How to declare a global variable in Python

In this tutorial, we will learn how to declare a global Variable in Python Program.

What is a Global Variable?

The global variables are the ones that are available both within and outside of any function, provided they are defined outside a function which is a global scope. Let's explore the creation of a global variable.

In this example, we will define and access a global variable.

Example:

# Simple Python program to show how to define a global variable  
# Here, we are defining a function  
def func():  
    print( "Inside the defined function the string is: ", var )
# Here, we are printing the local scope variable  
# Here, we are declaring the global scope  
var = "Declaring the Global Variable"  
# Here, we are calling the function  
func()   
print("Outside the defined function the string is: ", var)  
# Here, we are printing the global scope variable

Output:

Inside the defined function the string is:  Declaring the Global Variable
Outside the defined function the string is:  Declaring the Global Variable

Accessing the Global Variable

The variable var is accessed both within and outside the function func(), and since it is declared in the global scope, it is designated as a global variable.

Since we have not created local variables, the value of the global variable will be applied; however, we must ensure that the names of the local and global variables match.

What if a variable with the same identifier is initialized globally and inside a defined function? What will occur if we modify the value of a variable within the function func()? The problem is whether changing the local variable will change the global variable or, conversely, changing the global variable will change the local variable. The following example of code is used to evaluate it:

Example:

# Simple Python program to show how to access the global and the local variables  
# Here, we are defining a function  
def func():  
    var = "Global Variable"    # here, we have declared the variable in the local scope  
    print("The local scope variable is:", var )            
# here, we are printing the local scope variable
# Here, we have declared the variable in the global scope  
var = "I am learning Python"  
func()         # Here, we are calling the function
print("The global scope variable is:", var )     
# here, we are printing the global scope variable

Output:

The local scope variable is: Global Variable
The local scope variable is: I am leaning Python

Suppose a certain variable, having the name just as given to the variable in the global scope, is declared inside the function's local scope. In that case, it will only show the value provided within the defined function but not the global value.

Changing Global Variable inside the Local Scope

What would happen if we attempted to alter the value of a variable defined in the global scope inside the function? Let's examine it with the help of the example below.

Example:

# Here, we will try to change the value of the variable defined in the global scope in   # the local scope  
# Here, we are defining a function  
def func():  
    var = var + "Global Variable"  
    print("Inside the defined function", var )  
# Here, we are declaring a variable in the global scope  
var = "Python Tutorial"  
func()        # Here, we are calling the function

Output:

      4 def func():
----> 5     var = var + "Global Variable"
      6     print( "Inside the defined function", var )

UnboundLocalError: local variable 'var' referenced before assignment

We must include the "global" Python keyword for the preceding application to run. Let us now study the global variable.

Global Keyword

When assigning values to or changing the variables declared in global scope within a function, we must use the global Python keyword. The keyword is not required to display or access a global scope variable. Since we declared var inside the function func(), the Python interpreter "assumes" that we desire a local variable, which is why the first sentence raises the error. It is considered local if a variable is modified or declared inside a function without being declared a global variable using the global keyword. The following example demonstrates how to employ the keyword "global" to instruct the Python interpreter that we wish to access the global variable.

Example:

# Simple Python program to show the use of the global keyword to modify a variable, # defined in global scope, inside a local scope.  
# Here, we are defining a function  
def func():  
    global var         # here, we are declaring a global variable
    var = var + " " + "Global Variable"  
    print( "Inside the defined function: ", var )  
   
   
# Here, we are declaring a variable in the global scope  
var = "Python Tutorial"  
func()      # here, we are calling the function
print( "Outside the function: ", var ) 
# Here, the above will show if the variable var is changed in global scope also or not  

Output:

Inside the defined function:  Python Tutorial Global Variable
Outside the function:  Python Tutorial Global Variable

Example to Show Use of Local and Global Variables

Example

# Simple Python program to show how to use global and local variables  
var = 10       # here, we are declaring the variable
# Here, this will show the global value of var as there is no local var  
def func1():  
    print('Inside the first function: ', var)  
# Here, we are declaring a local variable named var  
def func2():  
    var = 15  
    print('Inside the second function: ', var)  
# Here, we are using the global keyword to modify var in global scope  
def func3():  
    global var  
    var += 3  
    print("Inside the third function: ", var)  
# Here, we are declaring the Global scope  
print('global value of variable: ', var)  
func1()  
print('global value of variable: ', var)  
func2()  
print('global value of variable: ', var)  
func3()  
print('global value of variable: ', var)  

Output:

global value of variable:  10
Inside the first function:  10
global value of variable:  10
Inside the second function:  15
global value of variable:  10
Inside the third function:  13
global value of variable:  13

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