Local and Global Variables in Python: Explained with Examples and Tasks

Learn the difference between local and global variables in Python with simple explanations, practical examples, and beginner-friendly tasks. Understand constants, best practices, and how to manage variables effectively in your programs.

Local Variables

What is a Local Variable?

• Local variables are variables declared inside a function. They can only be accessed within that function and are destroyed when the function completes execution..
• It only exists while the function is running.
• You cannot use it outside the function.

Characteristics:

• Created when the function starts
• Only accessible within the function
• Destroyed when the function exits

Example:

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

greet()

# This will cause an error:
# print(message)

message is local to greet() and can’t be accessed outside.

Key Points:

• Local variables are created when the function starts.
• They are destroyed when the function ends.
• They do not affect variables outside the function.

Task for Students:

Write a function called add_numbers that:

• Takes two numbers as input.
• Adds them together.
• Stores the result in a local variable.
• Prints the result.

Example Solution:

def add_numbers(a, b):
    result = a + b
    print("Sum:", result)

add_numbers(5, 10)

Global Variables and Constants

What is a Global Variable?

• A global variable is a variable that is declared outside all functions and can be accessed throughout the entire program..
• It can be used both inside and outside functions.

Example:

name = "Ali"  # global variable

def greet():
    print("Hello", name)

greet()
print("Goodbye", name)

name is accessible inside and outside the function.

Changing a Global Variable Inside a Function

• If you want to modify a global variable inside a function, you must use the global keyword.

count = 0

def increment():
    global count
    count += 1

increment()
print(count)  # Output: 1

Without global, Python would create a new local variable named count.

Python Tutorial: Local vs Global Variables in Python

Constants

• A constant is a variable that should not change once assigned.
• In Python, constants are typically declared as global variables with ALL_CAPS names by convention.
• Python does not force constants, but developers treat them as unchangeable.

In programming:

• A convention is a common habit or best practice that developers choose to follow to make code more readable and consistent.
• Python itself does not force it, but good programmers still follow it.

Example:

PI = 3.14159  # constant

def area_of_circle(radius):
    return PI * radius * radius

print(area_of_circle(5))

PI = 3.14159  # This is a constant (by convention)
TAX_RATE = 0.20  # Another constant

def calculate_tax(amount):
    return amount * TAX_RATE

print(calculate_tax(100))  # Output: 20.0

Best Practices:

1. Avoid excessive use of global variables as they can make code harder to maintain
2. Use ALL_CAPS for constants to indicate they shouldn’t be modified

3. When you must use global variables, declare them clearly at the top of your file

Tasks:

Task #1: Global Variable Task

• Create a global variable called language set to "Python".
• Write a function show_language() that prints "I love Python!" using the global variable.

Task #2:

• Define a constant GRAVITY = 9.8.
• Write a function weight_on_earth(mass) that calculates and returns the weight.

Task #3:

Create a program with:

1. A global constant DISCOUNT set to 0.10
2. A global variable total_purchases initialized to 0
3. A function make_purchase(amount) that adds to total_purchases and applies the discount
4. A function show_total() that prints the current total

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