Repeating with while

A while loop repeats a block as long as its condition is true.

count = 3
while count > 0:
    print(count)
    count = count - 1
print("Liftoff!")

Here is exactly what happens: Python checks count > 0. True, so it runs the indented block, then jumps back up and checks again. When the condition finally comes out false, it skips the block and carries on with the line after the loop. The condition is only checked at the top of each round, not halfway through.

The block must change something that the condition depends on. Delete the line count = count - 1 and count stays 3 forever, so the condition never becomes false. That is an infinite loop, and your program will hang until you stop it. Everyone writes one eventually; consider it a rite of passage.

A shortcut you will see everywhere: count -= 1 means count = count - 1. Same for +=, *= and friends.

Sometimes you do not know in advance when to stop. Then you can write while True: (a loop that would run forever) and use break to jump out from inside:

while True:
    guess = input()
    if guess == "42":
        break

while is the right tool when you are repeating "until something happens": until the user types the right password, until a number gets small enough, until the game is over. When you know how many times to repeat, or you are going through a collection of things, the for loop in the next lesson is usually cleaner.