made scan treat DisconnectedType as 0

theano/scan_module/scan_op.py

@@ -30,6 +30,7 @@ from theano.tensor import TensorType
 from theano import tensor
 from theano.tensor.opt import Shape_i
 from theano.gradient import grad_undefined
+from theano.gradient import DisconnectedType
 #from theano.sandbox import cuda
 from theano.compile.profiling import ScanProfileStats
 
@@ -1162,6 +1163,18 @@ class Scan(PureOp):
 
     ### GRAD FUNCTION
     def grad(self, args, g_outs):
+
+
+        #This discards information about whether incoming gradients are 0
+        #or disconnected from the cost
+        #TODO: upgrade scan op to report disconnection correctly
+        def strip_disconnected( g ):
+            if isinstance(g.type, DisconnectedType):
+                return None
+            return g
+
+        g_outs = [ strip_disconnected(g) for g in g_outs ]
+
         # 1. forward pass - get the outputs after applying scan
         scan_outputs = self(*args)
         # 2. make sure they are given as a list