Scope and Lifetime

Scope

Scope determines where in the program a variable is visible/accessible.

This is called the LEGB rule — Python looks up names in order: Local → Enclosing → Global → Built-in.

x = 100    # global scope

def show():
    x = 10   # local scope -- a DIFFERENT variable, shadows the global one
    print("Inside function:", x)

show()                  # Inside function: 10
print("Outside:", x)     # Outside: 100 (global x is untouched)

Local Variable

def calc():
    result = 50   # local -- exists only during this call
    print(result)

calc()
# print(result)   # NameError: result is not defined outside the function

global keyword

To modify a global variable from inside a function, use the global keyword.

counter = 0

def increment():
    global counter
    counter += 1

increment()
increment()
print(counter)   # 2

Without global, Python treats counter += 1 as creating a new local variable and raises UnboundLocalError.

Enclosing scope — nested functions

def outer():
    msg = "hello"
    def inner():
        print(msg)   # accesses the enclosing function's variable
    inner()

outer()   # hello

Lifetime of Variables

• Local variables are created when the function is called and destroyed when it returns — their lifetime = the function call's duration.
• Global variables live as long as the program (module) is running.

def demo():
    temp = "I exist only during this call"
    print(temp)

demo()
demo()   # each call creates a fresh, independent 'temp'