Python While Loops

A loop is a control structure that repeats code that belongs to it. A while-loop is a loop that repeats code while some condition is satisfied:

while <condition>:
    <code>

The condition is checked before every repetition (including the first) — if it’s false to begin with, the loop body never runs at all:

>>> x = 0
>>> while False:
...     print(x)
...     x += 1
>>> # nothing prints

Augmented assignment operators

Operator	Equivalent to
x += y	x = x + y
x *= y	x = x * y
x /= y	x = x / y
x %= y	x = x % y

break

The break keyword terminates the (innermost) loop immediately and continues with the rest of the program:

>>> while True:
...     print("hello")
...     break
...     print("world")
hello

Simulating a do-while

A do-while (or repeat-until) is a while-loop variant, available in other languages but not natively supported in Python, that runs its code at least once before checking the condition:

do:
    <code>
while <condition>   # not valid Python syntax

We can simulate one with while True (some programmers prefer 1 to True) and a break:

>>> x = 0
>>> while 1:
...     x += 1
...     if x > 0:
...         break
>>> x   # x retains its value outside the while
1

User input

x = input() waits for input from the keyboard and assigns it to x, always with type(x) is str.

Random number generation

Random number generation is handled by an external library (not built-in), so it must be imported:

>>> from random import randint
>>> randint(1, 6)   # chosen uniformly over the interval
2

or, importing the entire library:

>>> import random
>>> random.randint(1, 6)
2