added filter_update function to shared variable interface

theano/compile/sandbox/pfunc.py

@@ -86,12 +86,8 @@ def pfunc(params, outputs=None, mode=None, updates=[]):
     # Add update values as quantities that must be computed.
     new_updates = {}
     for (store_into, update_val) in iter_over_pairs(updates):
-        if not isinstance(update_val, Variable):
-            # The value for the update is not a Variable: we cast it into
-            # a shared Variable so that it can be used by 'function'. Note that
-            # it means the update value may change if it is mutable and its
-            # value is modified after the function is created.
-            update_val = shared(update_val)
+        assert isinstance(store_into, SharedVariable)
+        update_val = store_into.filter_update(update_val)
         computed_list.append(update_val)
         new_updates[store_into] = update_val
     updates = new_updates

theano/compile/sandbox/sharedvalue.py

@@ -83,6 +83,24 @@ class SharedVariable(Variable):
 
     """
 
+    def filter_update(self, update):
+        """When this shared variable is updated by a pfunc, the update value will be run through this function.
+        This is a good spot to cast or convert the update expression as necessary.
+
+        Default behaviour is to return `update` unmodified if it is a Variable, otherwise to create a SharedVariable for it by calling ``shared(update)``.
+
+        :param update: the new value for this shared variable when updated by a pfunc.
+
+        :returns: a Variable whose value will be assigned to this SharedVariable by a pfunc.
+        """
+        if not isinstance(update, Variable):
+            # The value for the update is not a Variable: we cast it into
+            # a shared Variable so that it can be used by 'function'. Note that
+            # it means the update value may change if it is mutable and its
+            # value is modified after the function is created.
+            update = shared(update)
+        return update
+
 def shared_constructor(ctor):
     shared.constructors.append(ctor)
     return ctor