Trying to get the value from a Tkinter scale and put it into a Label

Question

I have a small Python program that takes the value of a Tkinter scale and puts it into a label.

#!/usr/bin/python

from Tkinter import *

class App:

    strval = StringVar()
    def __init__(self,master):

        frame = Frame(master)
        frame.pack()
        self.slide = Scale(frame, command = self.up, from_ = 1, to = 100)
        self.out = Label(frame, textvariable = self.strval)
        self.slide.pack()
        self.out.pack()

    def up(self,newscale):
        amount = str(newscale)
        self.strval.set(amount)


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

When I run the program it gives me and error message:

Traceback (most recent call last):
  File "/Users/alex/Desktop/Python/Tkinter/scale_Entry.py", line 5, in <module>
    class App:
  File "/Users/alex/Desktop/Python/Tkinter/scale_Entry.py", line 7, in App
    strval = StringVar()
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/lib-tk/Tkinter.py", line 254, in __init__
    Variable.__init__(self, master, value, name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/lib-tk/Tkinter.py", line 185, in __init__
    self._tk = master.tk
AttributeError: 'NoneType' object has no attribute 'tk'
Exception exceptions.AttributeError: "StringVar instance has no attribute '_tk'" in <bound method StringVar.__del__ of <Tkinter.StringVar instance at 0x69f238>> ignored
logout

I'm not too sure what is going wrong and I'm completely crap at Tk interfacing. I would love if someone could explain what I'm doing wrong.

Answer 1

That happens because you are creating the StringVar before creating the Tk root element. If you move the statement root = Tk() before the definition of the class, you'll see how it works as expected.

However, the ideal solution would be write it in a way that you don't depend on the order to make it work, so I'd suggest you to create the StringVar in the constructor:

class App:
    def __init__(self,master):
        frame = Frame(master)
        frame.pack()
        self.strval = StringVar(frame)
        # ...