Variable Scoping, LEGB Rule & global / nonlocal Keywords

1Concept

Python resolves variables using the LEGB Rule (Local -> Enclosing -> Global -> Built-in). To reassign a variable outside local scope, use global (module-level) or nonlocal (nested enclosing function).

2Architecture Diagram

LEGB Scope Hierarchy:
 [L] Local Scope (Inside function)
      └──► [E] Enclosing Scope (Outer function closure)
            └──► [G] Global Scope (Module file)
                  └──► [B] Built-in Scope (len, range, print)

3Code Example

counter = 100 # Global

def outer_function():
    count = 10 # Enclosing

    def inner_function():
        nonlocal count # Modify enclosing variable
        count += 5
        print(f"Inner modified enclosing count: {count}")

    inner_function()
    print(f"Outer count after inner call: {count}")

outer_function()
print(f"Global counter remains: {counter}")

4Expected Output

Inner modified enclosing count: 15
Outer count after inner call: 15
Global counter remains: 100

5Key Takeaways

• ✓Variables declared in if-blocks or loops are NOT block-scoped; they exist at function scope.
• ✓Use nonlocal to modify variables in closure wrappers.
• ✓The LEGB rule searches from innermost to outermost scope.