Python Loops for Kids: for and while Explained

Imagine being asked to write "I will not shout in class" a hundred times. You would want a way to say "do this a hundred times." That is what a loop does: it repeats instructions so you don't have to write them again and again.

The for loop: repeat a set number of times

for i in range(3):
    print("Hello!")

This prints "Hello!" three times. Here is how to read it:

for starts the loop.
range(3) means "three times."
The colon : ends the line.
The indented line is the part that repeats. Those spaces matter.

The i is a variable that counts as the loop goes round. Try printing it:

for i in range(3):
    print("Round", i)

It prints Round 0, Round 1, Round 2. Notice Python starts counting at 0, not 1. That surprises everyone at first.

Looping over a list

A for loop is also great for going through a list:

pets = ["cat", "dog", "fish"]
for pet in pets:
    print("I have a", pet)

Each time round, pet becomes the next item in the list.

The while loop: repeat until something changes

A while loop keeps going as long as a condition is true.

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

This counts down from 3 and finishes with "Lift off!". Each time round, count gets smaller. When it reaches 0 the condition count > 0 is no longer true, so the loop stops.

Beware the endless loop

If nothing inside a while loop ever makes the condition false, it runs forever:

count = 3
while count > 0:
    print(count)

Here count never changes. If this happens, stop the program with the stop button, or press Ctrl+C in a terminal. It happens to every programmer, and it's a good bug to hunt together.

Which loop should I use?

Use for when you know how many times, or you're going through a list.
Use while when you want to repeat until something happens, such as until a player types "quit."

Common mistakes

Forgetting the colon at the end of the for or while line.
Forgetting to indent the lines that should repeat.
Expecting range(5) to include 5. It gives 0, 1, 2, 3, 4.

Challenges to try

Print your name five times.
Print the numbers 1 to 10. (Hint: range(1, 11).)
Print the 3 times table up to 12.
Make a countdown from 10 that ends with "Blast off!".

For more practice, try the For Loops lesson in the Python course, and read about Python if statements next.