Given the length of a deck of cards, how many perfect shuffles does it take to get to it's original order?

index.py

def shuffle(cards):
    return [i for sublst in zip(cards[:len(cards) // 2], cards[len(cards) // 2:]) for i in sublst]

def main(deck_len):
    cards = list(range(deck_len))

    start = shuffle(cards)
    counter = 1
    while start != cards:
        start = shuffle(start)
        counter += 1

    print("A {} card deck will take {} perfect shuffles".format(deck_len,counter))
    return

# The main() function returns the amount of perfect shuffles it takes to get back to the original deck. 
# Just pass it the size of the deck of cards

main(52)
# Returns: A 52 card deck will take 8 perfect shuffles

main(10000)
# Returns: A 10000 card deck will take 300 perfect shuffles