Section Overview

In this section, you’ll learn how Python stores information using variables. You’ll also be introduced to Python’s basic data types: strings, numbers, and booleans. These are the building blocks of most Python programs.

By the end of this section, you should be able to:

• Create and use variables in Python
• Identify and work with different data types
• Convert between strings and numbers when needed

Lesson 1: What Is a Variable?

A variable is a name that stores a value. Think of it like a labeled box: the label is the variable’s name, and the contents are the value it holds.

Example:

name = "Alice"
age = 30 

In this example:

• name stores a string ("Alice")
• age stores a number (30)

Once you've stored a value, you can use it elsewhere in your program:

print(name)
print(age + 5)

Lesson 2: Data Types in Python

Python has several built-in data types. Here are the three most important for now:

1. String (str) – text, surrounded by quotes
   Example: "hello", 'Python'
2. Integer (int) – whole numbers
   Example: 5, -42, 2024
3. Boolean (bool) – True or False values
   Example: True, False

You can check a variable’s type using the type() function:

height = 175 print(type(height))  # Output: <class 'int'> 

Lesson 3: Working with Strings and Numbers

Strings and numbers are not the same thing—even if they look similar.

age = "30"      # This is a string
real_age = 30   # This is a number

print(age + age)        
# Output: 3030 (string concatenation) print(real_age + real_age)  # Output: 60 (numeric addition) 

To convert between them, use:

• int() – convert string to number
• str() – convert number to string

Example:

age = input("What is your age? ")
future_age = int(age) + 5 print("In five years, you'll be " + str(future_age) + ".")

Lesson 4: Naming Variables

Variable names should be descriptive, and they must follow these rules:

• Use letters, numbers, and underscores only
• Must start with a letter or underscore (not a number)
• Avoid spaces or special characters

Good examples:

user_name = "Ella"
total_score = 92 

Bad examples:

2name = "Invalid"     # starts with a number
user-name = "Error"   # hyphens aren't allowed 

Stick to lowercase with underscores for readability (called "snake_case").

Quiz: Check Your Understanding

1. Which of these is a valid variable name?
a) user-name
b) 2score
c) total_score
Answer: c) total_score

2. What data type is returned by the input() function?
a) Integer
b) Boolean
c) String
Answer: c) String

3. What does this code output?

x = "4"
y = 4 print(x + str(y))

Answer: 44 (string concatenation)

4. True or False: You can add an integer directly to a string in Python.
Answer: False – you must convert the integer to a string first using str().

Practice Exercise: Age Calculator

Write a program that:

1. Asks the user for their name
2. Asks for their current age
3. Calculates how old they will be in 10 years
4. Prints a message like:
   "Hi Mia, in 10 years you will be 35."

Starter code:

name = input("What is your name? ")
age = input("How old are you? ")
age_in_10 = int(age) + 10 print("Hi " + name + ", in 10 years you will be " + str(age_in_10) + ".")

Challenge: Add one more line to print what year that will be.