--- a/wsgraph/model.py	Fri Dec 14 20:53:41 2012 -0800
+++ b/wsgraph/model.py	Sat Dec 15 23:22:19 2012 -0800
@@ -85,12 +85,18 @@
             return tuple(key) in self.edges()
 
 
+class DirectedGraph(Graph):
+    """mix-in class for directed graphs"""
+    # TODO: is this possible without super or other black magicks?
+
+
 class MemoryCache(Graph):
     """volatile in-memory representation of a graph"""
 
-    def __init__(self):
+    def __init__(self, node_type=dict):
         self._edges = {}
         self._nodes = {}
+        self.node_type = node_type
 
     def node(self, name, value=None):
         if value is not None:
@@ -109,7 +115,7 @@
             # setter
             self._edges[(node1, node2)] = deepcopy(value)
             for node in node1, node2:
-                self._nodes.setdefault(node, {})
+                self._nodes.setdefault(node, self.node_type())
         else:
             # getter
             # TODO: deepcopy
@@ -118,6 +124,7 @@
     def edges(self):
         return self._edges.keys()
 
+
 class FileCache(MemoryCache):
     """on-disk JSON file cache"""