class Validator:
    def __init__(self, name=None):
        self.name = name

    @classmethod
    def check(cls, value):
        return value

    def __set__(self, instance, value):
        instance.__dict__[self.name] = self.check(value)

    def __set_name__(self, cls, name):
        self.name = name


class Typed(Validator):
    expected_type = object

    @classmethod
    def check(cls, value):
        if not isinstance(value, cls.expected_type):
            raise TypeError(f"expected {cls.expected_type}")
        return super().check(value)


class Integer(Typed):
    expected_type = int


class Float(Typed):
    expected_type = float


class String(Typed):
    expected_type = str


class Positive(Validator):
    @classmethod
    def check(cls, value):
        if value < 0:
            raise ValueError("Expected >= 0")
        return super().check(value)


class NonEmpty(Validator):
    @classmethod
    def check(cls, value):
        if len(value) == 0:
            return ValueError("Must be non-empty")
        return super().check(value)


class PositiveInteger(Integer, Positive):
    pass


class PositiveFloat(Float, Positive):
    pass


class NonEmptyString(String, NonEmpty):
    pass


class Stock:
    name = String()
    shares = PositiveInteger()
    price = PositiveFloat()

    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    def __repr__(self):
        return f"Stock({self.name!r}, {self.shares!r}, {self.price!r})"

    def __eq__(self, other):
        if not isinstance(other, Stock):
            return False
        return (self.name, self.shares, self.price) == (
            other.name,
            other.shares,
            other.price,
        )

    @classmethod
    def from_row(cls, row):
        return cls(*row)

    @property
    def cost(self):
        return self.shares * self.price

    def sell(self, num):
        self.shares -= num
        if self.shares < 0:
            self.shares = 0