Include outside models and symbols

[shyamd]

It would be nice to have a means of making my own module outside of propnet that kept its own models and symbols, but propnet when started up, etc. could find them.

One way to do this might be ot have an environment varibale like PROPNET_MODULES that could reference places for it to look, or go the fireworks method and allow for some of configuration file that does this.

Something that might help with this pattern would be to move the Model/Symbol registry into class-registry pattern:

import abc


class ClassRegistry(abc.ABCMeta):
    def __init__(cls, name, bases, nmspc):
        super(ClassRegistry, cls).__init__(name, bases, nmspc)
        cls._instances = dict()

    def __call__(cls, *args, **kwargs):

        # First get the name
        if "name" in kwargs:
            name = kwargs["id"]
        elif len(args) < 1:
            raise TypeError("Can't get a propnet object without a name")
        else:
            id = args[0]
            args = args[1:]

        # If there isn't already an instance with this name
        # Let's make a new Model object
        if name not in cls._instances:
            new_object = super(ClassRegistry, cls).__call__(*args, **kwargs)
            cls._instances[id] = new_object

        return cls._instances[id]

class Model(metaclass=ClassRegistry):
    ...

[mkhorton]

I think Python explicitly supports this via the concept of plugins, I think this is a good idea:

https://packaging.python.org/guides/creating-and-discovering-plugins/

[mkhorton]

(and regardless of plugins, I think using a registry pattern is a good suggestion, would be more graceful than the current hard-coded default folder)