How to Write and Use Python While Loops

Materials Required: Latest version of Python (Python 3), an integrated development environment (IDE) of your choice (or terminal), stable internet connection

Prerequisites/helpful expertise: Basic knowledge of Python and programming concepts

While loops are control flow tools. Like for loops, they are used to iterate over a block of code. Unlike for loops, they will repeat that block of code for an unknown number of times until a specific condition is met. You’ll use while loops when you’re not sure how many times you need your code to repeat. You’ll use do while loops when you need it to repeat at least once.

How to use while loop in Python

While loops continuously execute code for as long as the given condition or, boolean expression, is true. They are most useful when you don’t know how many times you’ll need the code to execute. For example, you may want to run a piece of code as long as a given number is between 1 and 5. In that case, the loop will run while the number remains between 1 and 5. When the user chooses a 6 or any other number that isn’t between 1-5, the control flow will exit the while loop body.

How while loop works in python

First, the Python interpreter checks the test expression or, while condition. If the test expression evaluates to true, the body of the while loop will be entered. This process repeats until the test expression evaluates to false.

Try it yourself

1
2
3
4
5
numbers = [2, 4, 6, 8, 10]
total = 0
for num in numbers:
    total += num
print("The total is:", total)

What will the interpreter display at the end of the fourth loop?

How to write a while loop in Python

Remember that indentation is crucial in Python. Indentation tells Python where the loop body starts and ends. The while loop begins with indentation and ends at the first unindented line. First, let’s examine the syntax and structure of a Python while loop:

1
2
while count < 5:    # condition to check if the loop should continue
    print(count)    # body of the loop, will execute as long as the condition is true

Now, let’s look at an example:

In this example, the loop will print out the even numbers from 0 to 8, because we start at 0 and add 2 to the number for each iteration. Once num reaches 10, the condition num < 10 is no longer true, and the loop terminates.

How to end a while loop in Python

As mentioned above, the first unindented line of code marks the end of a while loop. However, you may need to end a loop early or terminate the current loop iteration. In that case, you can use one of three loop control statements:

1. Break

The break keyword is used to exit a loop early. It terminates the loop that contains it and redirects the program flow to the following statement.

Example:

In this example, we're using a while loop to print the numbers from 1 to 10. However, we're also using an if statement to check if the loop has reached the halfway point, which is when i is equal to 5. If we've reached the halfway point, we print a message to the console saying "We've reached the halfway point, stopping loop." and then immediately exit the loop using the break statement.

Learn more: How to Use Python Break: In For Loops, While Loops, and If Statements

2. Continue

continue is a Python keyword that you can use to end a for loop’s current iteration. The control flow will continue on to the next iteration. The continue statement is useful when you want to skip over certain iterations of a loop based on some condition, without exiting the loop entirely.

Example:

In this example, the while loop iterates over the numbers 1 through 10. The if statement checks whether the current value of i is even. If it is, the continue statement is executed, causing the loop to skip the current iteration and move on to the next one without executing the rest of the loop body. If i is odd, the print statement is executed, outputting the value of i.

3. Pass

The pass statement in Python intentionally does nothing. It can be used as a placeholder for future code or when a statement is required by syntax but you don’t want anything to happen. In a while loop, you can use it to ignore a certain condition during an iteration.

Example:

In this example, the while loop iterates over the numbers 1 through 5. The if statement checks whether the current value of i is equal to 3. The pass statement indicates that there is no specific action to be taken when i is equal to 3, but the program should continue executing the rest of the loop body.

Try it yourself

Write a program that repeatedly prompts the user to enter a number between 1 and 10, until the user enters a valid number. For each invalid input, the program should print an error message and prompt the user to try again. If the user enters a valid number, the program should print a message indicating that the number is valid and exit the loop.

Use a continue statement to skip over any input that is not a valid number between 1 and 10.

1
2
3
4
5
6
7
8
9
10
11
while True:
  num_str = input("Enter a number between 1 and 10: ")
  if not num_str.isdigit():
      print("Error: input must be a number.")
      continue
  num = int(num_str)
  if num < 1 or num > 10:
      print("Error: number must be between 1 and 10.")
      continue
  print("You entered a valid number:", num)
  break

Does Python have do while loops?

A do while loop is a variant of the while loop that checks the condition at the end of the loop rather than the beginning. Even if the condition is never true, the statements inside the loop body will be executed at least once. do while loops are useful when you need your code to run at least once. Python does not have a built-in do while loop like other programming languages. However, you can modify a regular Python while loop to accomplish the same functionality.

How to emulate do while loop in Python

A Python while loop only runs when the condition is met. Since it checks that condition at the beginning, it may never run at all. To modify a while loop into a do while loop, add true after the keyword while so that the condition is always true to begin with. Then, you can add if statements to check the inside condition and break statements to control the flow of the loop based on your terms.

Here’s an example:

This loop will print the numbers 1 to 5, which is equivalent to the behavior of a do while loop that iterates from 1 to 5. We use a break statement to exit the loop once we have printed all the numbers, since the while condition is always True and would otherwise result in an infinite loop.

Key takeaways

• while loops continuously execute code for as long as the given condition is true.
• There is no do while loop in Python, but you can modify a while loop to achieve the same functionality.
• There are three control statements you can use to break out of a while loop in Python: break, continue, and pass.
• Indentation tells Python which statements are inside or outside of the loop.

Resources

• Being a Python Developer: What They Can Do, Earn, and More
• Python control flow tools documentation
• Python Loops Cheat Sheet
• How to Use Python Break

Keep improving your Python skills with Coursera.

You can practice working with while loops with a Guided Project like Concepts in Python: Loops, Functions, and Returns. Or, take the next step in mastering the Python language and earn a certificate from the University of Michigan in Python 3 Programming.