Python while Loop

In the last lesson you learned the for Loop. It runs through a list of things one by one. That works great when you know how many times you want to repeat. But what about when you do not know? Think of asking someone for a password until they get it right. You have no idea if they will need one try or ten. That is where the while loop comes in.

🤔 Why do we need a while loop?

Picture this. You are building a login screen. You want to keep asking for the password until the person types the correct one. With a for loop you would have to guess a number. Ask three times? Five times? But people are unpredictable. Some get it on the first go. Some take a while.

The pain is simple. You need to repeat something. But you do not know how many times.

The fix is the while loop. It does not count. It just keeps going as long as a condition stays true. The moment that condition becomes false, it stops.

🧩 What is a while loop?

A while loop repeats a block of code over and over. But it only repeats while a condition is true.

Think of a kettle. It keeps heating while the water is not yet boiling. Once the water boils, the condition “not boiling” becomes false. So it stops. The kettle does not count how many seconds it ran. It just checks one thing again and again. Are we there yet?

That is exactly how a while loop thinks. Before each round it asks one question. If the answer is yes, it runs the code and asks again. If the answer is no, it walks away.

🧱 The syntax

Here is the basic shape of a while loop.

while condition:
    # code that runs each time the condition is true
    pass

Here is what each piece does.

• while is the keyword that starts the loop.
• condition is a question that gives back True or False. The loop runs while this condition is True.
• The colon : ends the line, just like with if and for.
• The indented block below is the code that repeats. Everything indented runs each round.

The loop checks the condition first. Then it runs the body. So if the condition is false right from the start, the body never runs at all.

🔢 A simple counter example

The clearest way to feel a while loop is to count. Here we print the numbers 1 through 5.

This code starts a counter at 1 and keeps printing while it stays at 5 or below.

count = 1

while count <= 5:
    print(count)
    count = count + 1

print("Done!")

Now let me walk through what happens, line by line.

• We set count to 1 before the loop. This is our starting point.
• The loop checks. Is count less than or equal to 5? Yes, 1 is. So it runs the body.
• Inside, it prints count. Then it adds 1 to it. Now count is 2.
• It goes back up and checks again. 2 is still <= 5, so it repeats.
• This keeps going until count becomes 6. Now 6 <= 5 is false, so the loop stops.
• After the loop ends, we print "Done!".

Here is exactly what this prints.

The most important line here is count = count + 1. That single line is what moves us toward the finish. Without it, count would stay 1 forever and the loop would never stop. More on that danger in a second.

🔁 You must change something inside the loop

This is the one idea that trips up most people. So let me say it plainly. A while loop only stops when its condition becomes false. So something inside the loop must change to make that happen.

In the counter example, that something was count = count + 1. Each round nudged count closer to the value that ends the loop.

A while loop usually has three moving parts.

Miss the update, and your loop runs forever. Let me show you what that looks like and why it matters.

⚠️ The infinite loop

An infinite loop is a loop that never stops, because its condition never becomes false. This is the classic while loop bug. Everyone writes one sooner or later.

Look at this code. Can you spot the problem?

count = 1

while count <= 5:
    print(count)
    # oops, we forgot to change count

Here count starts at 1 and stays 1. The condition count <= 5 is true on the first round. And it is still true on every round after, because nothing ever changes count. So Python prints 1 again and again and again, without end. Your screen fills up and the program never reaches anything below the loop.

Infinite loops are not always a mistake, though. Sometimes you want a loop to run “forever” until something special happens inside it. We will see exactly that pattern next.

💬 A real example: keep asking until they type “quit”

Now for something closer to real life. Let us write a tiny program that keeps reading what the person types. It only stops when they type quit.

This code reads input in a loop and breaks out of it the moment the word quit arrives.

command = ""

while command != "quit":
    command = input("Type a command (or 'quit' to stop): ")
    print(f"You typed: {command}")

print("Goodbye!")

Here is the walkthrough.

• We start command as an empty string "". We need it to exist before the loop checks it.
• The condition is command != "quit". That means “command is not equal to quit”. So the loop runs as long as the person has not typed quit.
• Inside, input() asks them to type something and stores it in command. This is the line that changes our variable each round.
• We print what they typed.
• The loop goes back and checks the condition again with the new value of command.
• The moment they type quit, the condition becomes false, the loop ends, and we print "Goodbye!".

If someone runs this and types hello, then python, then quit, the program prints this.

Notice that input() is doing the job that count = count + 1 did earlier. It is the thing that changes each round. That gives the loop a way to eventually stop.

⚠️ Common Mistakes

A few traps catch almost everyone when they start with while loops. Watch for these.

• Forgetting to update the variable. This is the big one. If nothing in the body changes the condition, you get an infinite loop.

# ❌ Avoid: count never changes, runs forever
count = 1
while count <= 5:
    print(count)

# ✅ Good: count grows each round, so the loop ends
count = 1
while count <= 5:
    print(count)
    count = count + 1

• Using the variable before creating it. The condition is checked before the first round, so the variable must already exist.

# ❌ Avoid: 'total' does not exist yet when the loop checks it
while total < 100:
    total = total + 10

# ✅ Good: create it first
total = 0
while total < 100:
    total = total + 10

• Mixing up the comparison. Writing count < 5 when you meant count <= 5 will stop one round too early. Read your condition slowly and ask “does this include the last value I want?”.

✅ Best Practices

Keep these small habits and your while loops will stay friendly.

• Set up your variable before the loop, on its own line, so the starting value is easy to see.
• Make sure the body clearly changes something that the condition depends on. If you cannot point to that line, you have a problem.
• Prefer a for loop when you already know how many times to repeat. Reach for while when the number of repeats is unknown, like reading input.
• When you do want a loop that runs until a special event, while True: paired with a break is a clean pattern. You will meet break in the next lesson.

🧩 What You’ve Learned

You now know how to repeat code when you do not know the count up front.

• ✅ A while loop repeats its body as long as the condition stays True.
• ✅ The condition is checked before each round, so it can run zero times.
• ✅ Something inside the loop must change, or you get an infinite loop that never stops.
• ✅ An infinite loop can be stopped with Ctrl+C in the terminal.
• ✅ while shines when the number of repeats is unknown, like asking until someone types quit.

🚀 What’s Next?

You can repeat code now. But sometimes you want to jump out of a loop early, the instant something happens. That is the job of one small but mighty keyword.

break Statement