Looping for pets

We're going to add validation loops to the tip program. It's a topic that confuses some people.

Ray

Uh oh.

Don't worry, we'll take it easy. Let's start with a simple task.

Just the dogs

Let's write a program asking users for the best pet. It keeps asking until they type the right pet, which is, of course, dogs.

Here's sample I/O:

The program keeps asking until you give it the right answer.

My friend ​Burt

Getting started

Here's one way to write the program:

Line 1 creates an MT string variable.

Line 2 starts a while loop. It has the form:

condition is either true or false, just like an if uses. Python will keep doing action while condition is true.

Ray

Not equal to, right?

Yes.

Ray

Hey. In the code...

1. best_pet = ''
2. while best_pet != 'dog':
3.     best_pet = input('What is the best pet? ');
4. print('Best pet: ', best_pet)

The first line, best_pet = '', why is that there?

Georgina

Ooo! I know! It's so line 2 works the first time.

If you try while best_pet != 'dog': but there is no best_pet variable, the program will crash.

So, make best_pet, and make it MT.

Aye! That's right.

Comments

Hey, Adela, can you add some comments to the code?

Adela

OK.

1. # Initialize best_pet to MT.
2. best_pet = ''
3. # While best_pet does not equal dog...
4. while best_pet != 'dog':
5.     # Ask the user what the best pet is.
6.     best_pet = input('What is the best pet? ');
7. # Output the best pet.
8. print('Best pet: ', best_pet)

Good! Thanks, Adela. We can see the purpose of each bit of code now.

Adela added one comment per line of code. That's not normally what you do. You add a comment for each chunk, a chunk being a sentence you would say when explaining the code to someone else. Like, "that's the input," and "that's processing."

Here, the chunks are so simple that each is just one line of code.

Georgina

That's not a very precise way of defining a chunk.

True. English is not a precise language. Write comments so that, if you just read the comments, you would understand how the code works.

Clean up user input

Georgina

What if they type "Dog" instead of "dog"?

Oh, good point. Or they might type DOG or doG or add some spaces.

Ethan

Normalize, right?

Aye. Georgina, how would you normalize user input?

Georgina

Take the user input, strip excess spaces, and make it lowercase. Like we did before.

OK, let me see... we had:

• service_level = input('Service level (g=great, o=ok, s=sucked)? ')
• service_level = service_level.strip().lower()

Normalization is on the second line, removing extra spaces, and making everything lowercase. So...

1. # Initialize best_pet to MT.
2. best_pet = ''
3. # While best_pet does not equal dog...
4. while best_pet != 'dog':
5.     # Ask the user what the best pet is.
6.     best_pet = input('What is the best pet? ');
7.     # Normalize
8.     best_pet = best_pet.strip().lower()
9. # Output the best pet.
10. print('Best pet: ', best_pet)

Ethan

Yes, it does.

To infinity, and beyond!

Here's some Evil Code:

It's the same as before, except I changed one letter.

Hey Georgina! Can you try running it?

Georgina

OK.

Georgina

What... It won't stop when I type dog. Or anything else, it looks like.

You said you changed one letter...

Oh, you changed...

• while best_pet != 'dog':

... to...

• while best_pet != 'Dog':

Oh. I see it now.

Ethan

See what?

Georgina

Nice trick, Kieran!

1. # Initialize best_pet to MT.
2. best_pet = ''
3. # While best_pet does not equal dog...
4. while best_pet != 'Dog':
5.     # Ask the user what the best pet is.
6.     best_pet = input('What is the best pet? ');
7.     # Normalize
8.     best_pet = best_pet.strip().lower()
9. # Output the best pet.
10. print('Best pet: ', best_pet)

Line 8 normalizes best_pet. It will always be lowercase. Like, Dog becomes dog.

On line 4, best_pet will always be not equal to Dog. There will never be an uppercase letter in best_pet! The test in the while will always be true, so it keeps looping forever.

Good! You worked it out.

This is an infinite loop. It will keep running around and around, no matter what the user types. The infinite loop is a common bug.

Ray

Umm... how do we stop the program? Reboot?

That'll work, but there's an easier way. Ctrl+C.

Georgina

Let me try it...

• What is the best pet? doggo
• What is the best pet?
• What is the best pet? stop
• What is the best pet? help!
• What is the best pet? Ctrl+C pressed here Traceback (most recent call last):
• ...
• KeyboardInterrupt: Interrupted by user

Good. If... no, when... you get stuck in an infinite loop, Ctrl+C will stop the program.

OK, that's the basics of a while loop. While a test is true, keep doing some code.

Ray

Wait, that's it? This is a short lesson.

Best to learn in small pieces. Each piece should be easy to understand.

Summary

• whiles have a condition and action. Python will keep doing the action while the condition is true.
• Variables used in the condition, like doggos in while doggos < 2:, should exist before the while is run for the first time.
• Break infinite loops with Ctrl+C.