Scope in python

In Python, scope refers to the region of a program where a particular variable or function is accessible.

Understanding scope is crucial because it determines the visibility and lifetime of variables.

There are four main types of scopes in Python:

1. Local Scope
2. Enclosing Scope
3. Global Scope
4. Built-in Scope

Local Scope

Variables defined inside a function are in the local scope. They are only accessible within that function.

def my_function():
    local_var = "I am local"
    print(local_var)

my_function()  # Output: I am local
# print(local_var)  # Uncommenting this will raise NameError: name 'local_var' is not defined

Enclosing Scope

This refers to the scope of enclosing functions, also known as nonlocal scope. It is relevant in nested functions.

def outer_function():
    outer_var = "I am outer"

    def inner_function():
        print(outer_var)  # Accessing variable from enclosing (outer) function
    
    inner_function()

outer_function()  # Output: I am outer

Global Scope

Variables defined at the top level of a module or script, outside any function, are in the global scope. They are accessible from any function within the module.

global_var = "I am global"

def my_function():
    print(global_var)

my_function()  # Output: I am global
print(global_var)  # Output: I am global

Built-in Scope

This is a special scope that contains built-in functions and exceptions provided by Python.

print(len([1, 2, 3]))  # len is a built-in function

Example Illustrating All Scopes

global_var = "I am global"

def outer_function():
    outer_var = "I am outer"

    def inner_function():
        inner_var = "I am inner"
        print(global_var)  # Accessing global scope
        print(outer_var)   # Accessing enclosing (outer) scope
        print(inner_var)   # Accessing local scope
    
    inner_function()
    # print(inner_var)  # Uncommenting this will raise NameError: name 'inner_var' is not defined

outer_function()

global and nonlocal Keywords

The global keyword is used to modify a global variable inside a function. The nonlocal keyword is used to modify a variable in the enclosing scope.

Using global Keyword

global_var = "I am global"

def my_function():
    global global_var
    global_var = "I have been modified globally"
    print(global_var)

my_function()  # Output: I have been modified globally
print(global_var)  # Output: I have been modified globally

Using nonlocal Keyword

def outer_function():
    outer_var = "I am outer"

    def inner_function():
        nonlocal outer_var
        outer_var = "I have been modified in enclosing scope"
        print(outer_var)
    
    inner_function()
    print(outer_var)

outer_function() 
# Output: I have been modified in enclosing scope
# Output: I have been modified in enclosing scope

Summary

• Local Scope: Variables defined inside a function. Accessible only within that function.
• Enclosing Scope: Variables in the local scope of enclosing functions. Relevant in nested functions.
• Global Scope: Variables defined at the top level of a module or script. Accessible from any function in the module.
• Built-in Scope: Special scope containing built-in functions and exceptions provided by Python.
• global keyword: Allows modification of global variables inside a function.
• nonlocal keyword: Allows modification of variables in the enclosing scope inside nested functions.
  

scope is essential for managing the visibility and lifetime of variables in Python programs, ensuring proper variable usage and avoiding unintended side effects.