It's clearly explained code that's shorter than you would have thought. Norvig showcases his expertise in manipulating both the basic data structure of the Python language, and a deep understanding of the underlying problem and methods for solving it.
Ouch. I admire Norvig, but that's one of the most horrible pieces of code I have ever seen. I've read that page several times, I've even translated it to C# - https://github.com/mdpopescu/public/blob/master/Sudoku/Sudok... - but I still think it's incredibly ugly. I will admit that my knowledge of Python is very basic but still, if I had to maintain that code without the entire description around it, I'd give up and rewrite it.
It's clearly explained code that's shorter than you would have thought. Norvig showcases his expertise in manipulating both the basic data structure of the Python language, and a deep understanding of the underlying problem and methods for solving it.