Python Tutorial 37 --- "Instance & Class Variables(OOPS 2)"

 Python Tutorial 37 --- "Instance & Class Variables(OOPS 2)"

When working with objects in Python, there are two types of variables we have to work with i.e. instance variables and class variables. But what do these types of variables mean, and how do they work? OOP allows the variables to be used at the class level or the instance level. In this tutorial, we are going to learn about the two different kinds of variables associated with a class and the difference between them. The variables are:

  • Instance variable
  • Class variable

Instance variable:

"Instance variable are the variables for which the value of the variable is different for every instance."

We can also say that the value is different for every object that we create. Let us dive into some in-depth explanation. When we create a class, we define a few variables along with it. For example, we have created a class of Students, and we have defined a variable age. All the students cannot have the same age in a class, so we have assigned the variable an average age of 16. Now, whenever we use an object to print the value of age, it will show 16. We can try to change the value of age, but it will create a new instance variable for the specific object that we are updating it for, hence defining the value to it.

The code for changing age for a particular object will be something like this:

Std1.age = 18

Class variable:

"Class attributes are owned by the class directly, which means that they are not tied to any object or instance."

 Same as in the above example, if we want to change the age for every instance from 16 to 17, then we can do it by using the class variable, which in this case is Student. 

"It is worth noting that updating the value of the class variable will not change it for the instance variables of the objects, such as in the case above."

The code for changing age using a class variable will be something like this:

Students.age = 18

The following are the notable differences between Class (static) and instance variables.

Following are the differences between Class and instance variables.

Instance variables

Class variables

When an object is created with the use of the new keyword, instance variables are created. They destroyed when the object is destroyed.

When the program starts, static variables are created and destroyed when the program stops.

Instance, variables can be accessed by calling the variable name inside the class. ObjectReference.VariableName.

 Static variables can be accessed by calling using a class name. ClassName.VariableName.

Every instance of the class has its own copy of that variable. Changes made to the variable don't affect the other instances of that class.

There is only one copy of that variable that is shared with all instances of the class. If changes are made to that variable, all other instances will be affected.


The __dict__ attribute

Every object in Python has an attribute which is denoted by __dict__, it maps the attribute name to its value. This dictionary is used to stores all the attributes defined for the object itself. Following is the syntax of using __dict__:

object.__dict__

A quick review:

            Instance, variables are created only for a specific object. The object can change, create, or update only its instance variables. While in the case of class variables, the variables and values we create or define are set as default for all the objects. The objects cannot change the value or variable in the class by just updating it using object_name.class_name .However, it can change the values of their particular instance variables. Making use of class and instance variables can ensure that our code adheres to the DRY (don't repeat yourself) principle to reduce repetition within code.


CODE --- 

# Instance & Class Variables

class employee:
    no_of_leaves = 52, "in a year"    # It is 'Class' variable
    pass

saswata = employee()
saswata.name = "Saswata Kayal"      # those are object variable
saswata.salary = 500, "$"
saswata.role = "Freelancer"
saswata.no_of_leaves = employee.no_of_leaves

tanmoy = employee()
tanmoy.name = "Tanmoy Mondol"
tanmoy.salary = 1000, "$"
tanmoy.role = "Mathematician"

print(employee.no_of_leaves)
# we can change no. leaves for all employee by employee.no_of_leaves
employee.no_of_leaves = 60
# to change only for tanmoy we have to use
tanmoy.no_of_leaves = 50, "in a year"   # it is a instance variable
print(tanmoy.no_of_leaves)
# now
print(saswata.__dict__,"\n",tanmoy.__dict__)

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