ক্লাস এবং ওওপি (Classes & OOP)

class Dog:
    # Class attribute (shared by ALL dogs)
    species = "Canis familiaris"

    def __init__(self, name, breed, age):
        # Instance attributes (unique to each dog)
        self.name = name
        self.breed = breed
        self.age = age
        self.tricks = []     # Each dog starts with no tricks

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

    def learn_trick(self, trick):
        self.tricks.append(trick)
        return f"{self.name} learned {trick}!"

# Create instances (objects)
buddy = Dog("Buddy", "Golden Retriever", 3)
max_dog = Dog("Max", "German Shepherd", 5)

print(buddy.bark())
print(max_dog.bark())
print(buddy.learn_trick("shake"))
print(buddy.learn_trick("roll over"))
print(f"{buddy.name}'s tricks: {buddy.tricks}")
print(f"{max_dog.name}'s tricks: {max_dog.tricks}")  # Still empty!

Buddy says: Woof!
Max says: Woof!
Buddy learned shake!
Buddy learned roll over!
Buddy's tricks: ['shake', 'roll over']
Max's tricks: []

class ShoppingCart:
    def __init__(self):
        self.items = []

    def add(self, item, price):
        self.items.append({"item": item, "price": price})

    @property
    def total(self):
        return sum(i["price"] for i in self.items)

    def __str__(self):
        lines = [f"  {i['item']}: ${i['price']:.2f}" for i in self.items]
        return "Shopping Cart:\n" + "\n".join(lines) + f"\n  Total: ${self.total:.2f}"

    def __len__(self):
        return len(self.items)

    def __contains__(self, item_name):
        return any(i["item"] == item_name for i in self.items)

cart = ShoppingCart()
cart.add("Coffee", 4.50)
cart.add("Muffin", 3.25)
cart.add("Juice", 5.00)

print(cart)               # Uses __str__
print(f"Items: {len(cart)}")  # Uses __len__
print(f"Has Coffee: {'Coffee' in cart}")  # Uses __contains__

Shopping Cart:
  Coffee: $4.50
  Muffin: $3.25
  Juice: $5.00
  Total: $12.75
Items: 3
Has Coffee: True

class Animal:
    def __init__(self, name, sound):
        self.name = name
        self.sound = sound

    def speak(self):
        return f"{self.name} says {self.sound}!"

    def __str__(self):
        return f"{self.name} the {self.__class__.__name__}"

class Cat(Animal):
    def __init__(self, name, indoor=True):
        super().__init__(name, "Meow")  # Call parent's __init__
        self.indoor = indoor

    def purr(self):  # New method, only for cats
        return f"{self.name} purrs softly..."

class Parrot(Animal):
    def __init__(self, name, vocabulary=None):
        super().__init__(name, "Squawk")
        self.vocabulary = vocabulary or []

    def speak(self):  # Override parent method
        if self.vocabulary:
            import random
            word = random.choice(self.vocabulary)
            return f"{self.name} says: '{word}'"
        return super().speak()  # Fall back to parent

# Create instances
whiskers = Cat("Whiskers")
polly = Parrot("Polly", ["Hello!", "Pretty bird!", "Want a cracker?"])

print(whiskers.speak())    # Inherited from Animal
print(whiskers.purr())     # Cat-only method
print(polly.speak())       # Overridden method
print()

# isinstance checks
print(f"Is Whiskers an Animal? {isinstance(whiskers, Animal)}")
print(f"Is Polly a Cat? {isinstance(polly, Cat)}")

Whiskers says Meow!
Whiskers purrs softly...
Polly says: 'Hello!'

Is Whiskers an Animal? True
Is Polly a Cat? False