Python Classes Guide | CMSC 105 Elementary Programming - Fall 2024

Python Classes Guide

What Are Classes in Python?

Classes are a way to group data and functions together. They act as a blueprint for creating objects. If you think of an object as a real-world thing (like a car), then a class is the template that describes what the car looks like (its properties) and what it can do (its methods).

Creating a Basic Class

Here’s how you define a class:

class Dog:
    # This is a class called Dog
    pass

The pass keyword means “do nothing.” This is useful when you’re setting up a class but don’t want to add anything to it yet.

Adding Properties and Methods

A property is a variable inside a class, and a method is a function inside a class.

class Dog:
    # Initialize the object with some properties
    def __init__(self, name, age):
        self.name = name  # Property: name
        self.age = age    # Property: age

    # Define a method
    def bark(self):
        return f"{self.name} says Woof!"

Explanation:

  1. __init__ method: This is the initializer. It runs when you create an object and sets up its properties.
  2. self: Refers to the specific object being created or used.

Creating Objects

You can create objects (instances) from the class:

# Create a Dog object
my_dog = Dog(name="Buddy", age=3)

# Access properties
print(my_dog.name)  # Outputs: Buddy
print(my_dog.age)   # Outputs: 3

# Call methods
print(my_dog.bark())  # Outputs: Buddy says Woof!

Adding More Methods

You can add as many methods as you want:

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says Woof!"

    def birthday(self):
        self.age += 1
        return f"Happy Birthday, {self.name}! You are now {self.age} years old."

my_dog = Dog(name="Buddy", age=3)
print(my_dog.birthday())  # Outputs: Happy Birthday, Buddy! You are now 4 years old.

Example: A Simple Student Class

Here’s how you might represent a student using a class:

class Student:
    def __init__(self, name, grade):
        self.name = name
        self.grade = grade

    def display_info(self):
        return f"Student: {self.name}, Grade: {self.grade}"

    def update_grade(self, new_grade):
        self.grade = new_grade
        return f"{self.name}'s grade has been updated to {self.grade}"

# Create a Student object
student1 = Student(name="Alice", grade="A")

# Use methods
print(student1.display_info())  # Outputs: Student: Alice, Grade: A
print(student1.update_grade("A+"))  # Outputs: Alice's grade has been updated to A+

Class vs. Object