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Python Global Variables

In Python, a global variable is a variable that is declared outside a function and can be accessed from anywhere in the program.
Global variables are useful when you want to share data across multiple functions.

What Is a Global Variable?

A variable created outside of all functions is called a global variable.
  • It belongs to the global scope
  • It can be accessed anywhere in the file
  • It exists for the entire lifetime of the program
python
x = 10

def show():
    print(x)

show()

Accessing Global Variables Inside a Function

You can read a global variable inside a function without any special keyword.
python
count = 5

def display():
    print(count)

display()
python
name = "Python"

def greet():
    print("Hello", name)

greet()
python
price = 100

def show_price():
    print(price)

show_price()

Local vs Global Variable

If a variable is created inside a function, it is a local variable and cannot be accessed outside.
python
def my_func():
    x = 20
    print(x)

my_func()
python
# print(x)  # Error: x is not defined

Using the global Keyword

To modify a global variable inside a function, you must use the global keyword.
python
x = 10

def change():
    global x
    x = 20

change()
print(x)
python
counter = 0

def increment():
    global counter
    counter += 1

increment()
print(counter)
python
status = "OFF"

def turn_on():
    global status
    status = "ON"

turn_on()
print(status)

Global Variables with Same Name as Local

If a variable with the same name exists inside a function, Python treats it as local by default.
python
x = 10

def my_func():
    x = 5
    print(x)

my_func()
print(x)

When to Use Global Variables

Global variables are useful when:
  • Multiple functions need shared data
  • Configuration values are required globally
  • Constants are defined once and reused
python
APP_NAME = "MyApp"

def show_app():
    print(APP_NAME)

show_app()

Best Practices for Global Variables

  • Use global variables sparingly
  • Prefer constants (uppercase names)
  • Avoid modifying globals frequently
  • Use function parameters when possible
python
PI = 3.14

def area(radius):
    return PI * radius * radius

print(area(5))

Common Mistake

Trying to modify a global variable without global keyword.
python
x = 10

def update():
    x = x + 1  # Error

update()

Summary

  • Global variables are declared outside functions
  • They can be accessed anywhere
  • Use global keyword to modify them inside functions
  • Overusing globals can make code hard to manage

Exercise

  • Create a global variable and access it inside a function
  • Modify a global variable using global keyword
  • Try creating a local variable with the same name