Python While Loop

A while loop is used when you want to perform a task indefinitely, until a particular condition is met. It’s a condition-controlled loop.

Syntax

Here’s the syntax of the while statement:

Basic Examples

Any non-zero value or nonempty container is considered TRUE; whereas Zero, None, and empty container is considered FALSE.

# Iterate until x becomes 0
x = 6
while x:
    print(x)
    x -= 1
# Prints 6 5 4 3 2 1

# Iterate until list is empty
L = ['red', 'green', 'blue']
while L:
    print(L.pop())
# Prints blue green red

# Iterate until string is empty
x = 'blue'
while x:
    print(x)
    x = x[1:]
# Prints blue
# Prints lue
# Prints ue
# Prints e

If the condition is false at the start, the while loop will never be executed at all.

# Exit condition is false at the start
x = 0
while x:
    print(x)
    x -= 1

Break in while Loop

Python break statement is used to exit the loop immediately. It simply jumps out of the loop altogether, and the program continues after the loop.

# Exit when x becomes 3
x = 6
while x:
    print(x)
    x -= 1
    if x == 3:
        break
# Prints 6 5 4

Continue in while Loop

The continue statement skips the current iteration of a loop and continues with the next iteration.

# Skip odd numbers
x = 6
while x:
	x -= 1
	if x % 2 != 0:
		continue
	print(x)
# Prints 4 2 0

Else in While Loop

Python allows an optional else clause at the end of a while loop. The else clause will be executed when the loop terminates normally (the condition becomes false).

x = 6
while x:
    print(x)
    x -= 1
else:
    print('Done!')
# Prints 6 5 4 3 2 1
# Prints Done!

The else clause will still be executed if the condition is false at the start.

x = 0
while x:
    print(x)
    x -= 1
else:
    print('Done!')
# Prints Done!

If the loop terminates prematurely with break, the else clause won’t be executed.

x = 6
while x:
    print(x)
    x -= 1
    if x == 3:
        break
else:
    print('Done!')
# Prints 6 5 4

Infinte Loop (while true)

The condition must eventually become false. Otherwise, the loop will execute forever, creating an infinite/endless loop.

# Infinte loop with while statement
while True:
    print('Press Ctrl+C to stop me!')

You can safely implement an infinite loop in your program using a break statement.

# Loop runs until the user enters 'stop'
while True:
    name = input('Enter name:')
    if name == 'stop': break
    print('Hello', name)

# Output: 
# Enter name:Bob
# Hello Bob
# Enter name:Sam
# Hello Sam
# Enter name:stop