oh, this made me feel dumb. Its so simple.

Same. I just wrote it in the order I thought it out though -- put the difficult part on the left, then filled in the simple part on the right. Would have changed it if I looked at it for more than a few seconds before submitting.

Honestly, I think it has to do with reading comprehension. I submitted this solution as well, yet for some reason I find the other one easier to read at a glance.

"set() method is used to convert any of the iterable to sequence of iterable elements with distinct elements, commonly called Set."
Meaning, a set is the same basic group of characters as the original, except it excludes duplicates and it is not in order.

Do you really need to ask? https://www.google.com/search?client=firefox-b-e&q=python+set

Ive seen people use the set() function for this. what does it do?

why line.index(m) if you can just enumerate()

I really like this. Very clean to read.

Maybe someone should search through all the katas and reduce the sudoku validators to one... I just copied my solution for NxN sudoku here...

for an even shorter solution: sum should work over a generator expression ;-)

I do not think that this is sufficient, since 'aabbc' and 'abccc' both have length 5 and {'a', 'b', 'c'}