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Exercises/soln4_2.md

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+# Exercise 4.2 - Solution
+
+Here is the validator code in its entirety:
+
+```python
+class Validator:
+    @classmethod
+    def check(cls, value):
+        return 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
+```
+
+## (c) Using the validators
+
+```python
+# validate.py
+...
+
+if __name__ == '__main__':
+    class Stock:
+        __slots__ = ('name', '_shares', '_price')
+        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 shares(self):
+            return self._shares
+        @shares.setter
+        def shares(self, value):
+            self._shares = PositiveInteger.check(value)
+
+        @property
+        def price(self):
+            return self._price
+        @price.setter
+        def price(self, value):
+            self._price = PositiveFloat.check(value)
+
+        @property
+        def cost(self):
+            return self.shares * self.price
+
+        def sell(self, nshares):
+            self.shares -= nshares
+```
+
+
+
+[Back](ex4_2.md)