard focus from the OK to the CANCEL button, and back again, by pressing the TAB key on the keyboard. In particular, you should experiment with invoking the OK button by pressing the key on the keyboard. If you invoke the OK button via a keypress of the key, you will be going through the buttonHandler_a function, and you will also get a message from it, printing information about the event that has been passed to it. In any case, whether you click on one of the button widgets with the mouse, or invoke a widget via a keypress on the keyboard, it will nicely print the arguments that were passed to the buttonHandler function. [revised: 2003-02-23] >""" from Tkinter import * class MyApp: def __init__(self, parent): self.myParent = parent self.myContainer1 = Frame(parent) self.myContainer1.pack() #------------------ BUTTON #1 ------------------------------------ button_name = "OK" # command binding self.button1 = Button(self.myContainer1, command = lambda arg1=button_name, arg2=1, arg3="Good stuff!" : self.buttonHandler(arg1, arg2, arg3) ) # event binding -- passing the event as an argument self.button1.bind("", lambda event, arg1=button_name, arg2=1, arg3="Good stuff!" : self.buttonHandler_a(event, arg1, arg2, arg3) ) self.button1.configure(text=button_name, background="green") self.button1.pack(side=LEFT) self.button1.focus_force() # Put keyboard focus on button1 #------------------ BUTTON #2 ------------------------------------ button_name = "Cancel" # command binding self.button2 = Button(self.myContainer1, command = lambda arg1=button_name, arg2=2, arg3="Bad stuff!": self.buttonHandler(arg1, arg2, arg3) ) # event binding -- without passing the event as an argument self.button2.bind("", lambda event, arg1=button_name, arg2=2, arg3="Bad stuff!" : self.buttonHandler(arg1, arg2, arg3) ) self.button2.configure(text=button_name, background="red") self.button2.pack(side=LEFT) def buttonHandler(self, argument1, argument2, argument3): print " buttonHandler routine received arguments:" \ , argument1.ljust(8), argument2, argument3 def buttonHandler_a(self, event, argument1, argument2, argument3): print "buttonHandler_a received event", event self.buttonHandler(argument1, argument2, argument3) print "\n"*100 # clear the screen print "Starting program tt078." root = Tk() myapp = MyApp(root) print "Ready to start executing the event loop." root.mainloop() print "Finished executing the event loop."