How to create socket that pass lists of POPO (Plain Old Python Objects)? I am building custom nodes that operate on messages generated by an external source. I have developed Python classes that encapsulate these messages. I have a Blender addon that gathers a collection of these message objects and a custom node that packages multiple messages into plain old Python lists. I want pass these lists down stream to other custom modes across a socket. I have having trouble understanding List Sockets. I get "NoneType is not iterable" errors on even the simplest of implementations. I understand what the error means, but not why its being generated. Any pointers on where I should start? Also, is there any way to increase the fidelity of the diagnostic messages in Python / Blender / AN? I find that the stack traces and other information provided by these errors are not very helpful.
Thanks All,
Mike
The solution is pretty obvious, but here it is in case someone else has a similar problem. The problem was not with the socket but in the node, and the problem was that the node's execute() method must  return a collection of N items where N is the number of output sockets.