Lesson 2: Variables and Data Types

Lesson 2: Variables and Data Types#

Variables are like labeled boxes that store information. They’re one of the most fundamental concepts in programming!

What You’ll Learn#

Creating and naming variables
Understanding different data types
Type checking and conversion
Variable operations and best practices

💡 Real-World Analogy#

Think of variables like labeled storage containers:

The label is the variable name (e.g., age)
The contents are the value (e.g., 25)
The type of container determines what you can store (numbers, text, etc.)

1. Creating Your First Variables#

A variable is created when you assign a value using the = sign.

Syntax: variable_name = value

# Creating variables
name = "Alice"
age = 25
height = 5.6
is_student = True

# Display the variables
print("Name:", name)
print("Age:", age)
print("Height:", height)
print("Is student:", is_student)

📝 Variable Naming Rules#

Rules you MUST follow:

Can contain letters, numbers, and underscores
Must start with a letter or underscore (not a number)
Case-sensitive (age and Age are different!)
Cannot use Python keywords (if, for, while, etc.)

Good naming conventions:

Use lowercase with underscores: user_name, total_score
Be descriptive: age is better than a
Avoid single letters (except for loops: i, j, k)

# Good variable names
user_name = "Bob"
total_score = 100
is_valid = True

# Bad variable names (but technically valid)
x = "Bob"  # Too vague
UsErNaMe = "Bob"  # Hard to read
name123 = "Bob"  # Numbers are okay, but not meaningful here

# Invalid variable names (will cause errors if uncommented)
# 123name = "Bob"  # Can't start with number
# user-name = "Bob"  # Hyphens not allowed
# for = "Bob"  # Can't use Python keywords

2. Data Types in Python#

Every value in Python has a type that determines what you can do with it.

Main Data Types#

Type	Description	Example
str (String)	Text data	`"Hello"`, `'Python'`
int (Integer)	Whole numbers	`42`, `-17`, `0`
float	Decimal numbers	`3.14`, `-0.5`, `2.0`
bool (Boolean)	True or False	`True`, `False`

Strings (Text Data)#

Strings store text and must be wrapped in quotes (single or double).

# Both single and double quotes work
message1 = "Hello, World!"
message2 = 'Hello, World!'

print(message1)
print(message2)

# Use single quotes if your text contains double quotes
quote = 'She said, "Python is awesome!"'
print(quote)

# Or use double quotes if your text contains single quotes
sentence = "It's a beautiful day!"
print(sentence)

Integers (Whole Numbers)#

Integers are whole numbers without decimal points.

score = 100
temperature = -5
year = 2024

print("Score:", score)
print("Temperature:", temperature)
print("Year:", year)

# You can use underscores for readability in large numbers
population = 1_000_000  # Same as 1000000
print("Population:", population)

Floats (Decimal Numbers)#

Floats represent numbers with decimal points.

pi = 3.14159
price = 19.99
height = 5.8

print("Pi:", pi)
print("Price: $", price)
print("Height:", height, "feet")

# Numbers with .0 are also floats
whole_number_float = 10.0
print(type(whole_number_float))  # This is a float, not an int!

Booleans (True/False Values)#

Booleans represent truth values. Must be capitalized!

is_raining = True
is_sunny = False
has_umbrella = True

print("Is it raining?", is_raining)
print("Is it sunny?", is_sunny)
print("Do I have an umbrella?", has_umbrella)

# Booleans are useful for decisions (we'll learn more about this later)
# Note: True and False must be capitalized
# true or false (lowercase) will cause an error

3. Checking Variable Types#

Use the type() function to check what type a variable is.

name = "Alice"
age = 25
height = 5.6
is_student = True

print("Type of name:", type(name))
print("Type of age:", type(age))
print("Type of height:", type(height))
print("Type of is_student:", type(is_student))

⚠️ Watch Out!#

Numbers in quotes are strings, not numbers!

number1 = 42        # This is an integer
number2 = "42"      # This is a string!

print("number1:", number1, "Type:", type(number1))
print("number2:", number2, "Type:", type(number2))

# They look the same when printed, but they're different types!
# This matters when you try to do math (you can't do math with strings)

4. Type Conversion#

Sometimes you need to convert between types. Python provides functions for this:

str() - Convert to string
int() - Convert to integer
float() - Convert to float
bool() - Convert to boolean

Converting to String#

age = 25
age_as_string = str(age)

print("Original age:", age, "Type:", type(age))
print("Age as string:", age_as_string, "Type:", type(age_as_string))

# This is useful when combining text and numbers
message = "I am " + str(age) + " years old."
print(message)

Converting to Integer#

# String to integer
text_number = "100"
actual_number = int(text_number)

print("Text number:", text_number, "Type:", type(text_number))
print("Actual number:", actual_number, "Type:", type(actual_number))

# Now we can do math with it!
result = actual_number + 50
print("100 + 50 =", result)

# Float to integer (removes decimal part)
decimal = 3.99
whole = int(decimal)
print(f"{decimal} as integer: {whole}")  # Note: it doesn't round, just truncates!

Converting to Float#

# Integer to float
whole_number = 42
decimal_number = float(whole_number)

print(f"{whole_number} as float: {decimal_number}")

# String to float
price_text = "19.99"
price = float(price_text)
print(f"Price: ${price}")

# Now we can calculate
with_tax = price * 1.08
print(f"Price with tax: ${with_tax:.2f}")  # .2f means 2 decimal places

⚠️ Conversion Errors#

Not all conversions work! Be careful:

# This works:
number1 = int("123")
print("Success:", number1)

# Uncomment to see errors:
# number2 = int("Hello")  # Error! Can't convert text to number
# number3 = int("12.5")   # Error! Can't convert decimal string to int
                           # Use float() first, then int()

# The correct way for decimal strings:
decimal_string = "12.5"
correct = int(float(decimal_string))  # First to float, then to int
print("Correct conversion:", correct)

5. Variable Operations#

Updating Variables#

Variables can change their values!

score = 10
print("Initial score:", score)

score = 20  # Change the value
print("New score:", score)

score = score + 5  # Use the old value to calculate the new value
print("Updated score:", score)

# Shorthand for updating
score += 10  # Same as: score = score + 10
print("Final score:", score)

Multiple Assignment#

You can assign multiple variables in one line!

# Assign multiple variables at once
x, y, z = 1, 2, 3
print("x:", x)
print("y:", y)
print("z:", z)

# Assign same value to multiple variables
a = b = c = 0
print("a:", a, "b:", b, "c:", c)

# Swap values (Python makes this easy!)
first = 100
second = 200
print("Before swap:", first, second)

first, second = second, first  # Swap!
print("After swap:", first, second)

6. None Type#

None represents “no value” or “nothing”. It’s useful for placeholders.

# Variable with no value yet
user_input = None
result = None

print("user_input:", user_input)
print("Type:", type(user_input))

# You can check if a variable is None
if user_input is None:
    print("No input received yet!")

📝 Exercises#

Exercise 1: Personal Profile#

Create variables to store your information:

Your name (string)
Your age (integer)
Your height in feet (float)
Whether you like Python (boolean)

Print all of them with labels.

# Your code here

Exercise 2: Favorite Book#

Create variables about your favorite book:

Title (string)
Number of pages (integer)
Your rating out of 5 (float)
Have you finished reading it? (boolean)

Print them all in a readable format.

# Your code here

Exercise 3: Type Conversions#

Create a string variable with the value "100"
Convert it to an integer
Add 50 to it
Convert the result to a string
Print the final result and its type

# Your code here

Exercise 4: Temperature Converter#

Create a variable for temperature in Fahrenheit (e.g., 72.5), then:

Convert it to Celsius using the formula: (F - 32) * 5/9
Print both temperatures with labels
Convert the Celsius temperature to an integer (whole number)
Print the rounded temperature

# Your code here

Exercise 5: Variable Swap#

Create two variables:

first = "Python"
second = "Programming"

Swap their values so that first contains “Programming” and second contains “Python”. Print before and after.

# Your code here

Exercise 6: Shopping Cart#

You’re building a shopping cart. Create variables for:

Item name (string)
Price (float)
Quantity (integer)
In stock? (boolean)

Calculate the total cost (price * quantity) and print a summary.

# Your code here

✅ Self-Check Quiz#

Before moving on, make sure you can answer these:

What is a variable?
What are the four main data types in Python?
How do you check the type of a variable?
What’s the difference between 42 and "42"?
How do you convert a string to an integer?
Can variable names start with numbers?
What does None represent?
How do you swap two variables in Python?

🎯 Key Takeaways#

Variables store data that you can use and modify
Data types determine what you can do with values:
- str for text
- int for whole numbers
- float for decimals
- bool for True/False
Use type() to check a variable’s type
Convert types with str(), int(), float(), bool()
Choose descriptive variable names
Variables can change their values

🚀 Next Steps#

Great work! You now know how to store and work with data in Python.

Next, you’ll learn about operations and conditionals - how to do math and make decisions in your code!

Continue to: 03_basic_operations_and_conditionals.ipynb

💡 Pro Tips#

Use meaningful names: user_age is better than x
Be consistent: Stick to one naming style (we use snake_case)
Watch your types: Can’t do math with strings!
Check types when debugging: print(type(variable)) is your friend
Practice type conversion: You’ll use it all the time in real programs