Fix dict KeyError (donnemartin#153)

solutions/object_oriented_design/lru_cache/lru_cache.py

@@ -34,7 +34,7 @@ def get(self, query):
 
         Accessing a node updates its position to the front of the LRU list.
         """
-        node = self.lookup[query]
+        node = self.lookup.get(query)
         if node is None:
             return None
         self.linked_list.move_to_front(node)
@@ -47,7 +47,7 @@ def set(self, results, query):
         If the entry is new and the cache is at capacity, removes the oldest entry
         before the new entry is added.
         """
-        node = self.lookup[query]
+        node = self.lookup.get(query)
         if node is not None:
             # Key exists in cache, update the value
             node.results = results