One other thing I am now trying is to take the task_instance object and 
call SerializeToString() on it, then use 
task_info_instance.MergeFromString(serialized_task_instance)

this is not raising any exception and does seem to be setting the field, 
but I'm not entirely convinced yet

On Friday, March 3, 2017 at 1:09:09 PM UTC-8, Tom Lichtenberg wrote:
>
> I’m trying to dynamically create objects from discovery of pb2 generated 
> protobuf files and assign appropriate values to their fields. If the field 
> types are simple, this is straightforward - e.g. for TYPE_BOOL, 
> setattr(obj, “field_name”, True) works fine - but if the field is 
> TYPE_MESSAGE, making an instance of that field and using setattr(obj, 
> “field_name”, instance) results in an error:
>       Assignment not allowed to composite field "task" in protocol message 
> object.
> from python2.7/site-packages/google/protobuf/internal/python_message.py", 
> line 736, in setter
>
>
> sample code: dummy/test_dummy.proto
> ============================
>
> syntax = "proto3";
>
> message task {
>    int32 id = 1;
>    string msg = 2;
> }
>
> message task_info {
>    task task = 1;
> }
>
>
> from this I used protoc to generate a test_dummy_pb2.py
>
> For example, I want to create a task_info object, so by exploring the 
> imported pb2 module, I find the task_info descriptor and create an instance 
> of it. I see that its field ‘task’ is of TYPE_MESSAGE so I create an 
> instance of that as well and call setattr(task_instance, “id”, 1) and 
> setattr(task_instance, “msg”, “some message”) to populate it, and then go 
> back and call setattr(task_info_instance, “task”, task_instance). Here I 
> get the error:  Assignment not allowed to composite field "task" in 
> protocol message object.
>
>
> programatically I can import this module and assign values well enough, 
> for example in the python shell:
> >> import importlib
> >> oobj = importlib.import_module("dummy.dummy.test_dummy_pb2", 
> package=None)
> >> obj = getattr(oobj, "task_info")
> >> task_info_instance = obj()
> >> task_info_instance.task.id = 1
> >> task_info_instance.task.msg = "some message"
> >> print task_info_instance
>
> task {
>   id: 1
>   msg: "some message"
> }
>   
>
> But dynamically I can’t explicitly assign it like that. I would need 
> something like a setattr() function that can set an object’s composite 
> object field. I’m hoping that someone can shed some light on this for me.
>

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