Variables & Scoping Rules

Expert Answer & Key Takeaways

Mastering Variables & Scoping Rules is essential for high-fidelity technical performance and advanced exam competency in 2026.

Python Variables & Scoping Rules: The LEGB Rule

Python resolves variable names using the LEGB rule (Local, Enclosing, Global, Built-in), determining where a variable can be accessed or modified within a script.

1. The Proof Code

# Global Scope
counter: int = 0

def outer_function() -> None:
    # Enclosing Scope
    text: str = "Initial"
    
    def inner_function() -> None:
        # Local Scope
        nonlocal text
        global counter
        
        text = "Modified by Inner"
        counter += 1
        print(f"Local access: {text}, Global access: {counter}")

    inner_function()
    print(f"Enclosing after inner: {text}")

if __name__ == "__main__":
    outer_function()
    print(f"Final Global counter: {counter}")

# Output:
# Local access: Modified by Inner, Global access: 1
# Enclosing after inner: Modified by Inner
# Final Global counter: 1

2. Execution Breakdown

Local Scope: Variables defined inside a function. text in inner_function would be local unless explicitly declared otherwise.
Enclosing Scope: Variables in the local scope of outer functions (relevant for nested functions).
Global Scope: Variables defined at the top level of a module or declared global inside a function.
Built-in Scope: Reserved names like len, print, and ValueError provided by Python automatically.
Keywords: The global keyword allows modification of variables in the module-level scope; nonlocal allows modification of variables in the nearest enclosing (non-global) scope.

3. Detailed Theory

Python's scoping is determined by where you assign a variable, not where you first use it. This is a common source of bugs for developers coming from languages like JavaScript or C++.

The LEGB Hierarchy

L (Local): Names assigned in any way within a function (def or lambda), and not declared global in that function.
E (Enclosing): Names in the local scope of any and all enclosing functions, from inner to outer.
G (Global): Names assigned at the top-level of a module file, or declared global in a def within the file.
B (Built-in): Names preassigned in the built-in names module: open, range, SyntaxError, etc.

Name Resolution vs. Assignment

When you read a variable, Python searches L -> E -> G -> B. When you assign a variable, Python creates a local name by default unless you use global or nonlocal.

[!TIP] Senior Secret: Minimize the use of global. It makes functions 'impure' and difficult to test because they depend on external state. Instead, pass the global variable as an argument and return the updated value.

Variables & Scoping Rules

Expert Answer & Key Takeaways

Python Variables & Scoping Rules: The LEGB Rule

1. The Proof Code

2. Execution Breakdown

3. Detailed Theory

The LEGB Hierarchy

Name Resolution vs. Assignment

Top Interview Questions

?Interview Question

?Interview Question

Course4All Engineering Team

Found an issue or have a suggestion?

Variables & Scoping Rules

Expert Answer & Key Takeaways

Python Variables & Scoping Rules: The LEGB Rule

1. The Proof Code

2. Execution Breakdown

3. Detailed Theory

The LEGB Hierarchy

Name Resolution vs. Assignment

Top Interview Questions

?Interview Question

?Interview Question

Course4All Engineering Team

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