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In this Python tutorial, we are going to explain the concept of global variables. Global variables are variables which are declared outside all the functions of a program and are accessible from anywhere in the program. i.e. from the inside or outside of any function defined in the program. Consequently, we could say that global variables have a global scope.

In order to understand the concept of global variables, we are going to explain how we can declare, initialize, access and even modify values of global variables. So, let us see how it is done in the upcoming programs.

• Example - Global Variables


In the upcoming program, we are going to declare and initialize some global variables and then we are going to access them from within a function named show_values(), when this function is called.

Note : The global variables are always defined(declared and initialized) anywhere outside all the functions of a program and before the first non-declaration and non-intialization statement in a program is executed.



# Python - Global variables



# Declaring and initializing global variables 
year = 2019
message = 'Hello'
gravity = 9.8



# Defining the function show_values()
def show_values():
    print('year : ', year)
    print('message : ', message)
    print('gravity : ', gravity)
	print('planet : ', planet)

	
# Declaring another global variable before the first statement in the program execution.
planet = 'Earth'


# Calling the show_values() function, this is the first statement in the program execution.
show_values()





Output-




year :  2019
message :  Hello
gravity :  9.8
planet : Earth


As you can see in the output, we have called a function show_values(), which has accessed and printed the values of global variables defined in the program.







• Shadowing issue while accessing the global variables


Local variables in a function having the same name as global variables will overshadow the global variables.



# Python - Shadowing issue while accessing the global variables

year = 2019
message = 'Hello'
gravity = 9.8


# Defining the function show_values()
def show_values():
    print('Global year : ', year)
    print('Global message : ', message)
    print('Global gravity : ', gravity)


# Declaring local variables with the same name as global variables in a function
# does not impact the original values of global variables
def put_values():
    year = 2020
    message = 'Have a good day'
    gravity = 10.8
	print('Local  year : ', year)
	print('Local message  : ', message)
    print('Local gravity : ', gravity)
    


# Calling the show_values() function
show_values()

# Calling the put_values() function
put_values()

# Calling the show_values() function
show_values()





Output

Global year :  2019
Global message :  Hello
Global gravity :  9.8
Local  year :  2020
Local message  :  Have a good day
Local gravity :  10.8
Global year :  2019
Global message :  Hello
Global gravity :  9.8


Within the function put_values(), we have declared some local variables with the same name as global variables, hence these local variables in the function, will overshadow the access to the global variables with the same name.




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• Trying to modify the value of a global variable in a function


In the upcoming program, we are going to declare and initialize a global variable and then we are going to try and modify its value from within a called function. Let us see what happens.



# Python - Trying to modify the value of a global variable in a function



# Declaring and initializing a global variable
message = 'Hello'



# Defining the function show_message()
def show_message():
    message = message + 'and goodbye!'
    print('message : ', message)

	

# Calling the show_message() function
show_message()





Output-




Traceback (most recent call last):
  File "C:/Users/RadheKrishna/Desktop/Python Programs/global7.py", line 21, in 
    show_values()
  File "C:/Users/RadheKrishna/Desktop/Python Programs/global7.py", line 14, in show_message
    message = message + 'and goodbye!'
UnboundLocalError: local variable 'message' referenced before assignment


The program has thrown an error on execution i.e.UnboundLocalError, which means, when the variable message is accessed from within a function show_message(), the interpreter has taken it as a local variable and not a global variable and hence, it has reported an error because according to it, we have tried to make a reference to a local variable named message, before even assigning a value to it.







• Using the global keyword in a function to modify values of global variables

To correctly modify the values of global variables from within any function, we must use the global keyword. In the upcoming program, we are going to define some global variables and then we are going to modify their values from within a called function by using the global keyword.

# Python - Using the global keyword in a function to modify values of global variables



# Defining global variables 
nickname = 'Mr Blue'
age = 30
height = 180



# Defining a function which modifies values of global variables
def modify_values():

    # Using global keyword to declare the same global variables
    global nickname,age, height	
	
    # Successfully modifying values of global variables
    nickname = nickname + ' Bond'
    age = age + 3
    height = height + 5
    
	
	
	
# Defining the function show_values() 
def show_values():
    print('nicnmame : ', nickname)
    print('age : ', age)
    print('height : ', height)
    
	
	

print('Original values of global variables :')

# Calling the show_values() function
show_values()


# Calling the modify_values() function to modify global variables
modify_values()


print('Modified values of global variables :')

# Calling the show_values() function to display modified values of global variables
show_values()

Output-

Original values of global variables :
nicnmame :  Mr Blue
age :  30
height :  180
Modified values of global variables :
nicnmame :  Mr Blue Bond
age :  33
height :  185

As you can see in the output, we have used the keyword global within the called function modify_values(), which has allowed us to modify the already defined values of global variables.

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