Use ChainMap to handle overriding default kwargs

[TomNicholas]

I just learned what collections.ChainMap does, and one of the examples of its use is to manage overriding default keyword arguments, e.g.

from collections import ChainMap

class CRMUser(User):
    def __init__(self, name, component, **kwargs):
        defaults = {"user_id": next(component.user_id), "role": "read"}
        super().__init__(name, **ChainMap(kwargs, defaults))

where kwargs is optional, and if provided a key in kwargs will end up overriding the corresponding default key in defaults.

We have this problem a lot in xGCM, with default kwargs that we allow the user to override. This pattern might simplify handling the options.