Python program on gopigo

Traceback (most recent call last):
File “”, line 10, in <module>
File “/usr/lib/python2.7/lib-tk/”, line 1813, in init = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: no display name and no $DISPLAY environment variable

If I use my own image, I run python, I can execute command=tk.Tk(), if I use your image, it will give an error. And if I modified my image to your image(option 3 of SD card ), it will still give this error. What is happened?
Looking forward your answer

Which file is this? Is this one of our example files?