Python Global Keyword

last modified February 25, 2025

The global keyword in Python is used to declare that a variable is a global variable. This allows you to modify a variable outside the current scope, typically within a function. This tutorial covers the usage of the global keyword, its scope, and practical examples.

The global keyword is used to indicate that a variable is defined in the global scope. Without the global keyword, a variable assigned within a function is considered local by default. Using the global keyword allows you to modify a global variable from within a function.

Modifying a Global Variable

This example demonstrates how to modify a global variable within a function using the global keyword.

modify_global.py

x = 10

def modify_global(): global x x = 20

modify_global() print(x) # Output: 20

The global keyword is used to declare that x is a global variable. The function modify_global modifies the value of x, and the change is reflected globally.

Accessing a Global Variable

This example demonstrates how to access a global variable within a function without modifying it.

access_global.py

x = 10

def access_global(): print(x) # Accessing the global variable

access_global() # Output: 10

The function access_global accesses the global variable x without modifying it. The global keyword is not required for accessing global variables.

Local vs Global Variables

This example demonstrates the difference between local and global variables.

local_vs_global.py

x = 10

def local_variable(): x = 5 # Local variable print(f"Local x: {x}")

local_variable() print(f"Global x: {x}") # Output: Local x: 5, Global x: 10

The function local_variable creates a local variable x that does not affect the global variable x. The local variable is only accessible within the function.

Using Global in Nested Functions

This example demonstrates how to use the global keyword in nested functions.

nested_global.py

x = 10

def outer_function(): x = 20 # Non-local variable

def inner_function():
    global x
    x = 30

inner_function()
print(f"Outer x: {x}")  # Output: Outer x: 20

outer_function() print(f"Global x: {x}") # Output: Global x: 30

The global keyword in the inner_function modifies the global variable x, while the x in outer_function remains a non-local variable.

Avoiding Global Variables

This example demonstrates how to avoid using global variables by passing variables as function arguments.

avoid_global.py

def modify_variable(value): value = 20 return value

x = 10 x = modify_variable(x) print(x) # Output: 20

Instead of using the global keyword, the variable x is passed as an argument to the function modify_variable. This approach avoids modifying global state and makes the code more modular.

Global Variables in Modules

This example demonstrates how global variables can be shared across multiple modules.

module_global.py

module1.py

x = 10

module2.py

import module1

def modify_global(): global x x = 20

modify_global() print(module1.x) # Output: 20

The global variable x is defined in module1.py and modified in module2.py using the global keyword. The change is reflected across both modules.

Best Practices for Using the Global Keyword

• Avoid Overuse: Overusing global variables can make code harder to understand and maintain.

• Use Function Arguments: Pass variables as function arguments instead of relying on global variables.

• Encapsulate State: Use classes or modules to encapsulate state and reduce reliance on global variables.

• Document Globals: Clearly document the purpose of global variables to improve code readability.

Source

Python Global Keyword Documentation

In this article, we have explored the Python global keyword and demonstrated its usage through practical examples.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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