set_value() for ExtensionObject

[scch1092]

Hi all, I wanted to know if anyone faced a similar problem. I have created a custom type (Struct) on my OPC-UA server. This one I can read. Unfortunately I don't know how to change values in this struct.

variants = client.load_type_definitions()
ex_node = client.get_node("ns=4;s=glb.exObj")
ex_node_value = ex_node.get_value()
ex_node_datavalue = ex_node.get_data_value()

For testing I created the object "exObj" from this Struct type. This contains two bool variables and one float. How can I change one or more values with "set_value()" now?

Thanks in advance! =)

[AndreasHeine]

should work similar like in asyncua https://github.com/FreeOpcUa/opcua-asyncio/blob/master/examples/server-extension-object-as-method-argument.py

[scch1092]

Hi, thanks for the quick reply!
The way I saw it, I have to define a custom class, because in "ua.uaprotocol_auto." my custom class is not defined. I have now created my own class, following the example in the documentation: https://python-opcua.readthedocs.io/en/latest/_modules/opcua/ua/uaprotocol_auto.html#XVType
Unfortunately it doesn't work, I get an Error, although the data looks the same.

My program code looks like this:

  class exStruc(FrozenClass):
  
      ua_types = [
          ('pEnable', 'Boolean'),
          ('pReset', 'Boolean'),
          ('pVal', 'Double'),
          ]
  
      def __init__(self):
          self.pEnable = False
          self.pReset = False
          self.pVal = 0.0
  
      def __str__(self):
          return 'exStruc(pEnable:{self.pEnable}, pReset:{self.pReset}, pVal:{self.pVal})'
  
      __repr__ = __str__
  
  ex = exStruc()
  
  ex.pEnable = True
  ex.pReset = None
  ex.pVal = None
  
  dv = ua.DataValue(ua.Variant(ex, ua.VariantType.ExtensionObject))

In the screenshot both ua.uatypes.dataValue look the same. Also the output like ex.Value.Value is identical. Unfortunately, when I send it (.set_value()) I get the error:

File "c:\users\chris\appdata\local\programs\python\python37\lib\site-packages\opcua\ua\ua_binary.py", line 135, in pack
return struct.pack(self.format, data)

error: required argument is not a float

Do I perhaps have a mistake in thinking?

[AndreasHeine]

why you dont use the server methods for creating a own DataType? creating just a class does not tell the server what to do with it and as you already found out will not register/create anything in ua.uaprotocol_auto!

python-opcua/opcua/server/server.py

Line 443 in 67f1555

def create_custom_data_type(self, idx, name, basetype=ua.ObjectIds.BaseDataType, properties=None, description=None):

see also the example from the examples folder:
https://github.com/FreeOpcUa/python-opcua/blob/master/examples/server-create-custom-structures.py

[scch1092]

My idea behind this was that the data type already exists on the server; the structure is already there. With "client.load_type_definitions()" I can load this also that with ".get_value()" the correct structure is present. Therefore I wanted to create the same object with the class to send it to the server with ".set_value()". The object should have the same structure as in ".get_value()".
I will try the suggested approach!

[scch1092]

Okay, I found my mistake: The cod works, but no variables must be =None.
My PLC does not accept this and probably the library does not work then, because this data has a NoneType. That means, all variables need a value.

class exStruc(FrozenClass):

    ua_types = [
        ('pEnable', 'Boolean'),
        ('pReset', 'Boolean'),
        ('pVal', 'Double'),
        ]

    def __init__(self):
        self.pEnable = False
        self.pReset = False
        self.pVal = 0.0

    def __str__(self):
        return 'exStruc(pEnable:{self.pEnable}, pReset:{self.pReset}, pVal:{self.pVal})'

    __repr__ = __str__


ex = exStruc() 

ex.pEnable = True  #example Values; !=None
ex.pReset = True  #example Values; !=None
ex.pVal = 2.2 #example Values; !=None

dv = ua.DataValue(ua.Variant(ex, ua.VariantType.ExtensionObject))

ex_node = client.get_node("ns=4;s=glb.exObj")
ex_node.set_value(dv)

Thanks for help!

[AndreasHeine]

not sure if this is the correct way to do it... it will work till the point you have optional struct fields i guess!
if you load typedefinitions from clientside it should create the classes for you on the fly.

edit:
a good generic way to encounter extension objects is to create a function which cheks the datatype of the variable first and if its a structure then trys to find it in the auto generated classes by name (generated while runtime) if avalible -> then check if it is a known exobj and uses factory-function's (prebuild for known exobj's) which then creates the value-obj with its fileds... in opc ua you can read and browse alot of information/attributes/references to implement generic ways to solve things!
One of opc ua's base concepts is that the server exposes all necessary information to describe its own data, the opc ua client has just to ask the questions ;)

[oroulet]

yes that approach does not seem to make sense. you should create the data type server side. Load it server side calling load_type_xxx() then the object is avalable as ua.MyObject() and you can use it to set values for example.
Same on client side. Call load_type_definition() then the type is available in ua namespace. Just use it

[scch1092]

Oh wow, I knew there had to be a more elegant solution to my problem! The proposed solution makes everything immensely easier, thx! Of course the proposal works :)

ex_node = client.get_node("ns=4;s=glb.exObj")
client.load_type_definitions() 

ex = ua.exStruc() 

ex.pEnable = True  #example Values
ex.pReset = True  #example Values
ex.pVal = 2.2 #example Values

dv = ua.DataValue(ua.Variant(ex, ua.VariantType.ExtensionObject))
ex_node.set_value(dv)