Learn Local, Global, and Nonlocal Variables Scope | Variable Scope, Nested Functions, and Closures

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Understanding how Python determines the scope of variables is crucial for writing clear and bug-free code. Python uses a specific set of rules, known as the LEGB rule, to decide where it should look for the value of a variable. LEGB stands for Local, Enclosing, Global, and Built-in. When you reference a variable, Python searches for its value in this order:

Local: variables defined within the current function or method;
Enclosing: variables in the nearest enclosing function scope (if using nested functions);
Global: variables defined at the top level of the module or declared global using the global keyword;
Built-in: names that are preassigned in Python, such as len or range.

This rule allows Python to resolve variable names efficiently and helps you predict where a variable's value will come from. Sometimes, you may want to modify variables outside the current function's scope. Python provides two keywords for this: global and nonlocal. The global keyword lets you assign a value to a variable at the module level, while nonlocal allows you to modify a variable in the nearest enclosing function scope that is not global.


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# Global scope
global_var = "I am global"

def outer():
    # Enclosing scope
    enclosing_var = "I am enclosing"
    
    def inner():
        # Local scope
        local_var = "I am local"
        print("Local:", local_var)
        print("Enclosing:", enclosing_var)  # Found in enclosing scope
        print("Global:", global_var)        # Found in global scope
        print("Built-in:", len([1, 2, 3]))  # 'len' comes from built-in scope
    
    inner()

outer()

The variable global_var is defined at the global scope and is accessible from anywhere in the module;
Inside the outer function, enclosing_var creates an enclosing scope variable that can be accessed by nested functions;
The nested inner function defines local_var, which exists only within that function and demonstrates access to variables from all enclosing scopes;
The len function is used to show how Python can access names from the built-in scope.

It shows how Python finds variable values by searching through local, enclosing, global, and built-in scopes in that order, making variable resolution predictable and organized.

1. What does the nonlocal keyword do in Python?

2. What is the LEGB rule?

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Section 3. Chapter 1

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Section 3. Chapter 1