Day 13/100: Variable Scope – Global vs Local in Python

Welcome to Day 13 of the 100 Days of Python series!
Today, we’re diving into a super important concept: variable scope.

Have you ever defined a variable inside a function and then tried to access it outside — only to get an error? That’s a scope issue!

Understanding the difference between local and global variables will help you write clean, bug-free code.

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📦 What You’ll Learn

• What variable scope means
• The difference between local and global variables
• How scope affects access to variables
• The global keyword
• Real-world examples and best practices

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🔍 What Is Scope?

Scope determines where a variable can be accessed in your code.

In Python, there are two main types:

• Local scope: Variables declared inside a function.
• Global scope: Variables declared outside all functions.

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🧪 1. Local Variables

A variable defined inside a function is local to that function.

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

greet()
print(name)  # ❌ Error: name is not defined

name only exists inside the greet() function. Trying to access it outside causes an error.

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🌍 2. Global Variables

A variable defined outside all functions is global and can be accessed from anywhere in the script.

message = "Welcome!"

def greet():
    print(message)

greet()
print(message)  # ✅ This works

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⚠️ 3. Modifying Global Variables Inside Functions

You can read global variables inside functions, but to modify them, you need to use the global keyword.

Without global:

count = 0

def increment():
    count += 1  # ❌ Error: UnboundLocalError

This gives an error because Python treats count as a new local variable.

With global:

count = 0

def increment():
    global count
    count += 1

increment()
print(count)  # ✅ 1

Use the global keyword only when necessary, as it can make debugging harder.

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🎯 Real-World Example: User Sessions

# global session
is_logged_in = False

def login():
    global is_logged_in
    is_logged_in = True

def logout():
    global is_logged_in
    is_logged_in = False

login()
print("User logged in?", is_logged_in)  # True

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🧠 Variable Resolution: LEGB Rule

Python uses the LEGB rule to resolve variable names:

• L – Local: Inside the current function
• E – Enclosing: In enclosing function(s) (for nested functions)
• G – Global: Defined at the top level of the script
• B – Built-in: Provided by Python itself (print, len, etc.)

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🧼 Best Practices

• ✅ Prefer local variables to avoid side effects
• ✅ Use function parameters and return values for data transfer
• ⚠️ Avoid using global unless absolutely necessary
• 🚫 Never use the same name for local and global variables (confusing!)

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🧠 Recap

Today you learned:

• What variable scope is and why it matters
• The difference between local and global variables
• How the global keyword works
• The LEGB rule for resolving variable names
• Why using local scope is often safer

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