Question

How can I get a returned object from a function that is executed as a Tkinter callback?

import Tkinter as Tk
from functools import partial

def square(x):
    return x*x

root = Tk.Tk()
var = Tk.IntVar(root, value=0) #the variable the gets passed to the class call
menu = Tk.OptionMenu(root, var, *[0,1,2,3,4,5]) #a drop-down list to choose a value for the variable
menu.pack()
button = Tk.Button(root, text='click', command = partial(square,var.get())) #a button that calls the class
button.pack()
root.mainloop()

Obviously this is a simplified example. In reality the function called by the button will return objects, which I wish to append to a list of objects that will be held in the main Python namespace for further operations.

Anyway, here the user is able to choose an argument for the function using a GUI, and press a button that will execute the function. The return value of the function, however, seems doomed to be lost to the aether, since the callback won’t accept returns. Can this be overcome without the use of an ugly global in the definition of square(x)?

Answer 1

The notion of “returning” values from callbacks doesn’t make sense in the context of an event driven program. Callbacks are called as the result of an event, so there’s nowhere to return a value to.

As a general rule of thumb, your callbacks should always call a function, rather than using functools.partial or lambda. Those two are fine when needed, but if you’re using an object-oriented style of coding they are often unnecessary, and lead to code that is more difficult to maintain than it needs to be.

For example:

def compute():
    value = var.get()
    result = square(value)
    list_of_results.append(result)

button = Tk.Button(root, text='click', command = compute)
...

This becomes much easier, and you can avoid global variables, if you create your application as a class:

class App(...):
    ...
    def compute():
        ...
        result = self.square(self.var.get())
        self.results.append(result)