Check the service level

Earlier, we used an if to decide on the tip rate:

Let's break it

Run the program again, but type an uppercase G for the service level. In the console:

Aye.

How do we fix that?

That would work, but you're right, it isn't the best way to do it.

To see why, run the program again, but type "g " (a space at the end) instead of "g".

Aye. There's another reason, too. Double-click on the word "birds" below.

Right.

But in Notepad++, a popular Windows editor, the space isn't selected.

Aye, that's so.

This is sometimes called normalization. Converting data that can have many forms to just one, then having a simpler if.

Python has a coupla things that will help.

strip

Remember, + can append strings. We did this in the name tag program:

Python took the value in first_name, appended a space, then appended the value in last_name.

Try this in the console:

Right!

Now, try this in the console:

print() and input() are functions that have been in Python for a long time.

However, .strip() is a method. It's like a function belonging to a data type. .strip() belongs to the string data type. It can be used on any string, but not on floats or ints.

Methods are part of object-oriented programming (OOP). OOP isn't part of this course, though. You won't be making your own methods.

Python's docs and tutorials are a bit confusing here. Sometimes methods are called functions. Ack!

I usually use my googly pal for that. I search for things I want to do, like:

• python remove extra spaces from string
• python square root
• python format a number to 2 decimal places

Often I find code to copy.

Aye.

Here's the code we started with.

Yes, that works.

Yep, that works, too.

You could argue for both ways, but I agree with Ethan. I'd use the second. Making code easier to understand is a Very Important Thing.

More to do

OK, we're talking about normalization, taking some data and converting it to a normal form.

Right. Instead of this:

We write

But we're not there yet. Run this code again, and type "G".

So, whether the user types "g" or "G", we want the if statement to work.

Aye, that's what we do.

Aye!

This...

... says to take what's in service_level, strip leading and trailing spaces, then convert what's left to lowercase, and then...

Yes, replacing whatever was there. You don't always want to do that. Sometimes you want to put the normalized version into a new variable. We'll see that later.

Normalization is so useful we should make a pattern out of it.

Wrong letter

The code's looking good. Just one more thing. What should the program do if someone types X, or Q, or doggos? Something that's not one of the valid choices, even after normalizing.

Right! Let's show an error message.

Here's the code again:

Good! Maybe like this?

Well, that didn't work. What now?

(Crickets)

Let's start by working out what the program did. How?

Aye, makes sense. Before I run the code, I'm going to add a breakpoint.

Instead of running one line at a time, as we did before, we can tell the debugger to run the program normally, until it gets to the breakpoint. Then pause, before running the line with the breakpoint.

Add a breakpoint by clicking in the gray area as shown. Start debugging. When we started debugging before, Spyder stopped on the first line of the program. This time, Spyder notices the breakpoint, and runs until it reaches it. Allowing for user input, as we have.

So, I click the debug button. The program starts running. In the console, I type the meal cost as usual. Then the service level. I'll type, say, q and Enter.

Next, Spyder stops at the breakpoint. In the editor pane, I see:

It's hard to tell, but there's an arrow underneath the red circle.

The console shows the breakpoint:

The program is still running. You can tell because the square button is red. Normally, it's black.

Here's the Variable Explorer:

Here's the line we're about to run:

Ray, can you break down that line, and tell me what it will do?

(Crickets.)

Right!

What now?

Right. We could do a coupla things here.

• We could stop the program.
• We could keep asking the user to enter a valid value.

The first is easier. Let's do that. You'll learn how to do the second one later in the course.

Some new code:

Line 12 stops the program. Notice it's indented, so it belongs to the code block under the else.

It tells Python where to find the exit function.

Python is a powerful language. You can use it to write programs for:

• Data analysis
• Games
• Web applications
• Audio analysis
• Cryptography
• Text analysis
• A bazillion other things

The Python memory footprint would be huge if all the code for this stuff was loaded all at once.

Instead, Python has a core with the most important stuff, like variables, ifs, print(), input(), etc. If you want to do more, you load in modules. A module is a code library for a specific thing. For example, code for making games is in the pygame module. Use the django module to write web apps. pyaudio is for audio analysis. And many more.

exit is in the sys module, perhaps the most used Python module. Load it by putting this at the top of your program:

Call its exit function like this:

The module name, then a ., then the function in the module.

Here's the program so far:

Cleaner pattern use

We learned about the IPO pattern:

Our code isn't as clean as it could be. It mixes input and processing.

Right! Good work!

You could see it that way, but the pattern says Input is "Code to put valid input data into variables." IPO programs are easier to change if each chunk is separate, if Processing knows all the data it gets from Input is valid. One less thing to think about when writing Processing.

Here's another way to write the Input chunk.

Check the if in Processing (lines 13 to 18). It's back to its simpler form. If service_level is not g, and it's not o, then it must be s. No need to test for s, if Processing is guaranteed to get valid data from Input.

Look at line 8. In English, it's "if the input is not g, and not o, and not s, it's invalid."

!= is not equal to. var1 != var2 is true if the two vars are different.

and is true if both sides are true.

You might think this would work:

Makes sense, since we're testing service_level three times. However, compys aren't as smart as we are. You have to repeat service_level in every comparison.

So, we have a good way of validating string inputs. Hooray!

Let's make a pattern of this, to make it easy to use in future code.

Summary

We worked on validation in this lesson.

• Normalizing converts data to a standard form.
• Normalizing input means we can use fewer ifs. We don't have to check for 'g', ' g', ' G ', etc.
• The strip() method removes spaces from the start and end of a string.
• The lower() method converts a string to lowercase.
• Use print() to show error messages.
• Use @sys.exit() to stop a program.
• Do all validation in the Input chunk if you can.

Exercise