How the program works

Tina's Louie

Here's the program again, without the syntax error we fixed.

Run it, and the first line will ask you a question.

What happens to the data you type? It goes into a variable.

Variables

A variable is a chunk of computer memory with a name. Knowing how to use variables is a Very Important Thing. Much of programming is putting data in and getting data from variables.

When Python runs this line...

first_name = input('What is your first name? ')

... the first thing it does is notice you've referred to a new variable, first_name. Python grabs some memory, and gives it the name first_name.

Next, it sees the =. The = is the assignment operator. It takes data from the right-hand side of the =, and puts it into the variable named on the left-hand side. For example:

legs = 4

This tells Python:

• If there's no legs variable already, create it in memory.
• Put 4 in it.

If I'd been designing the language, I'd have used an arrow instead:

legs <- 4

<- more clearly shows that whatever is on the right goes into the variable on the left. But we're stuck with = as the assignment operator.

No, "What is your first name? " is a prompt, a question for the user. The input function shows the prompt, and waits. Whatever the user types in the console is what input returns. So here...

Ray goes into first_name.

The console remembers variables in programs you run. So, I can ask the console about first_name.

The thing on the right doesn't have to be a fixed value. For example:

sales_tax = price * 0.06

This tells Python to:

• If it doesn't already exist, make a variable called sales_tax in memory.
• Take whatever value is in the variable price, multiply it by 0.06, and put the result into sales_tax.

Good question! Python won't create price. It only creates variables named on the left of the =. If price doesn't exist, Python won't know what value to multiply by 0.06.

The program gives you an error message, and stops. In other words, it crashes. The error message won't be something friendly, like "Sorry, I need a price." I'll be something computery that'll make regular people nervous.

In sales_tax = price * 0.06, * means multiply. There's +, -, / (means divide), and other things.

BTW, fixed values, like 4 and 0.06, are called constants because they never change. 4 is always 4, no matter how many times you run the program. 0.06 is, well, 0.06. It can't change. It's constant.

Yet another example:

total = price + sales_tax

Python...

• If the variable total doesn't exist, make it. Take a little piece of memory, and name it total.
• Take what's on the right (whatever's in price plus whatever's in sales_tax), and put it in the variable total.

You can put anything that returns a value on the right.

No, only the variable on the left of =, total, changes. Taking data from a variable to do a calculation doesn't change it.

Naming variables

Here are some good variable names:

• first_name
• total_legs
• best_doggo_name
• score
• sales_tax

A few things. First, they're all lowercase. Unfortunately, Python variable names are case-sensitive. So total and Total are different variables. Try this in the console:

It's easy if we use lowercase for all variable names. Let's make that a rule for ourselves.

Try this in the console:

The first line works, kinda, but blocks the print() function. So, another rule: don't use Python keywords as variable names.

The third rule. sales_tax is a two-word variable name. Makes sense. We could use salestax, but it's not as easy to understand.

Sometimes Python's error messages don't tell you what the error actually is. Anyone know what's wrong?

Indeed. Computers are stupid. They have no common sense at all. You have to allow for that.

Last, the most important rule of all!. What's the business purpose of this code?

Aye. How about this?

Yes. So:

Follow these rules in all your programs.

If you forget what they are, there's a page called Code standards in the Extras section:

String constants

Here's our line again:

first_name = input('What is your first name? ')

Remember, the question is a called a prompt, because it prompts the user to do something.

input is a function. (You'll learn more about functions later.) It's a bit o' code someone wrote. It shows the prompt in the console, and waits for you to type something.

Good question. No, it isn't. Remember earlier we said what a value that doesn't change is? Like 4.

Right! 4 and 3.14159 are numeric constants. In this code...

... you always get 4, not matter how many times you run the program.

'What is your first name? ' is a string constant. A string is a buncha characters, anything you can type on the keyboard.

A string constant doesn't change. All users see the same question every time (though their answers will be different). Python uses quotes, either single (') or double (") to mark string constants.

This program...

... always outputs the same thing.

Here's our line again:

first_name = input('What is your first name? ')

To run the statement, Python...

• If first_name doesn't exist, take some memory and name it first_name.
• Show What is your first name? in the console.
• Wait until the user types something and presses Enter.
• Put whatever they typed into the variable first_name.

Here's what I typed:

So Kieran went into first_name.

Expressions

OK, now line 3:

full_name = first_name + last_name

Like the first two lines, it works out a value on the right, and puts it into the variable named on the left.

first_name + last_name is an expression, a calculation of some sort. You'll use a lot of them in the course.

Makes sense, but it's not quite right. Here's some console action:

No space between the first and last names.

Right!

Obvious to you and me, but not to the computer. Here's an important thing to remember:

Computers have no common sense. They're stupid machines. Only the programs we write make them smart.

BTW, the names of the variables don't matter to the computer. This code...

... would do the same thing.

Yes. Computers don't care what variables are named, but humans do. It's easier to understand programs when variables have meaningful names.

Output

Here's the program again:

print outputs a line. It can show constants, like print('Hello!'), variables, like print(full_name), or expressions, like...

String constants have quotes around them. So 'What is your first name?' is a string constant. The quotes mark the beginning and end of a string.

Yes! You got it. You can run the code in Spyder, and see what you get.

' and " create a string constant. 'first_name + last_name' is a string constant, and will never change, no matter what you type for first and last name.

In our name tag program, we have:

No quotes. Python takes the value of the variable first_name, appends the value of the variable last_name, and puts the result into full_name.

Adding a space

We still have a problem. There's a missing space in full_name:

The problem is this line:

We need to change the expression to get a space between the first and last names. How?

Makes sense. Give it a try.

Right. When Python reads your code, it ignores extra spaces that aren't between quotes.

So these do the same thing:

Spaces in quotes? They show. Spaces in the rest of the code? Irrelevant. (Unless they're indenting, which we'll cover later. Forggedaboudit for now.)

We need to tell Python we want one space between the names.

Yes! You're on the right track!

Woohoo! That's it! But how do you get ' ' between the names?

Aye!

YES, THAT'S IT!

You guys rock!

A space is just another character you can type on the keyboard. If you want spaces in your output, you need to tell Python to put them in.

BTW, ' ' is not the same as '' (that's two quotes with nothing between them). The second is an MT (empty) string. It has no characters at all.

Blank lines

Here's some code:

That will output:

What if you wanted a blank line between them?

An easy way is to add another print, with no content:

That will give you:

Summary

Here's the main points. Reading them again helps link memories together, making your study time more effective.

• A variable is a small piece of memory with a name.
• = is the assignment operator, as in hooves = legs * 4. The value on the right is put into the variable on the left.
• 4 and 31.3 are numeric constants.
• 'Oakland wins the game!' and 'Doggos are the best things ever!' are string constants.
• Use the + operator to join strings together. + does other things, too, but with strings, it appends them.
• Computers don't know what you intend to do, only what you tell them to do.
• If you want spaces in the output, you need to add them somewhere inside quotes.
• This will add a blank line: print()

Exercises