Python Variable Scope

In Python, we can declare variables in three different scopes: local scope, global, and nonlocal scope.

A variable scope specifies the region where we can access a variable. For example,

def add_numbers():
    sum = 5 + 4

Here, the sum variable is created inside the function, so it can only be accessed within it (local scope). This type of variable is called a local variable.

Based on the scope, we can classify Python variables into three types:

1. Local Variables
2. Global Variables
3. Nonlocal Variables

Python Local Variables

When we declare variables inside a function, these variables will have a local scope (within the function). We cannot access them outside the function.

These types of variables are called local variables. For example,

def greet():

    # local variable
    message = 'Hello'
    
    print('Local', message)

greet()

# try to access message variable 
# outside greet() function
print(message)

Output

Local Hello
NameError: name 'message' is not defined

Here, the message variable is local to the greet() function, so it can only be accessed within the function.

That's why we get an error when we try to access it outside the greet() function.

To fix this issue, we can make the variable named message global.

Python Global Variables

In Python, a variable declared outside of the function or in global scope is known as a global variable. This means that a global variable can be accessed inside or outside of the function.

Let's see an example of how a global variable is created in Python.

# declare global variable
message = 'Hello'

def greet():
    # declare local variable
    print('Local', message)

greet()
print('Global', message)

Output

Local Hello
Global Hello

This time we can access the message variable from outside of the greet() function. This is because we have created the message variable as the global variable.

# declare global variable
message = 'Hello'

Now, message will be accessible from any scope (region) of the program.

Python Nonlocal Variables

In Python, nonlocal variables are used in nested functions whose local scope is not defined. This means that the variable can be neither in the local nor the global scope.

We use the nonlocal keyword to create nonlocal variables.For example,

# outside function 
def outer():
    message = 'local'

    # nested function  
    def inner():

        # declare nonlocal variable
        nonlocal message

        message = 'nonlocal'
        print("inner:", message)

    inner()
    print("outer:", message)

outer()

Output

inner: nonlocal
outer: nonlocal

In the above example, there is a nested inner() function. We have used the nonlocal keywords to create a nonlocal variable.

The inner() function is defined in the scope of another function outer().

Note : If we change the value of a nonlocal variable, the changes appear in the local variable.