[Tutor] Checkbox problem in Tkinter

[Peter Otten]

Alan Gauld wrote:

> 
> "ANKUR AGGARWAL" <coolankur2006 at gmail.com> wrote
> 
>> from Tkinter import *
>> root=Tk()
>> var=StringVar()
>> 
c=Checkbutton(root,text="hello",variable=var,onvalue="hhhh",offvalue="gggg")
>> c.pack()
>> root.mainloop()
> 
> FWIW I get a different problem, namely that the var does not seem
> to get updated nor does changling the var seem to alter the buuttons
> state. But at least the button displays OK with 'hello' in the label
> and
> I can select/deselect it with the mouse. its just the change to the
> variable that is broken.
> If I write an explicit event handler:
> 
> def f():
>       var = 'hhhh' if var == 'gggg' else 'gggg'
> 
> Then var changes OK. Here is my code(from Pythonwin):
> 
>>>> def f(): global var; var = 'on' if var == 'off' else 'off';print
>>>> var
> ...
>>>> top = Tk()
>>>> var = StringVar()
>>>> c = Checkbutton(top,text='txt', variable=var, onvalue='on',
>>>> offvalue='off', command=f)
>>>> c.pack()
>>>> top.mainloop()
> off
> on
> off
> on
> off
> on

I think that is confusing. Tkinter is not supposed to rebind the global name 
'var', it invokes its set() method. Try

from Tkinter import *

root = Tk()
var = StringVar()

def command():
    print var.get()

cb = Checkbutton(root, text="yadda", onvalue="on", offvalue="off", 
variable=var, command=command)
cb.pack()
root.mainloop()

You can also monitor changes to var's value directly:

from Tkinter import *

root = Tk()
var = StringVar()

def show(*args):
    print var.get()
var.trace_variable('w', show)
cb = Checkbutton(root, text="yadda", onvalue="on", offvalue="off", 
variable=var)
cb.pack()
root.mainloop()