ex 3.4

stock.py

@@ -1,10 +1,6 @@
-import csv
-
-from Decimal import Decimal
-
-
 class Stock:
-    types = (str, int, float)
+    _types = (str, int, float)
+    __slots__ = ["name", "_shares", "_price"]
 
     def __init__(self, name, shares, price):
         self.name = name
@@ -13,12 +9,37 @@ class Stock:
 
     @classmethod
     def from_row(cls, row):
-        values = [func(val) for func, val in zip(cls.types, row)]
+        values = [func(val) for func, val in zip(cls._types, row)]
         return cls(*values)
 
+    @property
     def cost(self):
         return self.shares * self.price
 
+    @property
+    def shares(self):
+        return self._shares
+
+    @shares.setter
+    def shares(self, value):
+        if not isinstance(value, self._types[1]):
+            raise TypeError("Expected int")
+        if value < 0:
+            raise ValueError("Expected positive integer")
+        self._shares = value
+
+    @property
+    def price(self):
+        return self._price
+
+    @price.setter
+    def price(self, value):
+        if not isinstance(value, self._types[2]):
+            raise TypeError("Expected float")
+        if value < 0.0:
+            raise ValueError("Expected positive float")
+        self._price = value
+
     def sell(self, num):
         self.shares -= num
         if self.shares < 0: