Variables

How it looks in code:

To create a variable, we give it a name, use the equals sign (=), and then give it a value.

• 'user_name' is our variable (the box)
• 'input()' is the information we are putting inside it

user_name = input("What is your name?")
print("Hello, " + user_name)
    

Expected output

What is your name?Fred
Hello, Fred
    

Rules for Naming Variables

In Python, you can name your variables almost anything, but there are a few important rules and "best practices" you must follow. If you break these rules, your program won't run!

1. The Starting Character

A variable name must start with a letter or an underscore (_). It cannot start with a number.

✅ user_name = 25
❌ 1st_place = "gold" # this will cause an error
    

2. Letters, Numbers, and Underscores

After the first character, you can use letters, numbers, and underscores. You cannot use spaces or special symbols (like !, @, #, or $).

✅ player_score_1 = 100
❌ player score = 100 # (Spaces are not allowed)
    

3. Case Sensitivity

Python is case-sensitive. This means Username, username, and USER_NAME are three completely different variables.

• age = 20
• Age = 30
• (In Python's eyes, these are two different boxes!)
4. Reserved Words

You cannot use words that Python already uses for its own instructions (these are called Keywords). For example, you can't name a variable print or if.

5. "Snake Case"

In Python, the most common way to name variables with multiple words is to use snake_case . This means using all lowercase letters and separating words with an underscore.

Good: total_price, is_logged_in, favorite_color

The Underscore Prefix

Sometimes, you might see a variable name that starts with a single underscore, like this:

_my_dog = "fat"

In Python, starting a variable name with an underscore is a special signal to other programmers. It indicates that the variable is intended to be "private." This means the variable is meant for internal use within a specific part of the code and shouldn't be messed with from the outside. Don't worry about the technical details of "private" data yet—we will dive much deeper into this in a later lesson! For now, just remember that the underscore acts like a "Do Not Disturb" sign.

Practice Naming Variables

• Create a variable that starts with a capital letter.
• Try to name a variable starting with a number and observe the error.
• What happens if you try to print a variable using a mismatched case (e.g., define 'score' but print 'Score')?
• Try naming a variable 'print' or another python keyword. "Follow the link to see a list of Python keywords"

Exercise 1: The Case-Sensitive Trap

Create a variable called user_score and set it to 100. On the next line, try to print the variable using a different casing, such as print(User_Score).

The Task: Run the code and look at your terminal. In a comment in your code, write down the specific name of the error that Python gives you when it can't find a variable due to a case mismatch.

Exercise 2: Illegal Names Hunt

Try to create three variables that intentionally break the naming rules: one starting with a number, one containing a space, and one using a special symbol (like $ or @).

The Task: Run your code after each attempt. In your script, use a comment to note whether Python gives you the same error type for all three, or if the errors are different.

Exercise 3: The "Snake Case" Reformat

In the professional world, consistency is key. You have been handed these "badly named" variables:

• UserAge = 25
• total-cost = 99.99
• PLAYERNAME = "Ace"

The Task: Rewrite these three variables in your script using the proper snake_case convention. Use print() to output each one to ensure your new names are valid and the script runs without errors.

Exercise 4: The Variable Update

Variables are dynamic! Create a variable called wallet and set it to 50. On the next line, "update" it by writing: wallet = wallet + 20.

The Task: Print the wallet variable to see the new total. Below that line, add a comment explaining in your own words how Python used the "old" value of the box to calculate the "new" value.

Exercise 5: Keywords & Syntax Highlighting

Python reserves certain words for its own use. Attempt to create a variable named pass = True or class = "Python".

The Task: Look up a list of Python Keywords. Pick three keywords you haven't used yet and try to use them as variable names in VS Code. In a comment, describe how the color of the text changed to warn you that these are special reserved words.

Advanced Naming Conventions

There are a couple more things to keep in mind when naming your variables as you progress. First, you might see names that use a double underscore (like: __init__). While it might look like a single long line, it is actually two underscores, and in Python, this has a distinct meaning. These are often called "Dunder" (Double-Under) variables, and they are reserved for special internal functions that we will cover in a later module.

Additionally, you may encounter variables written in ALL CAPS, such as MAX_SPEED = 100. In the programming world, this is a signal that the variable is a Constant—a value that is set once and is not intended to be changed while the program is running.