lesson1.md

Python Scope

Introduction

Scope in Python defines the accessibility of variables within different parts of a program. Understanding scope is essential to avoid unexpected behavior and errors.

Python has four types of scope, commonly referred to as LEGB:

1. Local Scope: Variables declared inside a function.
2. Enclosing Scope: Variables in the outer function of a nested function.
3. Global Scope: Variables declared at the top level of a script.
4. Built-in Scope: Variables that are built into Python (e.g., print, len).

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1. Local Scope

A variable declared inside a function is accessible only within that function.

def greeting():
    message = "Hello, World!"
    print(message)

greeting()
# print(message)  # This would raise an error because 'message' is not accessible outside the function

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2. Enclosing Scope

Variables in the outer function of a nested function are accessible inside the inner function.

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

    def inner_function():
        print(outer_var)  # Can access 'outer_var' from outer_function

    inner_function()

outer_function()

If we need to modify an enclosing variable inside the inner function, we use nonlocal:

def outer():
    color = "blue"

    def inner():
        nonlocal color
        color = "red"  # Modifies the outer function's variable
        print("Inside inner:", color)

    inner()
    print("Inside outer:", color)

outer()

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3. Global Scope

Variables declared at the top level of a script are accessible throughout the program.

name = "Alice"  # Global variable

def greet():
    print("Hello", name)  # Can access 'name'

greet()
print(name)  # Accessible outside the function as well

If we need to modify a global variable inside a function, we use global:

count = 0

def increment():
    global count  # Access and modify the global variable
    count += 1
    print("Count inside function:", count)

increment()
print("Count outside function:", count)

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4. Built-in Scope

Python provides built-in functions that are accessible globally.

print(len("Hello"))  # 'len' is a built-in function

Avoid overriding built-in names:

# Bad practice
len = 10  # Overrides built-in 'len' function
# print(len("Hello"))  # This will raise an error

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Summary

Scope Type	Description	Keyword
Local	Variable inside a function	None
Enclosing	Variable in outer function of a nested function	nonlocal
Global	Variable declared at the top level of the script	global
Built-in	Python’s built-in functions	None

Understanding scope ensures efficient variable usage and prevents unintended modifications in Python programs.