tkinter frame not expanding to fit window?

[Zobeid Zuma]

I've just started working through the tutorial here → http://
www.tkdocs.com/tutorial/firstexample.html and I already hit some behavior 
that I don't understand. The frame doesn't expand when I resize the 
window! The tutorial says these lines should do it:

	mainframe.columnconfigure(0, weight=1)
	mainframe.rowconfigure(0, weight=1) 

As an experiment I tried commenting those lines out, just to see if 
anything would change, and nothing did. So it seems they aren't working 
for me, but I don't know why. Any suggestions?

This is all running on Ubuntu MATE 17.10 and tk8.6.