Everything You Need to Know About the While Loop in Python

What is a Python while loop?

A while loop is made up of a condition or expression followed by a block of code to run. The condition or expression will be evaluated in a Boolean context. If it turns out to be true, the code within the block will be run.

This repeats until the condition evaluates as false. At this point, program execution will proceed to the first statement after the body of the loop.

As with if statements, a while loop can be specified on one line. If there are multiple statements in the loop body block, they can be separated by semicolons.

Python loops can have an else clause that can be included at the end of the loop. The else block of code runs only if the loop completes without encountering a break statement.

Basic syntax for the while loop in Python

A while loop in Python can be created as follows:

while <expression>:
    <statement(s)>

<statement(s)> indicates the section of code to be run with each iteration of the loop. It can also be known as the body of the loop.

Note: Python uses indentation for grouping statements: all statements indented by the same number of spaces after a construct are considered to be part of a single code block.

The controlling <expression> will be one or more variables that are set above the loop and then updated in the body of the loop:

i = 0
while i < 10:
    print(i)
    i += 1

The output of the above code would look like this:

0
1
2
3
4
5
6
7
8
9

Before the loop begins, i equals 0. The expression on line 2 is i< 10, which is true, so the body of the loop will run.

Moving into the loop body, on line 3, i is printed and then incremented by 1, on line 4, going from 0 to 1.

When the body of the loop finishes processing, the script returns to the top to re-evaluate the expression. As it is still True, the body runs again, and 2 is the output.

This process continues until i becomes 10. Then the expression will test as False and the loop will end.

Python break and continue statements

So far everything in the body of the loop has been run on each pass. To end the running of a while loop early, Python provides two keywords: break and continue.

A break statement will terminate the entire loop process immediately with the program moving to the first statement after the loop.

continue statements will immediately terminate the current iteration but return back to the top. There, the control expression will be re-evaluated again to determine whether to run or end the loop.

A couple of examples will help illustrate the difference between the two:

i = 0
while i < 10:
    if i == 4:
        break
    print(i)
    i += 1

print('The end')

The above code will output:

0
1
2
3
The end

When i = 4 the break statement runs and there is no more output.

We could also use the continue keyword with this script:

i = 0
while i < 10:
    if i == 5:
        i += 1
        continue
    print(i)
    i += 1

print('The end')

This implementation results in the output below:

0
1
2
3
4
6
7
8
9
The end

When i = 5, the continue statement is run and the program begins at the top again, skipping out the printing of 5 but printing the rest of the numbers in the loop.

While loop in Python: useful tips

• In Python, you can have infinite loops that are characterized by not having an explicit end.
• while loops can also be nested inside other loops.