made Apply and Variable into subclasses of Node

theano/gof/destroyhandler.py

@@ -160,13 +160,12 @@ def _contains_cycle(inputs, outputs, orderings):
                 fifo_queue.append(node)
             node_to_parents[node] = parents
 
-
     rset = set()
-    rlist = []
+    visit_count = 0
     while fifo_queue:
         node = fifo_queue.popleft()
         if node not in rset:
-            rlist.append(node)
+            visit_count += 1
             rset.add(node)
             for client in node_to_children.get(node, []):
                 node_to_parents[client] = [a for a in node_to_parents[client] if a is not node]
@@ -174,7 +173,7 @@ def _contains_cycle(inputs, outputs, orderings):
                     fifo_queue.append(client)
 
 
-    return len(rlist) != len(node_to_parents.keys())
+    return visit_count != len(node_to_parents.keys())
 
 
 

theano/gof/graph.py

@@ -21,7 +21,25 @@ is_same_graph_with_merge = None
 equal_computations = None
 
 
-class Apply(utils.object2):
+class Node(utils.object2):
+    """A Node in a theano graph.
+    Graphs contain two kinds of Nodes--
+    Variable and Apply.
+    Edges in the graph are not explicitly represented.
+    Instead each Node keeps track of its parents via
+    Variable.owner / Apply.inputs and its children
+    via Variable.clients / Apply.outputs.
+    """
+
+    def get_parents(self):
+        """ Return a list of the parents of this node.
+        Should return a copy--i.e., modifying the return
+        value should not modify the graph structure."""
+
+        raise NotImplementedError()
+
+
+class Apply(Node):
     """
     An :term:`Apply` instance is a node in an expression graph which represents the application
     of an `Op` to some input `Variable` nodes, producing some output `Variable` nodes.
@@ -213,7 +231,7 @@ class Apply(utils.object2):
     """property: Number of outputs"""
 
 
-class Variable(utils.object2):
+class Variable(Node):
     """
     A :term:`Variable` is a node in an expression graph that represents a variable.