better error message (and partial code clarification)

theano/tensor/basic.py

@@ -600,11 +600,27 @@ def get_scalar_constant_value(v):
 
             # This is needed when we take the grad as the Shape op
             # are not already changed into MakeVector
-            if (v.owner.inputs[0].owner and
-                isinstance(v.owner.inputs[0].owner.op,
+            owner = v.owner
+            leftmost_parent = owner.inputs[0]
+            if (leftmost_parent.owner and
+                isinstance(leftmost_parent.owner.op,
                            theano.tensor.Shape)):
-                if v.owner.inputs[0].owner.inputs[0].type.broadcastable[
-                    v.owner.op.idx_list[0]]:
+                owner = v.owner
+                op = owner.op
+                idx_list = op.idx_list
+                idx = idx_list[0]
+                grandparent = leftmost_parent.owner.inputs[0]
+                gp_broadcastable = grandparent.type.broadcastable
+                ndim = grandparent.type.ndim
+
+                assert ndim == len(gp_broadcastable)
+
+                if not (idx < len(gp_broadcastable)):
+                    raise ValueError("You are taking x.shape[i] on some tensor x, "
+                            "but x.ndim is %d and i is %d, so this is deterministically "
+                            "an index error." % (ndim, idx))
+
+                if gp_broadcastable[idx]:
                     return numpy.asarray(1)
 
     raise NotScalarConstantError(v)