Not knowing how many turns: while

for knows from the start how many turns it takes. Sometimes you cannot know: "until the number runs out", "until the wall". while asks a question and keeps going as long as the answer is true.

1steps = 0
2while key > 1:
3    key = key // 2
4    steps += 1
5return steps

Ask yourself two things about every while: is there something inside the loop that changes the condition, and is my answer right if the loop never runs? The second usually shows up at the edges, with 0 or an empty list.