[Tutor] The what and why of StringVar()

[Craig Hagerman]

I am wondering if someone can help explain "StringVar()" to me.  I was
trying to write a simple program using Tkinter to take input from an Entry
widget and then use a button command to pass it off to a function to do
something. But I was unsuccessful in accessing the variable attached to the
Entry widget itself. (I still don't understand why.)

After poking about in the "DEMO's" Folder I found a demo in the "Matt"
folder that allowed me to get around this problem by using StringVar().
However I don't understand why this is used, how it works or why I need it.
So far I can't find any documentation either on line of in my 3 Python
books (hope I didn't stupidly overlook something). Seeing as how this is a
necessary way to access the Entry widget variable I would like to
understand it. I thought that perhaps this method is part of the string
module (since Matt's script imports that module, and dir(string) shows
StringVar being a method) but it works without importing the string module.

Any help would be appreciated. I will copy Matt's example script below
minus comments so that you can see what I am talking about.

Craig Hagerman

		-----Matt's entry-with-shared-variable.py script-----


from Tkinter import *
import string

# This program  shows how to make a typein box shadow a program variable.

class App(Frame):
    def __init__(self, master=None):
	Frame.__init__(self, master)
	self.pack()

	self.entrythingy = Entry(self)
	self.entrythingy.pack()

	self.button = Button(self, text="Uppercase The Entry",
			     command=self.upper)
	self.button.pack()

	self.contents = StringVar()
	self.contents.set("this is a variable")
	self.entrythingy.config(textvariable=self.contents)

	self.entrythingy.bind('<Key-Return>', self.print_contents)

    def upper(self):


	str = string.upper(self.contents.get())
	self.contents.set(str)

    def print_contents(self, event):
	print "hi. contents of entry is now ---->", self.contents.get()

root = App()
root.master.title("Foo")
root.mainloop()