12.3 🧠 Understanding Variable Scope in Python: Local vs Global Made Easy

Once you’ve learned how to define and use functions in Python, the next important step is to understand where your variables live and how long they stay alive — this is what we call scope.

Let’s break this down with simple examples and clear explanations!

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🔍 What is Variable Scope?

In Python, scope refers to the region of a program where a variable is recognized. A variable’s scope determines:

• Where it can be accessed
• Where it can be modified
• Where it no longer exists

There are mainly two types:

• Local Scope
• Global Scope

Let’s explore each.

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🏠 Local Scope

A local variable is declared inside a function and is accessible only within that function.

def greet():
    message = "Hello!"
    print(message)

greet()        # Output: Hello!
print(message) # ❌ Error: message is not defined

👉 message is local to the greet() function. You can’t access it outside.

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🌍 Global Scope

A global variable is declared outside all functions and can be accessed anywhere in the program.

message = "Hi!"  # Global variable

def greet():
    print(message)

greet()        # Output: Hi!
print(message) # Output: Hi!

👉 Since message was defined outside, it's globally accessible.

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🔁 Modifying Global Variables Inside a Function

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

⚠️ Without global keyword:

count = 10

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

update_count()

Why the error? Python thinks you’re creating a new local count, but it sees you trying to use it before it's defined.

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✅ With global keyword:

count = 10

def update_count():
    global count
    count = count + 1
    print(count)

update_count()   # Output: 11
print(count)     # Output: 11

Now you’ve told Python clearly — “I’m referring to the global count!”

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🎯 Good Practice Tip

Using global too much can make your program hard to debug. It’s better to use function arguments and return values whenever possible.

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🚀 Bonus: Variable Scope in Action

x = 5  # Global

def demo():
    x = 10  # Local
    print("Inside:", x)

demo()              # Output: Inside: 10
print("Outside:", x)  # Output: Outside: 5

Same name, different values — because they exist in different scopes.

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📌 Optional: Nested Functions & nonlocal keyword

(For advanced learners)

When you define a function inside another function, you can use the nonlocal keyword to modify a variable in the enclosing function.
We’ll explore this in depth in future lessons!

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✅ Summary

• Local variable: Declared inside a function, accessible only there.
• Global variable: Declared outside functions, accessible everywhere.
• Use the global keyword only when necessary.

Understanding scope helps you write cleaner, bug-free programs and is a must-know for every Python programmer!