Cannot set value of Node

[ZachM05]

Hello all,

I have a piece of equipment which makes data available using OPCUA. I would like to use Python to ping the device and get data when it is available. To do so, I have started using Asyncua to create a loop allowing me to read the values on the machine. The reading has been no issue so far, but I am getting an error when I am writing to a node.

Here is my code:

import asyncio
import time
from asyncua import Client
from opcua import ua 
async def lifebyte():
    async with Client(url="opc.tcp://192.168.202.137:4840") as client:
        while True:
            lifeByte = client.get_node("ns=6;s=::Global.LifeByte")
            lifeByteAck = client.get_node("ns=6;s=::Global.LifeByteAck")
            value = await lifeByte.read_value()
            await lifeByteAck.set_value(ua.DataValue(value))
            print(type(value))
            time.sleep(5)


if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(lifebyte())
    loop.close()

This is quite simple and easy to do with the synchronous opcua Client, and works well. This error occurs though when I run the async code:

Traceback (most recent call last):
  File "C:\Users\melenz\Anaconda3\lib\dataclasses.py", line 1033, in fields
    fields = getattr(class_or_instance, _FIELDS)
AttributeError: type object 'DataValue' has no attribute '__dataclass_fields__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "c:\Users\melenz\Desktop\dev\opcua\async_client.py", line 19, in <module>
    loop.run_until_complete(lifebyte())
  File "C:\Users\melenz\Anaconda3\lib\asyncio\base_events.py", line 642, in run_until_complete
    return future.result()
  File "c:\Users\melenz\Desktop\dev\opcua\async_client.py", line 12, in lifebyte
    await lifeByteAck.set_value(ua.DataValue(value))
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\common\node.py", line 233, in write_value    
    await self.write_attribute(ua.AttributeIds.Value, dv)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\common\node.py", line 282, in write_attribute
    result = await self.server.write(params)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\client\ua_client.py", line 372, in write
    data = await self.protocol.send_request(request)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\client\ua_client.py", line 145, in send_request
    data = await asyncio.wait_for(self._send_request(request, timeout, message_type), timeout if timeout else None)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\client\ua_client.py", line 119, in _send_request
    binreq = struct_to_binary(request)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 301, in struct_to_binary
    return serializer(obj)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 290, in serialize
    return b''.join(
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 292, in <genexpr>
    else serializer(obj.__dict__[name])
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 290, in serialize
    return b''.join(
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 292, in <genexpr>
    else serializer(obj.__dict__[name])
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 347, in serialize
    return data_size + b''.join(type_serializer(el) for el in val)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 347, in <genexpr>
    return data_size + b''.join(type_serializer(el) for el in val)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 290, in serialize
    return b''.join(
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 292, in <genexpr>
    else serializer(obj.__dict__[name])
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 301, in struct_to_binary
    return serializer(obj)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 290, in serialize
    return b''.join(
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 292, in <genexpr>
    else serializer(obj.__dict__[name])
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 265, in <lambda>
    return lambda val: b'' if val is None else serializer(val)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 431, in variant_to_binary
    body = pack_uatype(var.VariantType, var.Value)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 188, in pack_uatype
    return create_uatype_serializer(vtype)(value)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 300, in struct_to_binary
    serializer = create_dataclass_serializer(obj.__class__)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\ua_binary.py", line 273, in create_dataclass_serializer
    data_fields = fields(dataclazz)
  File "C:\Users\melenz\Anaconda3\lib\dataclasses.py", line 1035, in fields
    raise TypeError('must be called with a dataclass type or instance')
TypeError: must be called with a dataclass type or instance

My Python version is 3.9.7, Asyncua version is 0.9.92, and OPCUA version is 0.98.13.

Any information on why this is happening?

[schroeder-]

Change your set_value to this:
await lifeByteAck.set_value(value)

[ZachM05]

I then get the following error:

Traceback (most recent call last):
  File "c:\Users\melenz\Desktop\dev\opcua\async_client.py", line 19, in <module>
    loop.run_until_complete(lifebyte())
  File "C:\Users\melenz\Anaconda3\lib\asyncio\base_events.py", line 642, in run_until_complete
    return future.result()
  File "c:\Users\melenz\Desktop\dev\opcua\async_client.py", line 12, in lifebyte
    await lifeByteAck.set_value(value)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\common\node.py", line 233, in write_value    
    await self.write_attribute(ua.AttributeIds.Value, dv)
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\common\node.py", line 283, in write_attribute
    result[0].check()
  File "C:\Users\melenz\Anaconda3\lib\site-packages\asyncua\ua\uatypes.py", line 318, in check
    raise UaStatusCodeError(self.value)
asyncua.ua.uaerrors._auto.BadWriteNotSupported: "The server does not support writing the combination of value, status and timestamps provided."(BadWriteNotSupported)

The server on the machine requires a timestamp, and for some reason when I pass just the value to the function, it does not create a correct timestamp? Not sure what the reason behind it is.

[ZachM05]

I noticed I am using the opcua version of "ua", instead of the asyncua version, I will try and determine the differences and if there is something I can do to combat the above error. Still, any help on it is appreciated.

[schroeder-]

You have to use the ua from asyncua they are fundamental different.

[schroeder-]

Some servers don't allow writing timestamps try this (ua should be from asyncua)

await lifeByteAck.set_value(ua.DataValue(value.Value))