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41. Scope and Lifetime of Variables

1. Introduction​

Scope defines where a variable can be accessed in a program.
Lifetime defines how long the variable exists in memory.

2. Local Scope​

Variables defined inside a function are local to that function.

def my_func():
    x = 10  # local variable
    print(x)

my_func()
# print(x)  # ❌ Error: x not defined here

3. Global Scope​

Variables defined outside any function are global.

y = 20  # global variable

def show():
    print(y)

show()
print(y)  # accessible everywhere

4. Modifying Globals Inside Functions​

Use global keyword.

count = 0

def increment():
    global count
    count += 1

increment()
print(count)  # 1

5. Enclosing Scope (Nonlocal)​

Nested functions can access variables from the enclosing function.

def outer():
    x = "outer"
    def inner():
        nonlocal x
        x = "inner"
    inner()
    print(x)

outer()  # inner

6. Built-in Scope​

Python has reserved keywords and built-in functions (like len, print).

print(len([1,2,3]))

7. Variable Lifetime​

  • Local variables live only during function execution.
  • Global variables live as long as the program runs.

8. Best Practices​

  • Minimize use of global variables.
  • Prefer returning values instead of modifying globals.
  • Use meaningful variable names.

9. Next Steps​

✅ You now understand scope and lifetime of variables.
In the next chapter, we’ll learn about importing modules.