Python while Loops

A while loop keeps running as long as its condition is true. It is the right tool when you do not know the number of repetitions in advance. The trade-off is that you must update the condition yourself, or the loop will never end. You will also use a loop invariant, bound retries, and drain a work queue.

A condition controls the loop

Before each iteration, Python checks the condition. If it is true, the body runs; if it is false, the loop stops. A counter that changes inside the body is the usual way to make the condition become false.

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

A countdown

break and continue

break leaves the loop immediately. continue skips the rest of the current iteration and goes back to the condition. Use both sparingly: too many exits make a loop harder to reason about.

n = 0
while n < 6:
    n += 1
    if n == 2:
        continue
    if n == 5:
        break
    print(n)

Skipping and stopping

A while loop over a sequence

You can use an index with while when you need manual control. Update the index at the end of every iteration. A for loop is usually clearer when you simply want every item.

orders = ["latte", "tea", "mocha"]
i = 0
while i < len(orders):
    print(orders[i])
    i += 1

Manual iteration

A loop invariant explains the total

Before each pass, total is the sum of the first i prices. The body adds one price and increments i, so the invariant is true again at the top. State that sentence before writing the loop; if you cannot, the loop probably needs a different shape.

prices = [4.5, 3.0, 2.5]
total = 0
i = 0
while i < len(prices):
    total += prices[i]
    i += 1
print(total)
print("processed", i, "of", len(prices))

An invariant makes the loop auditable

Give retries a budget

A validation loop needs a limit as well as a success path. Track attempts separately from the value; stop on success or when the budget is exhausted. Print the reason the loop stopped so a failure is visible instead of silent.

raw_values = ["many", "3", "5"]
attempts = 0
value = None
while attempts < len(raw_values):
    raw = raw_values[attempts]
    attempts += 1
    try:
        value = int(raw)
        break
    except ValueError:
        print("retry after", raw)
print("value", value, "attempts", attempts)

Stop on success or when attempts run out

Drain a queue one item at a time

A while loop fits work that shrinks. pop() removes one item, the list gets shorter, and the empty list is false. Do not append to a list while a for loop is walking it; remove work explicitly instead.

queue = ["latte", "tea", "mocha"]
served = []
while queue:
    order = queue.pop()
    served.append(order)
    print("serving", order)
print("served", served)

One removal per pass, empty ends the loop

Try it yourself

Change the values, predict the output, then run the code. The Python runtime is downloaded the first time you press Run, so the first run can take a few seconds.