Suppose I have a deck of cards, and I shuffle them

import random

cards = []

decks = 6

cards = list(range(13 * 4 * decks))

random.shuffle(cards)

So now I have an array of cards. I would like to cut these cards at some random point (between 1 and 13 * 4 * decks - 1, moving the lower half of that to the top half of the cards array.

For some reason, I can't see how this can be done (I know that it must be a simple line or two in Python, but I am really stuck here). Anyone have any direction they can give me on this? Thanks, RVic, python newbie