Python — Variables

What are Variables?

Variables store data values. In Python, you don't need to declare the type — Python figures it out automatically:

name = "Alice"      # string
age = 25            # integer
height = 5.7        # float
is_student = True   # boolean

Creating Variables

# Assignment
x = 10
name = "Bob"
pi = 3.14159

# Multiple assignment
a, b, c = 1, 2, 3

# Same value to multiple variables
x = y = z = 0

# Unpacking a list
coordinates = [10, 20, 30]
x, y, z = coordinates

Naming Rules

# Valid names
first_name = "Alice"    # snake_case (Python convention)
_private = True         # underscore prefix
MAX_SIZE = 100          # UPPER_CASE for constants
count2 = 5              # can contain numbers

# Invalid names
# 2count = 5            # can't start with number
# my-name = "Bob"       # no hyphens
# class = "Math"        # can't use reserved words

Rules:

• Start with a letter or underscore
• Letters, numbers, and underscores only
• Case-sensitive (name ≠ Name)
• Use snake_case for variables, UPPER_CASE for constants

Dynamic Typing

Python figures out the type automatically:

x = 10          # x is int
x = "hello"     # x is now string (reassigned)
x = [1, 2, 3]  # x is now list

# Check type
type(x)         # <class 'list'>
isinstance(x, list)  # True

Variable Scope

Local Variables

Created inside a function — only accessible there:

def greet():
    message = "Hello!"  # local variable
    print(message)

greet()    # works
# print(message)  # Error: name not defined

Global Variables

Created outside functions — accessible everywhere:

name = "Alice"  # global

def greet():
    print(f"Hello, {name}")  # can read global

greet()  # Hello, Alice

Modifying Global Variables

count = 0

def increment():
    global count  # declare intent to modify
    count += 1

increment()
print(count)  # 1

Variable Reassignment

x = 5
print(x)   # 5

x = 10
print(x)   # 10

# Python variables can change type
x = "hello"
print(x)   # hello

Multiple Assignment Patterns

# Swap values
a, b = 1, 2
a, b = b, a  # a = 2, b = 1

# Unpack strings
first, *rest = "hello"
# first = 'h', rest = ['e', 'l', 'l', 'o']

# Underscore for unused values
_, second, _ = (1, 2, 3)  # second = 2

Type Hints (Optional)

Python supports optional type hints for clarity:

name: str = "Alice"
age: int = 25
height: float = 5.7
active: bool = True

def greet(name: str) -> str:
    return f"Hello, {name}!"

Best Practices

• Use descriptive names (user_age, not ua)
• Use snake_case for variables, UPPER_CASE for constants
• Avoid single-letter names except in loops (i, j, x)
• Initialize variables before using them
• Use type hints for function signatures
• Don't use Python built-in names (list, str, int, type)

Common Mistakes

# Forgetting quotes
# message = Hello    # Error: name not defined
message = "Hello"    # Correct

# Confusing assignment and comparison
# if x = 5:          # Error
if x == 5:           # Correct

# Shadowing built-ins
# list = [1, 2, 3]   # Bad: shadows list()
my_list = [1, 2, 3]  # Good

Mini Practice

1. Create variables of different types and print them
2. Swap two variables without a temporary variable
3. Write a function with type hints
4. Practice variable scope with local and global variables
5. Create a simple calculator using variables

Up Next

Next: Data Types →