Python "while" Loops (Indefinite Iteration)

Python "while" Loops (Indefinite Iteration)

by John Sturtz basics python
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Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Mastering While Loops

Iteration means executing the same block of code over and over, potentially many times. A programming structure that implements iteration is called a loop.

In programming, there are two types of iteration, indefinite and definite:

  • With indefinite iteration, the number of times the loop is executed isn’t specified explicitly in advance. Rather, the designated block is executed repeatedly as long as some condition is met.

  • With definite iteration, the number of times the designated block will be executed is specified explicitly at the time the loop starts.

In this tutorial, you’ll:

  • Learn about the while loop, the Python control structure used for indefinite iteration
  • See how to break out of a loop or loop iteration prematurely
  • Explore infinite loops

When you’re finished, you should have a good grasp of how to use indefinite iteration in Python.

The while Loop

Let’s see how Python’s while statement is used to construct loops. We’ll start simple and embellish as we go.

The format of a rudimentary while loop is shown below:

Python 
while <expr>:
    <statement(s)>

<statement(s)> represents the block to be repeatedly executed, often referred to as the body of the loop. This is denoted with indentation, just as in an if statement.

The controlling expression, <expr>, typically involves one or more variables that are initialized prior to starting the loop and then modified somewhere in the loop body.

When a while loop is encountered, <expr> is first evaluated in Boolean context. If it is true, the loop body is executed. Then <expr> is checked again, and if still true, the body is executed again. This continues until <expr> becomes false, at which point program execution proceeds to the first statement beyond the loop body.

Consider this loop:

Python 
 1>>> n = 5
 2>>> while n > 0:
 3...     n -= 1
 4...     print(n)
 5...
 64
 73
 82
 91
100

Here’s what’s happening in this example:

  • n is initially 5. The expression in the while statement header on line 2 is n > 0, which is true, so the loop body executes. Inside the loop body on line 3, n is decremented by 1 to 4, and then printed.

  • When the body of the loop has finished, program execution returns to the top of the loop at line 2, and the expression is evaluated again. It is still true, so the body executes again, and 3 is printed.

  • This continues until n becomes 0. At that point, when the expression is tested, it is false, and the loop terminates. Execution would resume at the first statement following the loop body, but there isn’t one in this case.

Note that the controlling expression of the while loop is tested first, before anything else happens. If it’s false to start with, the loop body will never be executed at all:

Python 
>>> n = 0
>>> while n > 0:
...     n -= 1
...     print(n)
...

In the example above, when the loop is encountered, n is 0. The controlling expression n > 0 is already false, so the loop body never executes.

Here’s another while loop involving a list, rather than a numeric comparison:

Python 
>>> a = ['foo', 'bar', 'baz']
>>> while a:
...     print(a.pop(-1))
...
baz
bar
foo

When a list is evaluated in Boolean context, it is truthy if it has elements in it and falsy if it is empty. In this example, a is true as long as it has elements in it. Once all the items have been removed with the .pop() method and the list is empty, a is false, and the loop terminates.

The Python break and continue Statements

In each example you have seen so far, the entire body of the while loop is executed on each iteration. Python provides two keywords that terminate a loop iteration prematurely:

  • The Python break statement immediately terminates a loop entirely. Program execution proceeds to the first statement following the loop body.

  • The Python continue statement immediately terminates the current loop iteration. Execution jumps to the top of the loop, and the controlling expression is re-evaluated to determine whether the loop will execute again or terminate.

The distinction between break and continue is demonstrated in the following diagram:

Python while loops: break and continue statements
break and continue

Here’s a script file called break.py that demonstrates the break statement:

Python 
 1n = 5
 2while n > 0:
 3    n -= 1
 4    if n