Global and Local Variables in Python

Python Global variables are those which are not defined inside any function and have a global scope whereas Python local variables are those which are defined inside a function and their scope is limited to that function only. In other words, we can say that local variables are accessible only inside the function in which it was initialized whereas the global variables are accessible throughout the program and inside every function. 

Python Local Variables

Local variables in Python are those which are initialized inside a function and belong only to that particular function. It cannot be accessed anywhere outside the function. Let’s see how to create a local variable.

Creating local variables in Python

Defining and accessing local variables

def f():

    # local variable
    s = "I love Geeksforgeeks"
    print(s)


# Driver code
f()

I love Geeksforgeeks

Can a local variable be used outside a function?

If we will try to use this local variable outside the function then let’s see what will happen.

def f():
    
    # local variable
    s = "I love Geeksforgeeks"
    print("Inside Function:", s)

# Driver code
f()
print(s)

Output:

NameError: name 's' is not defined

Python Global Variables

These are those which are defined outside any function and which are accessible throughout the program, i.e., inside and outside of every function. Let’s see how to create a Python global variable.

Create a global variable  in Python

Defining and accessing Python global variables.

# This function uses global variable s
def f():
    print("Inside Function", s)

# Global scope
s = "I love Geeksforgeeks"
f()
print("Outside Function", s)

Inside Function I love Geeksforgeeks
Outside Function I love Geeksforgeeks

The variable s is defined as the global variable and is used both inside the function as well as outside the function.

Note: As there are no locals, the value from the globals will be used but make sure both the local and the global variables should have same name.

Why do we use Local and Global variables in Python?

Now, what if there is a Python variable with the same name initialized inside a function as well as globally? Now the question arises, will the local variable will have some effect on the global variable or vice versa, and what will happen if we change the value of a variable inside of the function f()? Will it affect the globals as well? We test it in the following piece of code: 

Example

If a variable with the same name is defined inside the scope of the function as well then it will print the value given inside the function only and not the global value. 

# This function has a variable with
# name same as s.
def f():
    s = "Me too."
    print(s)

# Global scope
s = "I love Geeksforgeeks"
f()
print(s)

Me too.
I love Geeksforgeeks

Now, what if we try to change the value of a global variable inside the function? Let’s see it using the below example.

# This function uses global variable s
def f():
    s += 'GFG'
    print("Inside Function", s)


# Global scope
s = "I love Geeksforgeeks"
f()

Output:

UnboundLocalError: local variable 's' referenced before assignment

To make the above program work, we need to use the “global” keyword in Python. Let’s see what this global keyword is.

The global Keyword

We only need to use the global keyword in a function if we want to do assignments or change the global variable. global is not needed for printing and accessing. Python “assumes” that we want a local variable due to the assignment to s inside of f(), so the first statement throws the error message. Any variable which is changed or created inside of a function is local if it hasn’t been declared as a global variable. To tell Python, that we want to use the global variable, we have to use the keyword “global”, as can be seen in the following example: 

Example 1: Using Python global keyword

# This function modifies the global variable 's'
def f():
    global s
    s += ' GFG'
    print(s)
    s = "Look for Geeksforgeeks Python Section"
    print(s) 

# Global Scope
s = "Python is great!" 
f()
print(s)

Python is great! GFG
Look for Geeksforgeeks Python Section
Look for Geeksforgeeks Python Section

Now there is no ambiguity. 

Example 2: Using Python global and local variables

a = 1

# Uses global because there is no local 'a'
def f():
    print('Inside f() : ', a)

# Variable 'a' is redefined as a local
def g():
    a = 2
    print('Inside g() : ', a)

# Uses global keyword to modify global 'a'
def h():
    global a
    a = 3
    print('Inside h() : ', a)


# Global scope
print('global : ', a)
f()
print('global : ', a)
g()
print('global : ', a)
h()
print('global : ', a)

global :  1
Inside f() :  1
global :  1
Inside g() :  2
global :  1
Inside h() :  3
global :  3

Difference b/w Local Variable Vs. Global Variables

Global and Local Variables in Python – FAQs

What Is the Difference Between Global and Local Variables in Python?

Global Variables:

• Scope: Accessible throughout the entire program or script, including all functions.
• Declaration: Defined outside any function or class.
• Lifetime: Exists for the duration of the program’s execution.
• Example:

global_var = 10  # Global variable

def my_function():
    print(global_var)  # Accessing global variable

Local Variables:

• Scope: Accessible only within the function or block where they are defined.
• Declaration: Defined inside a function or block.
• Lifetime: Exists only during the function’s execution.
• Example:

def my_function():
    local_var = 5  # Local variable
    print(local_var)  # Accessing local variable

my_function()
print(local_var)  # Raises an error because local_var is not accessible here

How Can We Change a Global Variable from Inside a Function in Python?

To modify a global variable inside a function, use the global keyword. This tells Python that you are referring to the global variable, not creating a new local one.

Example:

global_var = 10

def change_global():
    global global_var
    global_var = 20  # Modify the global variable

change_global()
print(global_var)  # Output: 20

What Is the Scope of a Local Variable in Python?

The scope of a local variable is limited to the function or block in which it is defined. It is not accessible outside that function or block.

Example:

def my_function():
    local_var = 5  # Local variable
    print(local_var)  # Accessible here

my_function()
print(local_var)  # Raises an error because local_var is not accessible outside the function

How Does Python Handle Variable Scopes in Nested Functions?

In nested functions, Python uses the LEGB rule (Local, Enclosing, Global, Built-in) to resolve variable scopes:

• Local: Variables defined in the current function.
• Enclosing: Variables in any enclosing functions (functions that are nested inside another function).
• Global: Variables defined at the top level of the script or module.
• Built-in: Python’s built-in names.

What Are the Best Practices for Using Global Variables in Python?

1. Minimize Usage: Use global variables sparingly to avoid potential issues with code readability and maintainability.
2. Encapsulation: Encapsulate global variables within classes or modules to limit their scope and manage their state more effectively.
3. Avoid Side Effects: Be cautious when modifying global variables from different functions or modules to avoid unintended side effects.
4. Use Constants: For immutable values, consider using constants and defining them in uppercase to signify their immutability.
5. Document Changes: Clearly document where and why global variables are being modified, especially if changes occur in multiple places.
6. Testing: Ensure thorough testing when using global variables to prevent unexpected behavior or bugs in different parts of the program.

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