Python Scope

Python Scope: LEGB Rule, Globals, & Closures

Variable scope determines the visibility and lifetime of variable labels. Python resolves variable lookups using the LEGB rule (Local, Enclosing, Global, Built-in).

The LEGB Scope Hierarchy

flowchart TD
    A["1. Local Scope (Inside current function/lambda)"] --> B["2. Enclosing Scope (Outer enclosing functions / closures)"]
    B --> C["3. Global Scope (Module-level variables)"]
    C --> D["4. Built-in Scope (Python built-in keywords/functions: len, print, int)"]

Scope Keywords

• global: Declares that a variable inside a function body refers to the module-level global variable.
• nonlocal: Declares that a variable inside a nested function refers to a variable in the nearest enclosing non-global scope.

Practical Code Example

# Global Scope Variable
app_status = "INITIALIZING"

def outer_service(service_name: str):
    # Enclosing Scope Variable
    connection_count = 0

    def inner_logger(msg: str) -> None:
        nonlocal connection_count  # Mutate enclosing variable
        connection_count += 1
        print(f"[{service_name}] (Log #{connection_count}): {msg}")

    return inner_logger

def update_global_status(new_status: str) -> None:
    global app_status  # Declare intention to modify module global
    app_status = new_status

if __name__ == "__main__":
    logger = outer_service("AuthService")
    logger("User login attempt")
    logger("User credentials verified")

    print(f"Status before update: {app_status}")
    update_global_status("RUNNING")
    print(f"Status after update: {app_status}")

Best Practices & Gotchas

• Minimize Global Variables: Relying on global state introduces coupling bugs. Pass parameters and return values explicitly instead.
• UnboundLocalError: If you assign a variable inside a function without global or nonlocal, Python treats it as a local variable across the entire function body.
• Do Not Shadow Built-ins: Avoid naming variables list, dict, str, id, or input to prevent overwriting Python's built-in namespace.

Self-Check Challenge

Write a function make_counter() returning an inner function that increments and returns a nonlocal count variable each time it is called.