when None is given as the outputs list, function and Method now return None

theano/compile/function_module.py

@@ -170,6 +170,9 @@ class Function(object):
     unpack_single = None
     """Bool: for outputs lists of length 1, should the 0'th element be returned directly?"""
 
+    return_none = None
+    """Bool: whether the function should return None or not"""
+
     maker = None
     """FunctionMaker instance"""
 
@@ -197,7 +200,7 @@ class Function(object):
     It maps container -> SymbolicInput
     """
 
-    def __init__(self, fn, input_storage, output_storage, indices, outputs, defaults, unpack_single, maker):
+    def __init__(self, fn, input_storage, output_storage, indices, outputs, defaults, unpack_single, return_none, maker):
         """
         Initialize attributes. create finder, inv_finder.
         """
@@ -209,6 +212,7 @@ class Function(object):
         self.outputs = outputs
         self.defaults = defaults
         self.unpack_single = unpack_single
+        self.return_none = return_none
         self.maker = maker
 
         # we'll be popping stuff off this `containers` object.  It's a copy
@@ -379,7 +383,9 @@ class Function(object):
         for i, (required, refeed, value) in enumerate(self.defaults):
             if refeed:
                 self[i] = value
-        if self.unpack_single and len(outputs) == 1:
+        if self.return_none:
+            return None
+        elif self.unpack_single and len(outputs) == 1:
             return outputs[0]
         else:
             return outputs
@@ -571,6 +577,10 @@ class FunctionMaker(object):
 
         # Handle the case where inputs and/or outputs is a single Variable (not in a list)
         unpack_single = False
+        return_none = False
+        if outputs is None:
+            return_none = True
+            outputs = []
         if not isinstance(outputs, (list, tuple)):
             unpack_single = True
             outputs = [outputs]
@@ -611,6 +621,7 @@ class FunctionMaker(object):
         self.expanded_inputs = expanded_inputs
         self.outputs = outputs
         self.unpack_single = unpack_single
+        self.return_none = return_none
         self.mode = mode
         self.accept_inplace = accept_inplace
         self.function_builder = function_builder
@@ -706,12 +717,13 @@ class FunctionMaker(object):
 
         # Get a function instance
         _fn, _i, _o = self.linker.make_thunk(input_storage = input_storage)
-        fn = self.function_builder(_fn, _i, _o, self.indices, self.outputs, defaults, self.unpack_single, self)
+        fn = self.function_builder(_fn, _i, _o, self.indices, self.outputs, defaults, self.unpack_single, self.return_none, self)
         return fn
 
 
 def _pickle_FunctionMaker(fm):
-    rval = (_constructor_FunctionMaker, (fm.inputs, fm.outputs[0] if fm.unpack_single else fm.outputs, fm.mode, fm.accept_inplace))
+    outputs = None if fm.return_none else (fm.outputs[0] if fm.unpack_single else fm.outputs)
+    rval = (_constructor_FunctionMaker, (fm.inputs, outputs, fm.mode, fm.accept_inplace))
     return rval
 
 def _constructor_FunctionMaker(*args):
@@ -794,9 +806,7 @@ def function(inputs, outputs, mode=None, accept_inplace = False):
     mode = mode if mode is not None else mode_module.default_mode
 
     inputs = map(convert_function_input, inputs)
-    if outputs is None:
-        outputs = []
-    else:
+    if outputs is not None:
         outputs = map(FunctionMaker.wrap_out, outputs) if isinstance(outputs, (list, tuple)) else FunctionMaker.wrap_out(outputs)
 
     defaults = [getattr(input, 'value', None) for input in inputs]

theano/compile/module.py

@@ -347,8 +347,6 @@ class Method(Component):
 
         """
         super(Method, self).__init__()
-        if outputs is None:
-            outputs = []
         self.inputs = inputs
         self.outputs = outputs
         self.updates = dict(updates)
@@ -483,7 +481,7 @@ class Method(Component):
         outputs = self.outputs
         _inputs = [x.variable for x in inputs]
         # Grab the variables that are not accessible from either the inputs or the updates.
-        outputs_list =  list(outputs) if isinstance(outputs, (list, tuple)) else [outputs]
+        outputs_list = [] if outputs is None else (list(outputs) if isinstance(outputs, (list, tuple)) else [outputs])
         outputs_variable_list = [o.variable if isinstance(o, io.Out) else o for o in outputs_list]
         for input in gof.graph.inputs(outputs_variable_list
                                       + [x.update for x in inputs if getattr(x, 'update', False)],