🧠Python Scope Explained – LEGB Rule, Global, Nonlocal, Best Practices

🧲 Introduction – Why Variable Scope Matters

In Python, scope refers to the area of a program where a variable is recognized and accessible. When your code contains nested functions, global declarations, or built-in names, understanding which variable Python is referring to is crucial.

Python follows a specific LEGB rule to resolve variable names:

• L – Local
• E – Enclosing
• G – Global
• B – Built-in

Misunderstanding scope often leads to hard-to-find bugs or UnboundLocalError. This guide will help you master scope and avoid such pitfalls.

🎯 In this guide, you’ll learn:

• The 4 scopes in Python defined by the LEGB rule
• How Python looks up variables
• Real-world examples of scope in nested functions
• When and how to use global and nonlocal

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🔍 The LEGB Rule Explained

📦 L – Local Scope

Defined inside a function and accessible only within that function.

def func():
    x = 10  # local scope
    print(x)

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🧩 E – Enclosing Scope

Variables from outer functions in nested functions.

def outer():
    y = "enclosing"
    def inner():
        print(y)  # enclosing scope
    inner()

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🌍 G – Global Scope

Defined at the top level of a script or module.

x = "global"

def func():
    print(x)  # global variable

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⚙️ B – Built-in Scope

Names preassigned in Python—like len, sum, print, etc.

print(len("Python"))  # Built-in function

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🔁 Scope Lookup Order

When you access a variable, Python searches in this order:

1. Local (inside current function)
2. Enclosing (in outer functions)
3. Global (at module level)
4. Built-in (Python reserved names)

As soon as Python finds a match, it stops searching.

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✅ Example: LEGB in Action

x = "global"

def outer():
    x = "enclosing"
    def inner():
        x = "local"
        print(x)
    inner()

outer()  # Output: local

📘 Explanation: The innermost assignment wins due to the LEGB search hierarchy.

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🔧 Using global Keyword

Modifies global variables inside functions.

count = 0

def increment():
    global count
    count += 1

⚠️ Use global sparingly—it makes code harder to debug and test.

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🔧 Using nonlocal Keyword

Modifies variables in enclosing (non-global) scope.

def outer():
    x = 10
    def inner():
        nonlocal x
        x += 1
        print(x)
    inner()

outer()  # Output: 11

✅ Useful for closures and decorators where the inner function needs to update outer variables.

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⚠️ Common Pitfalls

Issue	Example	Fix
UnboundLocalError	Using a variable before assigning	Declare variable before use
Overusing global	Frequent global mutations	Return values instead
Shadowing built-ins	Using sum = 10	Avoid overwriting built-ins

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💡 Best Practices

• ✅ Keep variables local whenever possible
• ✅ Use nonlocal only when necessary for closures
• ✅ Avoid using global unless absolutely required
• ✅ Never shadow built-in names like list, str, sum

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📌 Summary – Recap & Next Steps

Python uses the LEGB rule (Local, Enclosing, Global, Built-in) to determine variable scope and resolve names during execution. Understanding how this rule works ensures you write clean, predictable, and bug-free code.

🔍 Key Takeaways:

• ✅ Variables are resolved in the order: Local → Enclosing → Global → Built-in.
• ✅ global lets you write to global variables inside functions.
• ✅ nonlocal allows writing to variables in enclosing (outer) functions.
• ⚠️ Scope clarity is essential for nested functions and closures.

⚙️ Real-World Relevance:
Scope handling is critical in functional programming, decorators, closures, and frameworks like Flask/Django, where multiple nested functions share context.

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❓ FAQ Section – Python Scope (LEGB)

❓ What is scope in Python?

✅ Scope determines where a variable can be accessed. Python uses LEGB—Local, Enclosing, Global, and Built-in—to resolve variable names.

❓ What does LEGB stand for?

✅ LEGB refers to the order Python searches for variable names:
Local → Enclosing → Global → Built-in.

❓ What is a global variable?

✅ A global variable is defined at the top level of a Python script and is accessible from any function (unless shadowed).

❓ How do I modify a global variable inside a function?

✅ Use the global keyword:

global x

❓ What is nonlocal used for?

✅ The nonlocal keyword allows modification of variables from an enclosing (non-global) scope, typically used in nested functions.

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