from dataclasses import dataclass, field

@dataclass
class Person:
    name: str
    age: int
    email: str = ""
    tags: list[str] = field(default_factory=list)

    @property
    def is_adult(self):
        return self.age >= 18

p1 = Person("Alice", 30)
p2 = Person("Alice", 30)
print(p1 == p2)  # True (auto-generated __eq__)
print(p1)        # Person(name='Alice', age=30, email='', tags=[])

@dataclass(frozen=True)
class Point:
    x: float
    y: float

p = Point(3, 4)
# p.x = 5  # FrozenInstanceError!

# ❌ Inheritance (is-a)
class Engine:
    def start(self): return "Engine started"

class Car(Engine):  # A Car IS AN Engine? No!
    pass

# ✅ Composition (has-a)
class Car:
    def __init__(self):
        self.engine = Engine()  # A Car HAS AN Engine

    def start(self):
        return self.engine.start()

class PositiveNumber:
    def __set_name__(self, owner, name):
        self.name = f"_{name}"

    def __get__(self, obj, objtype=None):
        return getattr(obj, self.name, 0)

    def __set__(self, obj, value):
        if value < 0:
            raise ValueError("Must be positive")
        setattr(obj, self.name, value)

class Product:
    price = PositiveNumber()
    quantity = PositiveNumber()

    def __init__(self, price, quantity):
        self.price = price
        self.quantity = quantity

p = Product(10, 5)
# p.price = -5  # ValueError!

class LogMixin:
    def log(self, message):
        print(f"[{self.__class__.__name__}] {message}")

class JSONMixin:
    def to_json(self):
        import json
        return json.dumps(self.__dict__)

class User(LogMixin, JSONMixin):
    def __init__(self, name, email):
        self.name = name
        self.email = email
        self.log(f"Created user: {name}")

user = User("Alice", "alice@example.com")
print(user.to_json())

class Point:
    __slots__ = ('x', 'y')  # Limits attributes, saves memory

    def __init__(self, x, y):
        self.x = x
        self.y = y

p = Point(3, 4)
# p.z = 5  # AttributeError!