When should I create a class vs just using functions?

Use class when: you have data and operations that belong together (bank account with balance and deposit/withdraw methods), need to maintain state between operations (game character with health/inventory), or creating multiple instances of same type (multiple users, each with their own data). Use functions when: simple operations without state (calculate tax on amount), utility operations (format string, validate email), or one-time tasks. Example needing class: shopping cart (data: items, total; methods: add_item, remove_item, calculate_total—state changes over time). Example using functions: calculate_discount(price, percentage) (no state, just input→output). Don't overcomplicate with classes for simple tasks. Start with functions, refactor to class when you find yourself passing same data to multiple related functions.

What's the difference between instance variables and class variables?

Instance variables: belong to specific instance (each object has its own copy), defined in __init__ (self.name = name), different for each instance. Class variables: shared across all instances (one copy for all objects), defined at class level, same for every instance. Use instance variables for: data unique to each instance (user.name, user.email—each user has different values). Use class variables for: data shared by all instances (User.user_count—total users, same for all), constants (Math.PI—doesn't change per instance). Common mistake: modifying class variable (User.count += 1) from instance (user1.count += 1—creates instance variable shadowing class variable). Gotcha: mutable class variables (class User: items = []—all instances share same list—dangerous). Always use instance variables for mutable data.

What's __init__ and why do I need it?

__init__ is constructor—runs automatically when creating instance. Sets up initial state (instance variables, required setup). Example: class User: def __init__(self, name): self.name = name. Now User('Bob') creates user with name='Bob'. Without __init__: can create instances but they have no initial data (need to manually set every attribute). Use __init__ to: initialize instance variables (self.name = name), validate input (raise error if invalid), perform setup (open database connection, load configuration). self is instance reference (like 'this' in other languages)—always first parameter. Don't: perform slow operations in __init__ (makes object creation slow), have complex logic (keep __init__ simple, delegate to methods). Do: set instance variables, validate required parameters, keep it fast.

How do I make a class method vs instance method?

Instance method: operates on specific instance (self parameter), accesses instance data (self.name). Example: def get_name(self): return self.name. Class method: operates on class itself (@classmethod decorator, cls parameter), accesses class variables. Example: @classmethod def count_users(cls): return cls.user_count. Static method: no access to instance or class (@staticmethod), just function grouped with class for organization. Example: @staticmethod def validate_email(email): return '@' in email. Use instance methods for: operations on specific object (user.save(), car.start()). Use class methods for: factory methods (User.from_json(data)—creates User from JSON), operations on class itself. Use static methods for: utilities related to class (User.validate_password(pwd)). Most common: instance methods (90%+), occasional class methods, rare static methods.

Python Objects and Classes Guide

Today we will discuss what objects are, how objects relate to classes, and when you should use objects and classes in your Python applications. This guide provides practical examples to help you master these essential programming concepts.

What are Objects?

Objects are containers that hold a collection of attributes and functions. Think of objects as real-world entities that have properties (attributes) and can perform actions (methods). For example, you might create an application that tracks dogs.

Object Attributes (Properties)

For each dog you are tracking, you might create an object. Each dog object would have a collection of attributes like:

Color: The dog’s fur color
Age: How old the dog is
Breed: What type of dog it is

Object Methods (Actions)

Each dog object also has actions associated with it. For example, you might:

Take the dog for a walk
Cut the dog’s hair
Give the dog a bath

Working with Objects – Example

Let’s assume you have imported a library that gives you access to an object called dog. Here’s how you would work with it:

# Create a new dog object named Max
mydog = dog("Max")

# Set Max's breed to Chihuahua
mydog.breed = "Chihuahua"

# Shorten Max's hair length
mydog.cut_hair(2)

# Look at all of the attributes
print(mydog.name)
print(mydog.breed)
print(mydog.hairlength)

What are Classes?

As we discussed in the previous section, an object is a container that holds various attributes and functions. A class is the code that you use to create an object. Think of a class as a blueprint or template for creating objects.

Creating a Basic Class

To create a new class, we use the keyword class. Let’s create a new class called dog with two attributes: breed and name:

class dog:
    name = ""
    breed = ""

# Create a new instance of this class
mydog = dog()

# Set the name and breed
mydog.name = "Max"
mydog.breed = "Chihuahua"

# Print the dog's name
print(mydog.name)

Adding an Initialization Function

We probably want to treat the name of the dog as a unique attribute for each of our dogs and ensure that all dogs have names. To do this, we need to add an __init__ function that will be called every time we create a new dog object:

class dog:
    def __init__(self, name):
        self.name = name
        self.breed = ""

# Create a new dog object named Max
mydog = dog("Max")

print(mydog.name)  # Output: Max

Key Concept: The self.name call in the function tells the interpreter that this object is going to be named whatever we pass in when we declare our function.

Adding Methods to Classes

There are various things we will do with our dog. Perhaps we need to cut our dog’s hair on occasion. Let’s add a new variable called hairlength and a function called cut_hair:

class dog:
    breed = ""

    # Add hairlength variable to init function
    def __init__(self, name):
        self.name = name
        self.hairlength = 10

    # Declare hair cutting function
    def cut_hair(self, howmuch):
        self.hairlength = self.hairlength - howmuch

# Create a new Dog object named Max
mydog = dog("Max")

# Call the function to cut the dog's hair
mydog.cut_hair(2)

# Print how long the hair length is now
print(mydog.hairlength)  # Output: 8

As you can see above, we start by creating a new dog object, then we call the cut_hair function. The default value in the init function is to have a hairlength of 10. So the output from the print command should be 8.

When Should You Use Objects and Classes?

Objects and classes allow you to break up your application into smaller, manageable pieces. These smaller pieces can be independently modified and tested. If you have done things right, you can modify one class without worrying about breaking another class.

Key Benefits of Object-Oriented Programming

Modularity: Break complex programs into smaller, manageable pieces
Reusability: Create classes once and use them multiple times
Maintainability: Easier to modify and debug isolated components
Scalability: Better organization for larger teams and bigger projects

Best Practices

As your programs get bigger, and you work on larger teams, organization becomes increasingly important. Generally, the rule is that classes should only do one thing, and do that one thing really well. Think of each class as a mini-program within your main program.

Pro Tip: Single Responsibility Principle

For more advanced concepts, consider learning about the SOLID principles of Object Oriented Design, which provide guidelines for writing clean, maintainable object-oriented code.

Summary

Today we have discussed what Objects and Classes are in Python. We have covered how to use objects, how to create classes, and how the two topics are inter-related. Understanding these concepts is fundamental to writing effective Python code that can scale as your applications grow.

Key Takeaways

Objects are containers that hold attributes and methods
Classes are blueprints for creating objects
Use the __init__ method to initialize object attributes
Object-oriented programming improves code organization and maintainability

Inheritance: create new class based on existing class (subclass inherits parent's methods/attributes). Example: class Animal: def speak(self). class Dog(Animal): def speak(self): return 'Woof'. Dog inherits from Animal, overrides speak(). Use inheritance when: is-a relationship (Dog is-a Animal), code reuse (shared behavior across similar classes), extending existing classes. Don't use when: has-a relationship (Car has-a Engine—use composition: class Car: self.engine = Engine()), unrelated classes (forcing inheritance for code reuse creates confusing hierarchies). Python supports multiple inheritance but use sparingly (complex, confusing). Composition often better: class Car: def init(self): self.engine = Engine()—clearer than inheritance. Inheritance is powerful but overused. Start with simple classes, add inheritance only when clear is-a relationship exists.

Python Objects and Classes Guide | OOP Fundamentals