Variables

Resources

• Slides

Now that we’ve learned how to use the print(expression) statement, let’s focus on the next major concept in Python, as well as any other programming language: variables.

The word variable is traditionally defined as a value that can change. We’ve seen variables like $ x $ used in Algebraic equations like $ x + 4 = 7 $ to represent unknown values that we can try to work out. In programming a variable is defined as a way to store a value in a computer’s memory so we can retrieve it later. One common way to think of variables is like a box in the real world. We can put something in the box, representing our value. Likewise, we can write a name on the side of the box, corresponding to our variable’s name. When we want to use the variable, we can get the value that it currently stores, and even change it to a different value. It’s a pretty handy mental metaphor to keep in mind!

In a later lab, we’ll learn how to use operators to manipulate the values stored in variables, but for right now we’re just going to focus on storing and retrieving data using variables.

Creating Variables

To use a variable, we must first create one. In Python, we create a variable in a special type of statement called an assignment statement. The basic structure for an assignment statement is a = expression. When our Python interpreter runs this statement, it will first evaluate expression into a single value. Then, it will store that value in the variable named a.

For example, let’s consider the Python statement:

x = "Hello World"

In that statement, we are storing the string value "Hello World" in the variable named x. Pretty handy!

Now, let’s cover some important rules related to assignment statements:

1. Assignment statements are always written with the variable on the left, and an expression on the right. We cannot reverse the statement and say expression = x in programming like we can in math. In mathematical terms, this means an assignment statement is not commutative.
2. The left side of an assignment statement must be a location where a value can be stored. For now, we’re just going to work with single variables, so we don’t have to worry about this yet. In a later lab, we’ll introduce lists as another way to store data, and we’ll revisit this rule.
3. The right side of an assignment statement must be an expression that evaluates to a value that can be stored in the location given on the left side. Right now we’re only working with string values, so we don’t have to worry about this rule yet. We’ll come back to it in a future lab.

Using Variables

Once we’ve created a variable, we can use it in any expression to recall the current value stored in the variable. So, we can extend our previous example to store a value in a variable, and then use the print(expression) statement to display it’s value. Here’s what that would look like in Python:

x = "Hello World"
print(x)

Notice that we don’t put quotation marks " around the variable x in the print(expression) statement. This is because we want to display the value stored in the variable x, not the string value "x". So, when we run this code, we should get this output:

Hello World

To confirm, feel free to try it yourself! Copy the code above into a Python file, then use the python3 command in the terminal to run the file and see what it does. Running these examples is a great way to learn how a computer actually executes code, and it helps to confirm that your “mental model” of a computer matches how a real computer operates.

Updating Variable Values

Python also allows us to change the value stored in a variable using another assignment statement. For example, we can write some Python code that uses the same variable to print multiple outputs:

a = "Output 1"
print(a)
a = "Output 2"
print(a)

When we run this code, we’ll see the following output:

Output 1
Ouptut 2

So, just like we learned above, when we evaluate a variable in code, it will result in the value currently stored in that variable at the time it is evaluated. So, even though we are printing the same variable twice, each time it is storing a different value. Recall that this is why we call items like a a variable - their value can change!

Variable Names

Finally, Python has a few simple rules that determine what names can be used for variables in our code. Let’s quickly review those rules, as well as some conventions that most Python developers follow when naming variables.

First, the rules that must be followed:

1. Variable names must begin with either a letter or an underscore _.
2. Variable names may only contain letters, numbers, and underscores _.
3. Variable names are case sensitive.

Next, here are the conventions that most Python developers follow for variable names, which we will also follow in this course:

1. Variable names beginning with an underscore _ have a special meaning. So, we won’t create any variables beginning with an underscore right now, but later we’ll learn about what they mean and start using them.
2. Variables should have a descriptive name, like total or average, that makes it clear what the variable is used for.
3. Variables should be named using Snake Case. This means that spaces are represented by underscores _, as in number_of_inputs
4. Try to use traditional variable names only for their specific uses. Some examples of traditional variable names:
   1. tmp or temp are temporary variables.
   2. i, j, and k are iterator variables (we’ll learn about those later).
   3. x, y, and z are coordinates in a coordinate plane.
   4. r, g, b, a are colors in an RGB color system.
5. Variables should not have the same name as keywords or any built-in statements or expressions in the language.
   1. For example, Python has a print statement, so we should not name a variable print in our language.
6. In general, longer variable names are more useful than short ones, even if they are more difficult to type.

These conventions are not strict requirements enforced by the Python language itself, but they are general rules to help us write code that is meaningful and easy to read.

Finally, don’t forget that some of the code examples in this course will NOT follow these conventions, mainly because long, descriptive variable names might give away the purpose of the code itself. We’ll still follow the rules that are required, but in many cases we’ll use simple variable names so that the focus is learning to read the structure of the code, not inferring what it does based solely on the names of the variables.