📦 Lesson 3: Variables — Teaching Python to Remember

📦 What Are Variables?

Imagine you have a bunch of labeled boxes where you can store things. That's exactly what variables are in programming — labeled containers for storing information!

✏️ Creating Variables

Creating a variable is as simple as giving a name to a value:

# Creating variables
my_name = "Sarah"
my_age = 28
my_height = 5.6
likes_pizza = True

# Using variables
print("Hello, my name is", my_name)
print("I am", my_age, "years old")
print("I am", my_height, "feet tall")
print("I like pizza:", likes_pizza)

Hello, my name is Sarah
I am 28 years old
I am 5.6 feet tall
I like pizza: True

📏 Variable Naming Rules

Just like you can't name your pet "123" or "my-dog", Python has rules for naming variables:

Good Variable Names vs Bad Variable Names

# Good names - descriptive and follow rules
user_name = "John"
age_in_years = 30
total_score = 95.5
is_student = True

# Bad names - these will cause errors!
# 2fast = "car"         # Can't start with number
# my-name = "Bob"       # No hyphens allowed
# my age = 25           # No spaces allowed
# class = "Math"        # 'class' is a Python keyword

# Constants - values that shouldn't change
PI = 3.14159
MAX_USERS = 100
DEFAULT_COLOR = "blue"
TAX_RATE = 0.08

# Use them like regular variables
circle_area = PI * radius ** 2
final_price = price * (1 + TAX_RATE)

🗂️ Types of Data

Python variables can store different types of information, like different sections in your backpack:

# Check the type of any variable
age = 25
print(type(age))          # <class 'int'>

height = 5.9
print(type(height))       # <class 'float'>

name = "Python"
print(type(name))         # <class 'str'>

is_student = True
print(type(is_student))   # <class 'bool'>

# Useful for debugging!
mystery = "42"
print(f"mystery contains {mystery} of type {type(mystery)}")

🔄 Variables Can Change!

That's why they're called "variables" — they can vary! Watch this magic:

# Variables can change their values
mood = "happy"
print("I am feeling", mood)

mood = "excited"
print("Now I am feeling", mood)

mood = "sleepy"
print("After coding, I am feeling", mood)

# Variables can even change type!
mystery_box = 42
print("The box contains:", mystery_box)

mystery_box = "a surprise"
print("Now the box contains:", mystery_box)

I am feeling happy
Now I am feeling excited
After coding, I am feeling sleepy
The box contains: 42
Now the box contains: a surprise

⚙️ Working with Variables

Variables become powerful when you use them together:

# Variables in action
first_name = "Maria"
last_name = "Garcia"
full_name = first_name + " " + last_name
print("Full name:", full_name)

# Math with variables
width = 10
height = 5
area = width * height
print("The area of the rectangle is:", area)

# Updating based on other variables
savings = 100
paycheck = 500
savings = savings + paycheck  # Add paycheck to savings
print("New savings balance:", savings)

Full name: Maria Garcia
The area of the rectangle is: 50
New savings balance: 600

✨ The Power of F-Strings

Python has a super cool way to include variables in text called f-strings. It's like mail merge for your code!

# Without f-strings (the old way)
name = "Alex"
age = 25
print("Hello, my name is", name, "and I am", age, "years old")

# With f-strings (the modern way)
print(f"Hello, my name is {name} and I am {age} years old")

# F-strings can do math too!
items = 5
price = 10.99
print(f"Total cost for {items} items: ${items * price}")

Hello, my name is Alex and I am 25 years old
Hello, my name is Alex and I am 25 years old
Total cost for 5 items: $54.95

# Number formatting
price = 19.99
print(f"Price: ${price:.2f}")        # $19.99 (2 decimal places)

percentage = 0.856
print(f"Success rate: {percentage:.1%}")  # 85.6%

large_number = 1234567
print(f"Population: {large_number:,}")    # 1,234,567

# Alignment and padding
name = "Alice"
print(f"|{name:>10}|")   # |     Alice| (right align, 10 chars)
print(f"|{name:<10}|")   # |Alice     | (left align)
print(f"|{name:^10}|")   # |  Alice   | (center)

# Date formatting (with datetime)
from datetime import datetime
today = datetime.now()
print(f"Today is {today:%B %d, %Y}")  # Today is October 15, 2024

🐛 Common Mistakes and How to Fix Them

# Wrong
print(favorite_color)  # Error! Variable doesn't exist yet
favorite_color = "blue"

# Right
favorite_color = "blue"
print(favorite_color)

# Wrong
name = Sarah  # Error! Python thinks Sarah is a variable

# Right
name = "Sarah"  # Now Python knows it's text

🕵️ Interactive Exercise: Variable Detective

Can you predict what each print statement will show?

# Mystery Code #1
x = 10
y = 3
x = x + y
y = x - y
x = x - y
print(f"x = {x}, y = {y}")  # What will this print?

# Mystery Code #2
word = "Python"
word = word + " is"
word = word + " awesome!"
print(word)  # What will this print?

# Mystery Code #3
temperature = 32
temperature = (temperature - 32) * 5/9
print(f"Temperature in Celsius: {temperature}")  # What will this print?

# Mystery Code #1 — the values swap!
x = 3, y = 10

# Mystery Code #2 — string concatenation builds up
Python is awesome!

# Mystery Code #3 — Fahrenheit to Celsius
Temperature in Celsius: 0.0

🏋️ Practice Exercises

first_name = "Alex"
favorite_number = 7
height_inches = 68
likes_chocolate = True

print(f"Hi, I'm {first_name}!")
print(f"My favorite number is {favorite_number}")
print(f"I'm {height_inches} inches tall")
print(f"Do I like chocolate? {likes_chocolate}")

Hi, I'm Alex!
My favorite number is 7
I'm 68 inches tall
Do I like chocolate? True

# Shopping Calculator
item1_price = 12.99
item2_price = 24.50
item3_price = 7.99

subtotal = item1_price + item2_price + item3_price
TAX_RATE = 0.08
tax = subtotal * TAX_RATE
total = subtotal + tax

print("=== RECEIPT ===")
print(f"Item 1:   ${item1_price:.2f}")
print(f"Item 2:   ${item2_price:.2f}")
print(f"Item 3:   ${item3_price:.2f}")
print(f"Subtotal: ${subtotal:.2f}")
print(f"Tax (8%): ${tax:.2f}")
print(f"Total:    ${total:.2f}")

=== RECEIPT ===
Item 1:   $12.99
Item 2:   $24.50
Item 3:   $7.99
Subtotal: $45.48
Tax (8%): $3.64
Total:    $49.12

fahrenheit = 72
celsius = (fahrenheit - 32) * 5 / 9

print(f"{fahrenheit}°F is equal to {celsius:.1f}°C")

72°F is equal to 22.2°C

x = 5
x = x + 3
print(x)

Summary

🚀 What's Next?

Now that Python can remember things with variables, we'll learn how to get input from users. This will make your programs interactive — they'll be able to have conversations with people!