When I attempt to do a simple Hello World type example of using python and
tkinter (from the "Learning Python" book), I get an abort due to "no
display name or DISPLAY environment varible not set".  Since there's
nothing in the Python book, or on any tkinter web page I could find, about
this DISPLAY variable, it seems that it should be a standard assignment of
some sort.  What is missing from my configuration?

To reproduce:

python
>>>  from tkinter import *
>>>  w = Button(text="Hello World!", command='exit')