[Tutor] A Simple Tkinter Control Program--Slight Problem

[Wayne Watson]

Here's what I think the author meant in discussing a control variable sample
program. <http://effbot.org/tkinterbook/entry.htm>
========================
from Tkinter import *

v=StringVar()

e = Entry(master, textvariable=v)
e.pack()
e.focus_set()

v.set("a default value")
s = v.get()

mainloop()

The problem is that Python objects with the msg:
    AttributeError: 'NoneType' object has no attribute 'tk'

What corrects this? The full sample program below it runs fine:
=======================
from Tkinter import *

master = Tk()

e = Entry(master)
e.pack()

e.focus_set()

def callback():
     print e.get()

b = Button(master, text="get", width=10, command=callback)
b.pack()

mainloop()
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                                W. eWatson

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