added check that gradient of int is 0

theano/gradient.py

@@ -741,6 +741,18 @@ def _populate_grad_dict(var_to_node_to_idx,
 
             # Do type checking on the result
 
+            #List of bools indicating if each output is an integer dtype
+            output_is_int = [ hasattr(output.type,'dtype') and
+                    output.type.dtype.find('int') != -1
+                    for output in node.outputs]
+
+            #List of bools indicating if each input only has integer outputs
+            only_connected_to_int = [ (True not in
+                [ in_to_out and out_to_cost and not out_int
+                    for in_to_out, out_to_cost, out_int in
+                    zip(in_to_outs, outputs_connected, output_is_int) ])
+                for in_to_outs in connection_pattern ]
+
             for i, term in enumerate(input_grads):
 
                 # Disallow Nones
@@ -755,14 +767,35 @@ def _populate_grad_dict(var_to_node_to_idx,
 
                 if not isinstance(term.type,
                         (NullType,DisconnectedType)):
-                    if not hasattr(term.type,'dtype'):
-                        print term
-                        assert False
                     if term.type.dtype.find('float') == -1:
                         raise TypeError(str(node.op)+'.grad illegally '
                                 ' returned an integer-valued variable.'
                                 ' (Input index %d, dtype %s)' % (i,
                                     term.type.dtype))
+                    if only_connected_to_int[i]:
+                        # This term has only integer outputs and we know
+                        # it's not undefined or disconnected
+                        # The only other valid thing it can be is 0
+
+                        is_zero = False
+                        try:
+                            if tensor.get_constant_value(term) == 0:
+                                is_zero = True
+                        except:
+                            pass
+
+                        if not is_zero:
+                            msg = "%s.grad returned %s of type %s for input"
+                            msg += " %d. This input is only connected to "
+                            msg += "integer-valued outputs so it should be "
+                            msg += "NullType, DisconnectedType, or some form "
+                            msg += "of zeros."
+
+                            msg = msg % (str(node.op), str(term),
+                                    str(type(term)), i)
+
+                            raise ValueError(msg)
+
 
             #Check that op.connection_pattern matches the connectivity
             #logic driving the op.grad method