diff --git a/TODO b/TODO
index b4962d0c..77a2452c 100644
--- a/TODO
+++ b/TODO
@@ -1,9 +1,19 @@
-- python/try.py
+- Vips.cache_operation_build() is not very wrapper friendly
 
-	- have a ref leak
+  make it work like:
+
+  op2 = build(op1)
+
+  return NULL on error, returns a new op on success
+
+  always unref the old op
+
+
+
+
+- grep for other vips_class_find() problems: do we use it for simple class
+  lookup anywhere else?
 
-	- try looping over operator props and looking up class and instance; 
-	  need to be able to get INPUT, OUTPUT, ASSIGNED etc
 
 
 
diff --git a/libvips/arithmetic/measure.c b/libvips/arithmetic/measure.c
index 4ba953a0..7fc800ef 100644
--- a/libvips/arithmetic/measure.c
+++ b/libvips/arithmetic/measure.c
@@ -111,11 +111,11 @@ vips_measure_build( VipsObject *object )
 
 	/* left/top/width/height default to the size of the image.
 	 */
-	if( !vips_argument_get_assigned( object, "width" ) )
+	if( !vips_object_get_assigned( object, "width" ) )
 		g_object_set( object, 
 			"width", vips_image_get_width( measure->in ),
 			NULL );
-	if( !vips_argument_get_assigned( object, "height" ) )
+	if( !vips_object_get_assigned( object, "height" ) )
 		g_object_set( object, 
 			"height", vips_image_get_height( measure->in ),
 			NULL );
diff --git a/libvips/deprecated/dispatch_types.c b/libvips/deprecated/dispatch_types.c
index 82447244..696d0f63 100644
--- a/libvips/deprecated/dispatch_types.c
+++ b/libvips/deprecated/dispatch_types.c
@@ -872,7 +872,7 @@ input_interpolate_init( im_object *obj, char *str )
 {
 	GType type = g_type_from_name( "VipsInterpolate" );
 	VipsObjectClass *interpolate_class = 
-		VIPS_INTERPOLATE_CLASS( g_type_class_ref( type ) );
+		VIPS_OBJECT_CLASS( g_type_class_ref( type ) );
 	VipsObject *object;
 
 	g_assert( interpolate_class );
diff --git a/libvips/foreign/tiffsave.c b/libvips/foreign/tiffsave.c
index 34c373a3..8c0b116b 100644
--- a/libvips/foreign/tiffsave.c
+++ b/libvips/foreign/tiffsave.c
@@ -93,14 +93,14 @@ vips_foreign_save_tiff_build( VipsObject *object )
 
 	/* Default xres/yres to the values from the image.
 	 */
-	if( !vips_argument_get_assigned( object, "xres" ) )
+	if( !vips_object_get_assigned( object, "xres" ) )
 		tiff->xres = save->ready->Xres * 10.0;
-	if( !vips_argument_get_assigned( object, "yres" ) )
+	if( !vips_object_get_assigned( object, "yres" ) )
 		tiff->yres = save->ready->Yres * 10.0;
 
 	/* resunit param overrides resunit metadata.
 	 */
-	if( !vips_argument_get_assigned( object, "resunit" ) &&
+	if( !vips_object_get_assigned( object, "resunit" ) &&
 		vips_image_get_typeof( save->ready, 
 			VIPS_META_RESOLUTION_UNIT ) &&
 		!vips_image_get_string( save->ready, 
diff --git a/libvips/include/vips/object.h b/libvips/include/vips/object.h
index 93ea17ee..5cf28702 100644
--- a/libvips/include/vips/object.h
+++ b/libvips/include/vips/object.h
@@ -293,7 +293,8 @@ int vips_object_get_argument( VipsObject *object, const char *name,
 	GParamSpec **pspec,
 	VipsArgumentClass **argument_class,
 	VipsArgumentInstance **argument_instance );
-gboolean vips_argument_get_assigned( VipsObject *object, const char *name );
+gboolean vips_object_get_assigned( VipsObject *object, const char *name );
+VipsArgumentFlags vips_object_get_flags( VipsObject *object, const char *name );
 
 /* We have to loop over an objects args in several places, and we can't always
  * use vips_argument_map(), the preferred looper. Have the loop code as a
diff --git a/libvips/iofuncs/object.c b/libvips/iofuncs/object.c
index 7993e643..56cff603 100644
--- a/libvips/iofuncs/object.c
+++ b/libvips/iofuncs/object.c
@@ -459,8 +459,15 @@ vips__argument_get_instance( VipsArgumentClass *argument_class,
 
 /**
  * vips_object_get_argument: (skip)
+ * @object: the object to fetch the args from
+ * @name: arg to fetch
+ * @pspec: (transfer none): the pspec for this arg
+ * @argument_class: (transfer none): the argument_class for this arg
+ * @argument_instance: (transfer none): the argument_instance for this arg
  *
  * Look up the three things you need to work with a vips argument.
+ *
+ * Returns: 0 on success, or -1 on error.
  */
 int
 vips_object_get_argument( VipsObject *object, const char *name,
@@ -497,10 +504,17 @@ vips_object_get_argument( VipsObject *object, const char *name,
 	return( 0 );
 }
 
-/* Convenience: has an argument been assigned.
+/**
+ * vips_object_get_assigned:
+ * @object: the object to fetch the args from
+ * @name: arg to fetch
+ *
+ * Convenience: has an argument been assigned.
+ *
+ * Returns: %TRUE if the arguent has been assigned.
  */
 gboolean
-vips_argument_get_assigned( VipsObject *object, const char *name )
+vips_object_get_assigned( VipsObject *object, const char *name )
 {
 	GParamSpec *pspec;
 	VipsArgumentClass *argument_class;
@@ -513,6 +527,29 @@ vips_argument_get_assigned( VipsObject *object, const char *name )
 	return( argument_instance->assigned );
 }
 
+/**
+ * vips_object_get_flags:
+ * @object: the object to fetch the args from
+ * @name: arg to fetch
+ *
+ * Convenience: get the flags for an argument.
+ *
+ * Returns: The #VipsArgmentFlags for this argument.
+ */
+VipsArgumentFlags
+vips_object_get_flags( VipsObject *object, const char *name )
+{
+	GParamSpec *pspec;
+	VipsArgumentClass *argument_class;
+	VipsArgumentInstance *argument_instance;
+
+	if( vips_object_get_argument( object, name,
+		&pspec, &argument_class, &argument_instance ) )
+		return( 0 );
+
+	return( argument_class->flags );
+}
+
 static void
 vips_object_clear_member( VipsObject *object, GParamSpec *pspec, 
 	GObject **member )
diff --git a/python/try.py b/python/try.py
index 1255bd37..a9915a47 100755
--- a/python/try.py
+++ b/python/try.py
@@ -30,13 +30,23 @@ print 'call operation:'
 
 op = Vips.Operation.new("add")
 for prop in op.props:
-    print 'prop =', prop
+    print 'prop.name =', prop.name
+    flags = op.get_flags(prop.name)
+    if flags & Vips.ArgumentFlags.OUTPUT:
+        print '\toutput'
+    if flags & Vips.ArgumentFlags.INPUT:
+        print '\tinput'
+    if flags & Vips.ArgumentFlags.REQUIRED:
+        print '\trequired'
+    print '\tassigned', op.get_assigned(prop.name)
+
 op.props.left = a
 op.props.right = a
 if op.build() != 0:
     print Vips.error_buffer()
     sys.exit(-1)
 out = op.props.out
+op.unref_outputs()
 
 print 'out.get_format() =', out.get_format()
 print 'out.props.format =', out.props.format