Added solution code

Solutions/6_15/portfolio.py

@@ -0,0 +1,31 @@
+# portfolio.py
+
+class Portfolio(object):
+    def __init__(self, holdings):
+        self._holdings = holdings
+
+    def __iter__(self):
+        return self._holdings.__iter__()
+
+    def __len__(self):
+        return len(self._holdings)
+
+    def __getitem__(self, index):
+        return self._holdings[index]
+
+    def __contains__(self, name):
+        return any(s.name == name for s in self._holdings)
+
+    @property
+    def total_cost(self):
+        return sum(s.shares * s.price for s in self._holdings)
+
+    def tabulate_shares(self):
+        from collections import Counter
+        total_shares = Counter()
+        for s in self._holdings:
+            total_shares[s.name] += s.shares
+        return total_shares
+
+
+