How to get() a widget’s IntVariable value from the widget?

I am trying to get the value of a Tkinter.Checkbutton‘s variable using .get(), but I get an error.

import Tkinter as tk

class myApp(tk.Tk):
    def __init__(self, parent):
        tk.Tk.__init__(self, parent)
        self.parent = parent
        self.grid()
        self.var = tk.IntVar()
        self.cb = tk.Checkbutton(self.parent, variable=self.var)
        self.cb.bind('<Button-1>', self.useValue)
        print self.var.get() # works
        print self.cb.get() # does not work
        print self.cb.cget('variable') # prints something like PYNUMVAR1 
        print self.cb.cget('variable').get() # error I mention below
        print self.cb.var.get() # error
        print self.cb.val.get() # error

    def useValue(self, event):
        print event.widget.cget('variable').get()) # why not?

if __name__ == '__main__':
    runit = myApp(None)
    runit.mainloop()

I tried plenty of combinations of everything I’ve seen all over stackoverflow and other tutorial sites. The error I get is:

AttributeError: '_tkinter.Tcl_Obj' object has no attribute 'get'

It should be an IntVar, not a _tkinter.Tcl_Obj object, which doesn’t have a get attribute, when I try, it raises an attribute error on self.var like this:

print self.var.crumpet()

The console lets me know that it is also that type of object. (It did this on my first experiment, but I cannot get it to recreate this error message on the IntVar instance in the example code above, so I think this might be wrong.)

I know I can just use get() on self.var, but if I’m passing the widget to a callback function via the event parameter, I would like to be able to get the value of it.

How can I get the value of the Checkbutton without having to do anything with the variable? Should I avoid assigning it a variable?

Answer

print event.widget.cget(‘variable’).get()) # why not?

It’s simply a limitation of tkinter. Conceptually it should work, and you can do the equivalent in tcl/tk, but it won’t work in tkinter.

In other words, it’s a bug, either in the design of tkinter or it’s implementation, I’m not sure which.

How the hell can I get the value of the Checkbutton without having to do anything with the variable? Should I avoid assigning it a variable?

A simple but effective solution is to attach a reference to the widget:

class myApp(tk.Tk):
    def __init__(...):
        ...
        self.cb.var = self.var
        ...
    def useValue(self, event):
        print event.widget.var.get()

Another solution is the under-documented getvar method which takes a tcl variable name as an argument and returns the value.

For example:

def useValue(self, event):
    widget = event.widget
    varname = str(widget.cget("variable"))
    print widget.getvar(varname)

---

Note: binding to <Button-1> means your callback will be called before the checkbutton is set or unset. It will always show the previous value. Either bind to <ButtonRelease-1>, or put a trace on the variable. The latter is preferable since a binding on <ButtonRelease-1> won’t fire if the user changes the checkbutton via the keyboard.