while

Updated May 19, 2026· Keywords

pythonwhileloopscontrol-flow

Executes a block of code repeatedly as long as a given condition evaluates to a truthy value. The condition is checked before each iteration.

Syntax

while condition:
    # body

condition is any expression. The body is indented one level (four spaces). No parentheses are required around the condition.

n = 3
while n > 0:
    print(n)
    n -= 1
# output: 3, 2, 1

`while...else`

Python allows an optional else clause on while loops. The else block runs once when the condition first evaluates to falsy — it does not run if the loop was exited via break or return.

i = 0
while i < 3:
    print(i)
    i += 1
else:
    print("done")
# output: 0, 1, 2, done

i = 0
while i < 3:
    if i == 1:
        break
    i += 1
else:
    print("done")
# output: (nothing after break — else is skipped)

A common use case is the “search or not found” pattern:

nums = [1, 2, 3, 4, 5]
while nums:
    if nums.pop() == 99:
        print("found 99")
        break
else:
    print("99 not found")
# output: 99 not found

`break` and `continue`

break exits the innermost loop immediately and skips the else clause if present.

count = 0
while count < 10:
    count += 1
    if count == 5:
        break
    print(count)
# output: 1, 2, 3, 4

continue skips the rest of the current iteration and jumps straight to the condition check.

count = 0
while count < 5:
    count += 1
    if count == 3:
        continue
    print(count)
# output: 1, 2, 4, 5

Infinite Loops

while True: creates an infinite loop that runs until explicitly broken.

import random
attempts = 0
while True:
    attempts += 1
    if random.random() > 0.95:
        break
print(attempts)
# output: varies (stopped when random() > 0.95)

When nesting while loops, break only exits the innermost level. Python does not have labeled break. To break out of multiple loop levels, use a flag variable, return from a function, or raise an exception.

found = False
i = 0
while i < 3 and not found:
    j = 0
    while j < 3:
        if (i, j) == (1, 2):
            found = True
            break
        j += 1
    i += 1
print(i, j)
# output: 1 2

Truthiness

Any expression can be used as a condition. Python checks truthiness directly — not just boolean values.

Falsy values: False, None, 0, "", [], {}, ().

Everything else is truthy.

# Loop until list is empty
items = [1, 2, 3]
while items:
    print(items.pop())
# output: 3, 2, 1

# Loop until None (cache miss sentinel)
cache = {"key": "value"}
while cache.get("key"):
    print(cache.pop("key"))
# output: value

Walrus Operator (`:=`)

The walrus operator (Python 3.8+, PEP 572) assigns and returns a value in a single expression. Useful in while conditions to avoid repeated function calls.

# Read lines until blank — no double call to readline()
import sys
data = sys.stdin
while (line := data.readline().strip()):
    print(f"got: {line}")

# Prompt until valid input
while (resp := input("(y/n): ")) not in ("y", "n"):
    pass
print("thanks")

Summary

Feature	Syntax
Basic loop	`while condition:`
Exit on condition	`while condition: ... else:`
Exit immediately	`break`
Next iteration	`continue`
Infinite loop	`while True:`
Assignment in condition	`while (x := expr):`

Syntax

while...else

break and continue