Merge pull request #946 from goodfeli/type_check

doc/extending/op.txt

@@ -109,7 +109,7 @@ following methods:
   elements of inputs[input_idx] have an effect on the elements of
   outputs[output_idx].
 
-  The ``node'' parameter is needed to determine the number of
+  The ``node`` parameter is needed to determine the number of
   inputs. Some ops such as Subtensor take a variable number of
   inputs.
 
@@ -159,7 +159,11 @@ following methods:
 
   If the output is not differentiable with respect to an input
   then this method should be defined to return a variable of type
-  NullType for that input.
+  NullType for that input. Likewise, if you have not implemented the
+  grad computation for some input, you may return a variable of type
+  NullType for that input. theano.gradient contains convenience methods
+  that can construct the variable for you: :func:`theano.gradient.grad_undefined` and
+  :func:`theano.gradient.grad_not_implemented`, respectively.
 
   If an element of output_gradient is of type theano.gradient.DisconnectedType,
   it means that the cost is not a function of this output. If any of the

theano/gradient.py

@@ -446,11 +446,21 @@ def grad(cost, wrt, g_cost=None, consider_constant=None,
                 raise TypeError('Elements of consider_constant must be '
                                 'variables, but got ' + str(type(elem)))
 
+    if isinstance(wrt, set):
+        raise TypeError("wrt must not be a set. sets have no defined "
+                "iteration order, so we can't return gradients in a matching"
+                " order.")
+
     using_list = isinstance(wrt, list)
     using_tuple = isinstance(wrt, tuple)
     if not using_list and not using_tuple:
         wrt = [wrt]
 
+    for elem in wrt:
+        if not isinstance(elem, Variable):
+            raise TypeError("Expected Variable, got " + str(elem) +
+                    " of type "+str(type(elem)))
+
     var_to_node_to_idx = _populate_var_to_node_to_idx([cost], wrt)
 
     # build a dict mapping var to the gradient of cost with respect to var