it's a generator expression. Instead of creating a list of all elements it creates an iterable object that returns a single element at a time, which sum() can still operate on. You could also add square brackets and it should still work (because both lists and generator objects are iterable) but will use more memory.

Likewise

Why does the init function have only self and data as inputs, but has data and head as assignments in the body?

I get if editing that is part of the kata, but this kata seems focused on the Push and BuildOneTwoThree functions.

Can this kata get a less "girls like diamonds herp" prompt?

Make sure you're set to python 2.7.6, and maybe try refreshing the page? (save your solution just in case)

It's not about the total index, just the index of the number that comes 2nd.

Yes, but time out is at 7000ms, not 2496ms (a reasonable amount of time for this to run, my solution took that long)

Cool, that worked once I made sure it only appended unique items. 2.7s!

Hmm, could it have to do with my list slicing? Not sure how efficient that is. Going to build a list of previous items with append rather than reslicing the whole list each time.

Can you let people know that Python 3 is implemented now? I had no idea why it was asking for parentheses on my print statements. You probably shouldn't default to 3 for people who were already using 2.