Python global, local and nonlocal

Python global Variable

In Python, a variable which is declared in global scope (outside functions) is known as a global variable

A global variable can be accessed inside or outside of functions

Python global keyword

The global keyword allows to modify a variable of the global scope

It can be used to declare a variable as global in a local context/namespace and make changes to the variable

• When a variable is created inside a function, it’s local by default
• When a variable is declared outside of a function, it’s global by default, without requiring global keyword
• global keyword is used to read and write a global variable inside a function
• Use of global keyword in the global scope is redundant

Accessing global Variable From Inside a Function

Global variables can be read inside a function

x = 12
def printx():
    print(x)
printx()

Output when above program is run :

12 

There might be scenarios requiring to modify the global variable from inside a function

Modifying global variable from inside a Function

Consider following example, where modification to a global variable is attempted :

x = 12
def printx():
    x = x + 1
    print(x)
printx() 

When above code is run, Python shows error like:

UnboundLocalError: local variable 'x' referenced before assignment 

This is because the global variable can only be accessed inside a function but cannot be modified if not declared with global keyword

By using global keyword to declare a global variable, the variable can be modified inside the function

x = 12
def printx():
    global x
    x = x + 1
    print("value of x inside printx():", x)
print("value of x outside:", x)
printx()
print("value of x outside, after printx():", x)

Output

value off x outside: 12
value of x inside printx(): 13
value of x outside, after printx(): 13 

In the above example, x is declared as a global keyword inside printx() function, allowing it to be modified inside the function

Change also gets reflected outside of the printx() function

Global Variables Across Python Modules

global variables can be shared across Python modules(or files) within a program

Following example stores global variables in a file named constants.py and uses update.py to update them

constants.py file is used to store global variables as

count = 0
b = ""
c = False 

An update.py file is created, to change global variables

import constants

constants.a = 10
constants.b ="letters"

def change_c():
    constants.c = True 

A test.py file is created to test changes in value

import constants
import update

print("a :", constants.a)
print("b :", constants.b)
print("c :", constants.c)
update.change_c()
print("c after constants.change_c() :", constants.c) 

When test.py file is run, its output is as follows

a : 10
b : letters
c : False
c after constants.change_c() : True 

In update.py file, the constants.py module is imported and the values of a and b are modified

Similarly, in test.py file both config.py and update.py modules are imported and values of the global variables are tested

change_c() method of update.py module is called to change value of c, which is then printed

Local Variables

A variable declared inside a function's body or in a local scope is known as local variable

Creating a Local Variable

Any variable declared or assigned inside a function is a local variable for that function

def printy():
    y = "locally declared"
    print(y)

foo() 

Output

locally declared

Nonlocal Variables

Nonlocal variables are used in nested function whose local scope is not defined

This means, the variable can be neither in the local nor the global scope

The nonlocal keyword is used to create nonlocal variable

Example

def printx():
    x = 12
    print("initial x:", x)
    def change():
        nonlocal x
        x = "twelve"
        print("x in nested fn change():", x)
    change()
    print("x in printx():", x)

printx()

Output

initial x: 12
x in nested fn change(): twelve
x in printx(): twelve 

In the above example, there is a nested function change() inside a function printx()

nonlocal keyword to declare x as a nonlocal variable in printx() to refer to x declared in printx()

Due to this, change to value of x done in change() method gets reflected in printx()