Global and Local Variables in Python

Global variables are those which are not defined inside any function and have a global scope whereas local variables are those which are defined inside a function and its 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. Local variables 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.

Example: Creating local variables

Python3

def f():

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


# Driver code
f()

Output

I love Geeksforgeeks

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

Example:

Python3

def f():

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

# Driver code
f()
print(s)

Output:

NameError: name 's' is not defined

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 global variable.

Example: Defining and accessing global variables

Python3

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

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

Output

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.

Now, what if there is a 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: 

Python3

# 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)

Output

Me too.
I love Geeksforgeeks

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

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

Example: 

Python3

# 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. Let’s see what this global keyword is.

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 global keyword

Python3

# 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)

Output

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

Now there is no ambiguity. 

Example 2: Using global and local variables

Python3

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)

Output

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

This article is contributed by Shwetanshu Rohatgi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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