SCAPY: Using objects (classes) multiple times in a packet/layer

Question

I'm new to scapy and I'm trying to implement a protocol. Within this protocol a type, lets call it "Attribute" is used multiple times (with an other name ofcourse). Now I'm trying to implement this, but it won't work.

I've created my "Attribute" as follows:

class Attribute(Packet):
    name = "Attribute"
    fields_desc = [
        ByteField("Tag_ID_Open", 0xa3),                     # 0xa3 = Attribute
        ShortField("ObjectVariableTypeName", 0x8169),
        ByteField("Datatype_Flags", 0x00),
        ShortField("Datatype", 0x15),                       #0x15 = String
        ByteField("Length", 0x00),
        StrField("Value","SubscriptionContainer"),
        ByteField("Tag_ID_Close", 0xa2),                    # 0xa2 = Terminating Object
    ]

The class where this "Attribute" is used is as follows: (I know there are some issues with conditional fields)

class ClassServerSession_GetNewRIDOnServer(Packet):
    name = "ClassServerSession_GetNewRIDOnServer"
    fields_desc = [
        ByteField("Tag_ID_Open", 0xa3),                         # 0xa3 = Attribute
        NBytesField("RelationID",0x000000,8),
        ShortField("ClassID",0x0000),
        ByteField("ClassFlags",0x00),
        ByteField("AttributeID",0x00),
        PacketListField("ObjectVariableTypeName", None, Attribute(ID_Number = 0x8169, Value="ServerSession_1C9C381",Length=len("ServerSession_1C9C381"))),
        PacketListField("ServerSessionClientString", None, Attribute(ID_Number = 0x8221, Value="1:::6.0::Intel(R) PRO/1000 MT Desktop Adapter.TCPIP.1", Length=len("1:::6.0::Intel(R) PRO/1000 MT Desktop Adapter.TCPIP.1"))),
        PacketListField("ServerSessionUser", None, Attribute(ID_Number = 0x8228, Value="", Length=len(""))),
        PacketListField("ServerSessionApplication", None, Attribute(ID_Number = 0x8229, Value="", Length=len(""))),
        PacketListField("ServerSessionHost", None, Attribute(ID_Number = 0x822a, Value="DESKTOP-3DTU8UA_1340031", Length=len("DESKTOP-3DTU8UA_1340031"))),
        PacketListField("ServerSessionRole", None, Attribute(ID_Number = 0x822b, Datatype = 0x04, Value=1, Length=1)),
        PacketListField("ServerSessionClientRID", None, Attribute(ID_Number = 0x822c, Datatype = 0x12, Value=0x01c9c381)),
        PacketListField("ServerSessionClientComment", None, Attribute(ID_Number = 0x822d, Value="", Length=0)),
        ByteField("Tag_ID_Close", 0xa2),                    # 0xa2 = Terminating Object
    ]

When printing this class I get the following:

pPacket = ClassServerSession_GetNewRIDOnServer()
pPacket.show2()

###[ ClassServerSession_GetNewRIDOnServer ]### 
  Tag_ID_Open= 163
  RelationID= 0
  ClassID   = 0
  ClassFlags= 0
  AttributeID= 0
  \ObjectVariableTypeName\
   |###[ Raw ]###
   |  load      = '\\xa2'
  \ServerSessionClientString\
  \ServerSessionUser\
  \ServerSessionApplication\
  \ServerSessionHost\
  \ServerSessionRole\
  \ServerSessionClientRID\
  \ServerSessionClientComment\
  Tag_ID_Close= 162

I've tried with FieldLenList, without succes:

FieldLenField("ObjectVariableTypeName", 22, length_of=Attribute, fmt='B'),
PacketListField("Attribute", None, Attribute(ID_Number = 0x8169, Value="ServerSession_1C9C381",Length=len("ServerSession_1C9C381"))),

Also searched stackoverflow for similar issues, but couldn't find any. Looked at examples found on stackoverflow and other sites, but none of them were helpfull.

Can somebody help me?