What are Variables?

Variables are containers that store data values. Think of them as labeled boxes where you put information.

name = "Alice"  # A box labeled 'name' contains "Alice"
age = 25        # A box labeled 'age' contains 25

Why Use Variables?

Without variables (repeating values):

print("Alice")
print("Alice is 25 years old")
print("Alice likes Python")

With variables (reusable):

name = "Alice"
age = 25

print(name)
print(name, "is", age, "years old")
print(name, "likes Python")

Change the value once, it updates everywhere!

Creating Variables

Basic Syntax

variable_name = value

Examples

# Numbers
score = 100
temperature = 98.6
pi = 3.14159

# Text (strings)
greeting = "Hello"
name = "Bob"
city = 'New York'

# True/False (booleans)
is_active = True
is_logged_in = False

Rules for Variable Names

✅ Allowed:

age = 25
user_name = "Alice"
total_score = 100
_private = "hidden"
number2 = 42

❌ Not Allowed:

2nd_place = "Silver"    # Can't start with number
user-name = "Bob"       # No hyphens
first name = "Alice"    # No spaces
class = "Python"        # Can't use reserved words

Naming Conventions (PEP 8)

Use snake_case for variables:

# Good
user_name = "Alice"
total_amount = 100
is_valid = True

# Avoid
userName = "Alice"      # camelCase
TotalAmount = 100       # PascalCase

Make names descriptive:

# Good - clear meaning
student_count = 30
max_temperature = 75
user_email = "alice@example.com"

# Bad - unclear
x = 30
temp = 75
ue = "alice@example.com"

Data Types

Python has several built-in data types. Let's explore the basics:

1. Integers (int)

Whole numbers without decimals:

age = 25
year = 2025
temperature = -10
large_number = 1000000

Operations:

x = 10
y = 3

print(x + y)   # 13 (addition)
print(x - y)   # 7 (subtraction)
print(x * y)   # 30 (multiplication)
print(x / y)   # 3.333... (division)
print(x // y)  # 3 (integer division)
print(x % y)   # 1 (remainder/modulus)
print(x ** y)  # 1000 (exponentiation)

2. Floats (float)

Numbers with decimal points:

price = 19.99
temperature = 98.6
pi = 3.14159
negative = -5.5

Operations:

x = 10.5
y = 2.5

print(x + y)   # 13.0
print(x - y)   # 8.0
print(x * y)   # 26.25
print(x / y)   # 4.2

3. Strings (str)

Text enclosed in quotes:

name = "Alice"
message = 'Hello, World!'
address = """123 Main Street
Apt 4B
New York, NY"""

String Operations:

first_name = "John"
last_name = "Doe"

# Concatenation
full_name = first_name + " " + last_name
print(full_name)  # John Doe

# Repetition
laugh = "ha" * 3
print(laugh)  # hahaha

# Length
print(len(full_name))  # 8

4. Booleans (bool)

True or False values:

is_student = True
is_graduated = False
has_license = True

print(5 > 3)   # True
print(10 == 5) # False
print(7 < 2)   # False

Checking Types

Use type() to check a variable's type:

age = 25
print(type(age))        # <class 'int'>

price = 19.99
print(type(price))      # <class 'float'>

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

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

Type Conversion

Convert between types:

# String to Integer
age_str = "25"
age_int = int(age_str)
print(age_int + 5)  # 30

# String to Float
price_str = "19.99"
price_float = float(price_str)
print(price_float * 2)  # 39.98

# Number to String
score = 100
score_str = str(score)
print("Your score is " + score_str)

# Integer to Float
x = 5
y = float(x)
print(y)  # 5.0

Multiple Assignment

Assign multiple variables at once:

# Multiple assignment
x, y, z = 10, 20, 30
print(x, y, z)  # 10 20 30

# Same value to multiple variables
a = b = c = 100
print(a, b, c)  # 100 100 100

# Swap values
x, y = 5, 10
x, y = y, x
print(x, y)  # 10 5

Practice Exercises

1. Create variables for your name, age, and city
2. Calculate and print your age in months
3. Create a greeting message using string concatenation
4. Convert a string number to an integer and perform math
5. Check the types of different variables

Example Solution:

# Exercise 1
name = "Alice"
age = 25
city = "New York"

# Exercise 2
age_in_months = age * 12
print("Age in months:", age_in_months)

# Exercise 3
greeting = "Hello, my name is " + name
print(greeting)

# Exercise 4
num_str = "100"
num_int = int(num_str)
result = num_int + 50
print("Result:", result)

# Exercise 5
print(type(name))
print(type(age))
print(type(city))

Key Takeaways

✅ Variables store data values
✅ Use descriptive names in snake_case
✅ Main types: int, float, str, bool
✅ Use type() to check data types
✅ Convert types with int(), float(), str()

Next Lesson: Basic Operations