A simpler version of PyMC3's questionable idea

model_context.py

import sys
from typing import ClassVar, Dict, List, Type

class Model:
    _ctx: 'ClassVar[List[Model]]' = list()

    def __init__(self):
        self.nodes: 'Dict[Node]' = dict()

    def __enter__(self):
        Model._ctx.append(self)
        return self

    def __exit__(self, typ, value, traceback):
        Model._ctx.pop()

    @classmethod
    def context(cls: 'Type[Model]'):
        if not cls._ctx:
            raise ValueError('empty context stack')
        return cls._ctx[-1]

class Node:
    def __init__(self, name: str):
        self.name = name
        Model.context().nodes[self.name] = self

def main(argv: List[str]) -> int:
    with Model() as mdl:
        x = Node('x')
        y = Node('y')
        z = Node('z')
        print(mdl.nodes)

    return 0

if __name__ == '__main__':
    sys.exit(main(sys.argv))