Allow None as a valid context.

theano/gof/cc.py

@@ -516,7 +516,7 @@ class CLinker(link.Linker):
         self.contexts = dict()
         for node in self.node_order:
             ctx = node.run_context()
-            if ctx is not None:
+            if ctx is not graph.NoContext:
                 # try to avoid creating more than one variable for the
                 # same context.
                 if ctx in self.contexts:
@@ -665,7 +665,7 @@ class CLinker(link.Linker):
             sub = dict(failure_var=failure_var)
 
             ctx = node.run_context()
-            if ctx is not None:
+            if ctx is not graph.NoContext:
                 context_var = symbol[self.contexts[ctx]]
 
             # The placeholder will be replaced by a hash of the entire
@@ -682,15 +682,15 @@ class CLinker(link.Linker):
             # Make the CodeBlock for c_code
             sub['id'] = id
             sub['fail'] = failure_code(sub)
-            if ctx is not None:
+            if ctx is not graph.NoContext:
                 sub['context'] = context_var
 
             sub_struct = dict()
             sub_struct['id'] = id + 1
             sub_struct['fail'] = failure_code_init(sub)
-            if ctx is not None:
+            if ctx is not graph.NoContext:
                 # Since context inputs are always constants they are
-                # guarenteed to be available in the struct init code.
+                # guaranteed to be available in the struct init code.
                 sub_struct['context'] = context_var
 
             struct_support = ""

theano/gof/graph.py

@@ -21,6 +21,7 @@ from theano.misc.ordered_set import OrderedSet
 is_same_graph_with_merge = None
 equal_computations = None
 
+NoContext = object()
 
 class Node(utils.object2):
     """A Node in a theano graph.
@@ -120,7 +121,7 @@ class Apply(Node):
         """
         if hasattr(self.op, 'get_context'):
             return self.op.get_context(self)
-        return None
+        return NoContext
 
     def default_output(self):
         """Returns the default output for this node.

theano/gof/op.py

@@ -742,9 +742,9 @@ class Op(utils.object2, PureOp, CLinkerOp):
 
         p = node.op.perform
 
-        ctx = node.get_context()
+        ctx = node.run_context()
 
-        if ctx is None:
+        if ctx is graph.NoContext:
             # default arguments are stored in the closure of `rval`
             def rval(p=p, i=node_input_storage, o=node_output_storage, n=node):
                 r = p(n, [x[0] for x in i], o)