1 Answer
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It's a good question. From my prospective, the most difficult problems are the problems, where you have to use more rich libraries functions, classes, data structures and algorithms. And of course classes, functions and so on, defined by yourself. I don't have some very gut example, because I also just learn python currently, but you can find a great examples on github ("100+ Python challenging programming exercises"). You can also find some great exercises on exercism.io
I hope, it helps :)