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Learn Object-Oriented Programming in Python

Class and Instance Variables

Explore the concepts of class and instance variables in Python's object-oriented programming. Understand how class variables are shared among all instances while instance variables are unique to each object. This lesson helps you learn proper usage to avoid common mistakes and leverage these variables effectively.

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In Python, properties can be defined into two parts:

  • Class variables
  • Instance variables

Definitions

Class variables

The class variables are shared by all instances or objects of the classes. A change in the class variable will change the value of that property in all the objects of the class.

Instance variables

The instance variables are unique to each instance or object of the class. A change in the instance variable will change the value of the property in that specific object only.

Defining class variables and instance variables

Class variables are defined outside the initializer and instance variables are defined inside the initializer.

Python 3.5
class Player:
    teamName = 'Liverpool'  # class variables
    def __init__(self, name):
        self.name = name  # creating instance variables
p1 = Player('Mark')
p2 = Player('Steve')
print("Name:", p1.name)
print("Team Name:", p1.teamName)
print("Name:", p2.name)
print("Team Name:", p2.teamName)

In line 2, we have created a class variable and in line 5, we have created an instance variable.

Wrong use of class variables

It is imperative to use class variables properly since they are shared by all the class objects and can be modified using any one of them. Below is an example of wrongful use of class variables:

Python 3.5
class Player:
    formerTeams = []  # class variables
    teamName = 'Liverpool'
    def __init__(self, name):
        self.name = name  # creating instance variables
p1 = Player('Mark')
p2 = Player('Steve')
p1 = Player('Mark')
p1.formerTeams.append('Barcelona') # wrong use of class variable
p2 = Player('Steve')
p2.formerTeams.append('Chelsea') # wrong use of class variable
print("Name:", p1.name)
print("Team Name:", p1.teamName)
print(p1.formerTeams)
print("Name:", p2.name)
print("Team Name:", p2.teamName)
print(p2.formerTeams)

In the example above, while the instance variable name is unique for each and every object of the Player class, the class variable, formerTeams, can be accessed by any object of the class and is updated throughout. We are storing all players currently playing for the same team, but each player in the team may have played for different former teams. To avoid this issue, the correct implementation of the example above will be the following:

Python 3.5
class Player:
    teamName = 'Liverpool'  # class variables
    def __init__(self, name):
        self.name = name  # creating instance variables
        self.formerTeams = []
p1 = Player('Mark')
p1.formerTeams.append('Barcelona')
p2 = Player('Steve')
p2.formerTeams.append('Chelsea')
print("Name:", p1.name)
print("Team Name:", p1.teamName)
print(p1.formerTeams)
print("Name:", p2.name)
print("Team Name:", p2.teamName)
print(p2.formerTeams)

Now the property formerTeams is unique for each Player class object and can only be accessed by that unique object.

Using class variables smartly

Class variables are useful when implementing properties that should be common and accessible to all class objects. Let’s see an example of this:

Python 3.5
class Player:
    teamName = 'Liverpool'      # class variables
    teamMembers = []
    def __init__(self, name):
        self.name = name        # creating instance variables
        self.formerTeams = []
        self.teamMembers.append(self.name)
p1 = Player('Mark')
p2 = Player('Steve')
print("Name:", p1.name)
print("Team Members:")
print(p1.teamMembers)
print("")
print("Name:", p2.name)
print("Team Members:")
print(p2.teamMembers)

Explanation

  • In the example above, we have defined a class variable teamMembers, which is a list that will be shared by all the objects of the class Player.

  • This list, teamMembers, will contain names of all the instances created of the Player class.

  • As you can see in line 8, whenever a new object is created, its name is appended in teamMembers.

  • In lines 16 and 20, we can see that teamMembers is accessed by p1 and p2 respectively, and both produce the same output.