ex 3.1 a

stock.py

@@ -1,4 +1,3 @@
-
 class Stock:
     def __init__(self, name, shares, price):
         self.name = name
@@ -7,3 +6,8 @@ class Stock:
 
     def cost(self):
         return self.shares * self.price
+
+    def sell(self, num):
+        self.shares -= num
+        if self.shares < 0:
+            self.shares = 0