🐍 Class

class

Good to know

⭐️ A class is a blueprint for creating objects (instances) with data (attributes) and behavior (methods). Most of the time you’ll work with instance methods and init to set up data.

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Basic Class & Object

Define a class with class, then create an object (instance) by calling it like a function.

class Dog:
    def bark(self): # Function
        print("Woof!")
 
buddy = Dog()      # create an instance
buddy.bark()       # call method -> Woof!

init and Instance Attributes (Constructor)

init runs automatically when you create an object and usually sets up attributes.

class Dog:
    def __init__(self, name, age):
        self.name = name    # instance attribute
        self.age = age
 
dog = Dog("Buddy", 5)
print(dog.name, dog.age)    # Buddy 5

The self Parameter

self is a reference to the current object and is how methods access attributes. It’s needed when you wanna access instance variables inside the class itself just like this in java.

class Dog:
    def __init__(self, name):
        self.name = name
 
    def speak(self):
        print(f"{self.name} says woof!")
 
dog = Dog("Luna")
dog.speak()     # Luna says woof!

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Class Attributes vs Instance Attributes

Class attributes are shared by all instances; instance attributes belong to one object.

class Dog:
    species = "Canis familiaris"   # class attribute
 
    def __init__(self, name):
        self.name = name           # instance attribute
 
dog1 = Dog("Max")
dog2 = Dog("Bella")
 
print(dog1.species)   # Canis familiaris
print(dog2.name)      # Bella

Methods: Instance, Class, Static

Use @classmethod and @staticmethod for alternate constructors or utility functions.

class Dog:
    def __init__(self, name):
        self.name = name
 
    def speak(self):              # instance method
        print(self.name, "barks")
 
    @classmethod
    def from_dict(cls, data):     # class method
        return cls(data["name"])
 
    @staticmethod
    def is_dog_food(food):        # static method
        return food in ["meat", "bones"]
 
dog = Dog.from_dict({"name": "Rex"})
dog.speak()                       # Rex barks
print(Dog.is_dog_food("meat"))    # True

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** Quick Data Classes**

For simple “data holder” classes, dataclasses reduce boilerplate. This gives you init, repr, comparisons, and more for free.

from dataclasses import dataclass
 
@dataclass
class Dog:
    name: str
    age: int
 
dog = Dog("Kira", 2)
print(dog.name, dog.age)

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Inheritance (Extending a Class)

A subclass reuses and extends behavior of a parent class.

class Animal:
    def __init__(self, name):
        self.name = name
 
    def speak(self):
        print(self.name, "makes a sound")
 
class Dog(Animal):                # Dog inherits from Animal
    def speak(self):
        print(self.name, "barks!")
 
dog = Dog("Milo")
dog.speak()                       # Milo barks!

Using super() to Call Parent Logic

super() lets a subclass call methods (like init) from its parent.

class Animal:
    def __init__(self, name):
        self.name = name
 
class Dog(Animal):
    def __init__(self, name, age):
        super().__init__(name)    # call Animal.__init__
        self.age = age
 
dog = Dog("Nala", 3)
print(dog.name, dog.age)          # Nala 3

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Nice String Representation (str) (to_string)

Define str for a human-readable representation (e.g. print(obj)).

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
 
    def __str__(self):
        return f"{self.name} ({self.age} years)"
 
dog = Dog("Rocky", 4)
print(dog)                        # Rocky (4 years)