Question

I am trying to figure out exactly how to implement a callback function which does something more meaningful than just print output. I am fairly inexperienced, so I am not sure how callback functions should or can be implemented in Python (or in any other language, for that matter).

Consider the following Python code:

from Tkinter import *

def callbackfunc(*args):
  print "Hello World!"

class App:

  def __init__(self, master):
    frame = Frame(master)
    frame.pack()

    optionvalue = IntVar(master)
    optionvalue.set(2)
    optionvalue.trace("w", callbackfunc)
    self.optionmenu = OptionMenu(master, optionvalue, 1, 2, 3, 4)
    self.optionmenu.pack()

I am trying to implement an OptionMenu (a Tkinter widget) such that when its selected value is changed, my callback function does something meaningful—more specifically, it will change a global variable value, defined somewhere else in the program. As it is implemented above, it simply prints output (albeit successfully).

I cannot figure out how to pass parameters to my callback function. I do not want this particular callback function to return anything; however, I am curious as to how I would make my callback function return something, and how I would implement the rest of my program so that it could utilize those returned results, whatever those might be. Am I trying to implement a Python callback function in a way in which it was not intended to be implemented? If not, how do I make this one work?

Answer 1

It’s a little bit unclear what you mean by “pass parameters to my callback function.” You’re already doing that! For example:

from Tkinter import *

def callbackfunc(*args, **kwargs):
    print args, kwargs
    print "Hello World!"

class App(object):
    def __init__(self, master):
        frame = Frame(master)
        frame.pack()

        optionvalue = IntVar(master)
        optionvalue.set(2)
        optionvalue.trace("w", callbackfunc)
        self.optionmenu = OptionMenu(master, optionvalue, 1, 2, 3, 4)
        self.optionmenu.pack()

root = Tk()
app = App(root)
root.mainloop()

When run…

$ python foo.py 
('PY_VAR0', '', 'w') {}
Hello World!

So you see, when Tkinter calls your callback, it passes parameters to it. If you wanted to do something other than print them, you could store them in some state, by passing a method instead of a function.

from Tkinter import *

class App(object):
    def __init__(self, master):
        frame = Frame(master)
        frame.pack()

        optionvalue = IntVar(master)
        optionvalue.set(2)
        optionvalue.trace("w", self.callbackfunc)
        self.optionmenu = OptionMenu(master, optionvalue, 1, 2, 3, 4)
        self.optionmenu.pack()
        self.state = []

    def callbackfunc(self, *args):
        self.state.append(args)
        print self.state


root = Tk()
app = App(root)
root.mainloop()

When run…

$ python foo.py 
[('PY_VAR0', '', 'w')]
[('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w')]
[('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w')]

Also, perhaps you want to access the value of optionvalue. You could save a reference to it then:

from Tkinter import *

class App(object):
    def __init__(self, master):
        frame = Frame(master)
        frame.pack()

        self.optionvalue = IntVar(master)
        self.optionvalue.set(2)
        self.optionvalue.trace("w", self.callbackfunc)
        self.optionmenu = OptionMenu(master, self.optionvalue, 1, 2, 3, 4)
        self.optionmenu.pack()
        self.state = []

    def callbackfunc(self, *args):
        self.state.append(args)
        print self.state
        print self.optionvalue.get()


root = Tk()
app = App(root)
root.mainloop()

When run…

$ python foo.py 
[('PY_VAR0', '', 'w')]
1
[('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w')]
2
[('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w'), ('PY_VAR0', '', 'w')]
3

You could also use root.getvar(name) with name = 'PY_VAR0' (the first arg passed to the callback), as noob oddy suggests.