Return -1 for errors.

Objects/moduleobject.c

@@ -107,7 +107,7 @@ module_setattr(m, name, v)
 {
 	if (strcmp(name, "__dict__") == 0 || strcmp(name, "__name__") == 0) {
 		err_setstr(NameError, "can't assign to reserved member name");
-		return NULL;
+		return -1;
 	}
 	if (v == NULL)
 		return dictremove(m->md_dict, name);

Objects/object.c

@@ -157,7 +157,7 @@ setattr(v, name, w)
 			err_setstr(TypeError, "attribute-less object");
 		else
 			err_setstr(TypeError, "object has read-only attributes");
-		return NULL;
+		return -1;
 	}
 	else {
 		return (*v->ob_type->tp_setattr)(v, name, w);

Python/structmember.c

@@ -130,5 +130,5 @@ setmember(addr, mlist, name, v)
 	}
 	
 	err_setstr(NameError, name);
-	return NULL;
+	return -1;
 }