# Exercise 4.3 - Solution

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

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

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

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

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('must be >= 0')
        return super().check(value)

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

class PositiveInteger(Integer, Positive):
    pass

class PositiveFloat(Float, Positive):
    pass

class NonEmptyString(String, NonEmpty):
    pass

# Examples
if __name__ == '__main__':
    def add(x, y):
        Integer.check(x)
        Integer.check(y)
        return x + y

    class Stock:
        name = NonEmptyString()
        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})'

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

        def sell(self, nshares):
            self.shares -= nshares
```



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