code wars is ruined by the 13 to 20 demographic

You are absolutely right. Don't know what I was thinking hahaha

would be best practice if documented & explained accordingly; after all, green computing and suitable for use in library code

While I'll agree that "best practice" isn't necessarily the best tag, I think your critique here is flawed.

This solution is O(n) time complexity in best, average, and worst cases. There are solutions that are O(1) in the best case (no solution is better than O(n) in the worst case), but whether they are going to be faster or not is largely going to depend on the actual data they receive in theory.

In practice, I've yet to see any solution in pure python that has tested faster than this solution with the exception of Blind4Basics' forks of this solution, which do greatly narrow down the second step.

Part of the reason this solution is fast, is because set is a bit of a cheat in that it uses a lot of C code under the hood. Assuming your unique number is not within 0.32% of the beginning of the array (a pretty safe bet), it's going to be faster than just iterating over the array in python.

That the code is compact is merely a nice bonus.

In any case, I'm vigorously against downvoting. Having a meaningful discussion about disagreements (as has happened here, repeatedly, even if people don't always read the other comments) is far more useful for everyone than a -1.

this is a minor task i think, so way he solved it is the best instead of split toString and etc etc etc. I im not really sure that his solution can be useful in commercial development, but in this case - good solution, very clever!