added instantiate_optional support on create object method and possible fix for #307 (#323)

* added instantiate_optional

* Update uatypes.py

* Revert "Update uatypes.py"

This reverts commit 8e9f907a.

* Update uatypes.py

* Revert "Update uatypes.py"

This reverts commit f073371a.

* Update uatypes.py

asyncua/common/manage_nodes.py

@@ -50,7 +50,7 @@ async def create_folder(parent, nodeid, bname):
     )
 
 
-async def create_object(parent, nodeid, bname, objecttype=None):
+async def create_object(parent, nodeid, bname, objecttype=None, instantiate_optional=True):
     """
     create a child node object
     arguments are nodeid, browsename, [objecttype]
@@ -61,7 +61,7 @@ async def create_object(parent, nodeid, bname, objecttype=None):
     if objecttype is not None:
         objecttype = make_node(parent.server, objecttype)
         dname = ua.LocalizedText(qname.Name)
-        nodes = await instantiate(parent, objecttype, nodeid, bname=qname, dname=dname)
+        nodes = await instantiate(parent, objecttype, nodeid, bname=qname, dname=dname, instantiate_optional=instantiate_optional)
         return nodes[0]
     else:
         return make_node(

asyncua/common/node.py

@@ -684,8 +684,8 @@ class Node:
     async def add_folder(self, nodeid, bname):
         return await create_folder(self, nodeid, bname)
 
-    async def add_object(self, nodeid, bname, objecttype=None):
-        return await create_object(self, nodeid, bname, objecttype)
+    async def add_object(self, nodeid, bname, objecttype=None, instantiate_optional=True):
+        return await create_object(self, nodeid, bname, objecttype, instantiate_optional)
 
     async def add_variable(self, nodeid, bname, val, varianttype=None, datatype=None):
         return await create_variable(self, nodeid, bname, val, varianttype, datatype)

asyncua/ua/uatypes.py

@@ -750,7 +750,10 @@ class Variant(FrozenClass):
         if self.VariantType is None:
             self.VariantType = self._guess_type(self.Value)
         if self.Value is None and not self.is_array and self.VariantType not in (VariantType.Null, VariantType.String, VariantType.DateTime, VariantType.ExtensionObject):
-            raise UaError(f"Non array Variant of type {self.VariantType} cannot have value None")
+            if self.Value == None and self.VariantType == VariantType.NodeId:
+                self.Value = NodeId(0,0)
+            else:
+                raise UaError(f"Non array Variant of type {self.VariantType} cannot have value None")
         if self.Dimensions is None and isinstance(self.Value, (list, tuple)):
             dims = get_shape(self.Value)
             if len(dims) > 1: