gradient - added some asserts and comments

theano/gradient.py

@@ -32,6 +32,8 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
     """
     gmap = {}
     for (r, g_r) in sources:
+        if not hasattr(r, 'type'):
+            raise TypeError('sources must be Variables', r)
         if g_r is not None:
             if r in gmap:
                 gmap[r] = gmap[r] + g_r
@@ -52,6 +54,10 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
         output_arg = g_outputs
         input_arg = node.inputs
 
+        # Each Op's grad function requires inputs and output_grads
+        # If the Op destroys any input, but the grad expression uses it, then chances are the
+        # resulting graph will have a dependency cycle.  We avoid this cycle by passing
+        # (symbolic) copies of each destroyed input.
         try:
             dinputs = [node.inputs[x[0]] for x in node.op.destroy_map.values()]
         except AttributeError:
@@ -93,6 +99,7 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
             if g_r and len(sources) == 1 and sources[0][0].name and r.name:
                 g_r.name = "(d%s/d%s)" % (sources[0][0].name, r.name)
             if g_r is not None: 
+                assert r is not None
                 if r in gmap:
                     gmap[r] = gmap[r] + g_r
                 else: