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Global and Local Variables in Python

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

Python Local Variables

Local variables in Python 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.

Creating local variables in Python

Defining and accessing local variables

Python3




def f():
 
    # local variable
    s = "I love GeeksforLazyroar"
    print(s)
 
 
# Driver code
f()


Output

I love GeeksforLazyroar

Can a local variable be used outside a function?

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

Python3




def f():
     
    # local variable
    s = "I love GeeksforLazyroar"
    print("Inside Function:", s)
 
# Driver code
f()
print(s)


Output:

NameError: name 's' is not defined

Python 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 Python global variable.

Create a global variable  in Python

Defining and accessing Python global variables.

Python3




# This function uses global variable s
def f():
    print("Inside Function", s)
 
# Global scope
s = "I love GeeksforLazyroar"
f()
print("Outside Function", s)


Output

Inside Function I love GeeksforLazyroar
Outside Function I love GeeksforLazyroar

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.

Why do we use Local and Global variables in Python?

Now, what if there is a Python 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: 

Example

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

Python3




# This function has a variable with
# name same as s.
def f():
    s = "Me too."
    print(s)
 
# Global scope
s = "I love GeeksforLazyroar"
f()
print(s)


Output

Me too.
I love GeeksforLazyroar

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

Python3




# This function uses global variable s
def f():
    s += 'GFG'
    print("Inside Function", s)
 
 
# Global scope
s = "I love GeeksforLazyroar"
f()


Output:

UnboundLocalError: local variable 's' referenced before assignment

To make the above program work, we need to use the “global” keyword in Python. Let’s see what this global keyword is.

The 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 Python global keyword

Python3




# This function modifies the global variable 's'
def f():
    global s
    s += ' GFG'
    print(s)
    s = "Look for GeeksforLazyroar Python Section"
    print(s)
 
# Global Scope
s = "Python is great!"
f()
print(s)


Output

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

Now there is no ambiguity. 

Example 2: Using Python 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

Difference b/w Local Variable Vs. Global Variables

Comparision Basis Global Variable Local Variable
Definition declared outside the functions declared within the functions
Lifetime They are created  the execution of the program begins and are lost when the program is ended They are created when the function starts its execution and are lost when the function ends
Data Sharing Offers Data Sharing It doesn’t offers Data Sharing
Scope Can be access throughout the code Can access only inside the function
Parameters needed parameter passing is not necessary parameter passing is necessary
Storage  A fixed location selected by the compiler They are  kept on the stack
Value Once the value changes it is reflected throughout the code once changed the variable don’t affect other functions of the program
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