Added solution code

Solutions/7_4/stock.py

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+# stock.py
+
+class Stock(object):
+    '''
+    An instance of a stock holding consisting of name, shares, and price.
+    '''
+    __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):
+        if not isinstance(value,int):
+            raise TypeError("Must be integer")
+        self._shares = value
+
+    @property
+    def cost(self):
+        '''
+        Return the cost as shares*price
+        '''
+        return self.shares * self.price
+
+    def sell(self, nshares):
+        '''
+        Sell a number of shares and return the remaining number.
+        '''
+        self.shares -= nshares