Changing Class Members in Python

In the previous fact, we have seen that Python doesn’t have the static keyword. All variables that are assigned a value in the class declaration are class variables.

We should be careful when changing the value of a class variable. If we try to change a class variable using an object, a new instance (or non-static) variable for that particular object is created and this variable shadows the class variables. Below is a Python program to demonstrate the same.

Python3

# Class for Computer Science Student
class CSStudent:
    stream = 'cse'     # Class Variable
    def __init__(self, name, roll):
        self.name = name
        self.roll = roll

# Driver program to test the functionality
# Creating objects of CSStudent class
a = CSStudent("Geek", 1)
b = CSStudent("Nerd", 2)

print ("Initially")
print ("a.stream =", a.stream )
print ("b.stream =", b.stream )

# This thing doesn't change class(static) variable
# Instead creates instance variable for the object
# 'a' that shadows class member.
a.stream = "ece"

print ("\nAfter changing a.stream")
print ("a.stream =", a.stream )
print ("b.stream =", b.stream )

Output: 

Initially
a.stream = cse
b.stream = cse

After changing a.stream
a.stream = ece
b.stream = cse

We should change class variables using class names only.

Python3

# Program to show how to make changes to the
# class variable in Python

# Class for Computer Science Student
class CSStudent:
    stream = 'cse'     # Class Variable
    def __init__(self, name, roll):
        self.name = name
        self.roll = roll

# New object for further implementation
a = CSStudent("check", 3)
print "a.stream =", a.stream

# Correct way to change the value of class variable
CSStudent.stream = "mec"
print "\nClass variable changes to mec"

# New object for further implementation
b = CSStudent("carter", 4)

print "\nValue of variable steam for each object"
print "a.stream =", a.stream
print "b.stream =", b.stream

Output: 

a.stream = cse

Class variable changes to mec

Value of variable steam for each object
a.stream = mec
b.stream = mec