variables_class_instance.md

Variables (class & instance)

There are two types of variables in the class, one are class variables, and the other are instance variables.

Table of Contents

• Variables (class & instance)
  • Table of Contents
  • Class Variables
  • Instance Variables
  • Combined (class & instance)

Class Variables

The class variables will always stick to the class, and can be called by instance. One use is as a counter.

class Employee:
    num_emp = 0

    def __init__(self):
        Employee.num_emp += 1  # <----- Calling variables with class is class variable.
        

e1 = Employee()
e2 = Employee()
print(Employee.num_emp)  # 2
print(e1.num_emp)        # 2
print(e2.num_emp)        # 2

We can also use instance to get the class variable.

Instance Variables

An instance variable exists only on an instance.

class Employee:
    def __init__(self, pay):
        self.pay = pay


e1 = Employee(10000)
e2 = Employee(20000)
print(Employee.pay)  # AttributeError: type object 'Employee' has no attribute 'pay'
print(e1.pay)        # 10000
print(e2.pay)        # 20000

Combined (class & instance)

A variable can be both a class variable and an instance variable, although this should not be done in general.

class Employee:
    count = 0

    def __init__(self):
        Employee.count += 1
        self.count += 1


e1 = Employee()
e2 = Employee()

print(Employee.count)  # 2
print(e1.count)        # 2
print(e2.count)        # 3

Now you should be able to see the reason for the value of these variables.

1. Employee.count is a class variable, no problem.
2. e1.count is an instance variable which, when created, is created by taking the class variable Employee.count (1) + 1 = 2.
3. e2.count is also an instance variable which is created by taking the class variable Employee.count (2) + 1 = 3.

In the above example, instance and class variable are completely separated, so the changes do not affect each other.

e1.count = 0
print(Employee.count)  # 2
print(e1.count)        # 0
print(e2.count)        # 3

Employee.count = 0
print(Employee.count)  # 0
print(e1.count)        # 0
print(e2.count)        # 3

This is because the instance will first go to its own __dict__ to see if any variables match, and then to the class's __dict__ if there are none.

Now, if only the class variable exists in an instance, then when the class variable is modified, the variables called from the instance will be modified as well.

class Employee:
    count = 0

    def __init__(self):
        Employee.count += 1


e1 = Employee()
Employee.count = 0
print(Employee.count)  # 0
print(e1.count)        # 0