Scope & Closures (LEGB)

Introduction

Python resolves names using LEGB: Local → Enclosing → Global → Built-in. Closures capture enclosing-scope variables — essential for decorators.

Understanding the topic

Core concepts to understand:

• Local: inside the function.
• Enclosing: outer function (closure).
• Global: module level.
• Built-in: print, len, etc.
• nonlocal and global declare write-through.

Syntax reference

Visual flow / code:

def make_counter():
    count = 0
    def increment():
        nonlocal count
        count += 1
        return count
    return increment

c = make_counter()
print(c())  # 1
print(c())  # 2
print(c())  # 3

Execution workflow

Real-world use

Closures power decorators, factory functions, and stateful callbacks. Understand LEGB before tackling decorators.

Best practices

• Avoid global — return values instead.
• Use nonlocal for closures.
• Keep state in classes when complex.

Common mistakes

• Forgetting nonlocal creates a new local var, not the outer one.

Hands-on exercise

Interview preparation — practice these questions:

• What's LEGB?
• What's a closure?
• nonlocal vs global?

Summary

In summary: LEGB rules name lookup. Closures = functions + captured scope.