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.