Class vs. instance attributes

Introduction to Object-Oriented Programming in Python

George Boorman

Curriculum Manager, DataCamp

Core principles of OOP

Encapsulation:

Bundling of data and methods

Inheritance:

Extending the functionality of existing code

Polymorphism:

Creating a unified interface

Instance-level attributes

class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

emp1 = Employee("Teo Mille", 50000) 
emp2 = Employee("Marta Popov", 65000)

name and salary values are specific to each object
self assigns to an object

Class-level attributes

Data shared among all instances of a class

Define class attributes in the body of class

"Global variable" within the class

Implementing class-level attributes

class Employee:
    # Define a class attribute
    # No self. syntax
    MIN_SALARY = 30000 

    def __init__(self, name, salary):
        self.name = name
        # Use class name 
        # to access class attribute
        if salary >= Employee.MIN_SALARY:
            self.salary = salary

        else:
            self.salary = Employee.MIN_SALARY

MIN_SALARY is shared among all instances
Don't use self to define class attribute
Convention is to use capital letters
Use ClassName.ATTR_NAME to access the class attribute value

Class-level attributes

class Employee:
    # Define a class attribute
    MIN_SALARY = 30000    

    def __init__(self, name, salary):
        self.name = name
        # Use class name 
        # to access class attribute
        if salary >= Employee.MIN_SALARY:
            self.salary = salary
        else:
            self.salary = Employee.MIN_SALARY

emp1 = Employee("John", 40000)
print(emp1.MIN_SALARY)

emp2 = Employee("Jane", 60000)
print(emp2.MIN_SALARY)

Modifying class-level attributes

emp1 = Employee("John", 40000)
emp2 = Employee("Jane", 60000)


# Update MIN_SALARY of emp1
emp1.MIN_SALARY = 50000


# Print MIN_SALARY for both employees
print(emp1.MIN_SALARY)
print(emp2.MIN_SALARY)

50000
30000

Modifying class-level attributes

MIN_SALARY is created in the class definition

Updating MIN_SALARY of an object will not affect the value in the class definition

Security - prevent changes being made to software!

Why use class attributes?

Minimum and maximum values for attributes
- Prevent invalid data
Commonly used values and constants, e.g. host, port for a Database class
- Avoid repetition when creating objects

Let's practice!

Introduction to Object-Oriented Programming in Python

Class vs. instance attributes

Core principles of OOP

Encapsulation:

Inheritance:

Polymorphism:

Instance-level attributes

Class-level attributes

Implementing class-level attributes

Class-level attributes

Modifying class-level attributes

Modifying class-level attributes

Why use class attributes?

Global constants related to the class

Let's practice!