Python ChainMap that treats items with None as the value the same as if the associated key was not set

nonechainmap.py

from collections import ChainMap

class NoneChainMap(ChainMap):
    """Modified ChainMap to allow items to be treated as unset if they have a specific value (e.g. None)

    (C) 2021, [email protected], BSD-3-Clause
    LICENSE: https://opensource.org/licenses/BSD-3-Clause

    Problem
    -------
    A ChainMap works great for scoping dictionaries but it has one annoying limitation. It will happily
    prioritize a None value over a "real" value. This means that if you have a value initialized to None,
    it will override other values in the scoped list of dict items

    Solution
    --------
    Override ChainMap as simply as possible to cause items with None values to be treated the same as if
    they were unset

    Example Behavior
    ----------------
    In: from collections import ChainMap
    In: class FChainMap(ChainMap):
            ... this class ...

    # Create three dictionaries, d0 is the highest priority, d2 is the lowest
    In: d0 = {'a': None, 'b': None}
    In: d1 = {'a': 'default', 'b': None}
    In: d2 = {'a': 'fallback', 'b': 'something'}

    The undesired behavior provided by ChainMap; the None overrides scope1:
      In: print(ChainMap(d0, d1, d2)['a'])
      None

    The desired behavior provided by NoneChainMap; the None doesn't override scope1:
      In: print(FChainMap(d0, d1, d2)['a'])
      default

    Implementation
    --------------
    It's very simple to override ChainMap to behave how we want

    There are two simple ways to do this:
      1. Filtering set operations, never setting the items that have a None value
      2. Filtering get operations, never returning the items that have a None value

    This uses #2 as it saves memory and it's very simple and compact. By hooking __init__()
    and __setitem__(), every possible angle is covered

    This class is still a fully functional ChainMap. The only thing that is different is how
    it treats items with a None value. If that is not the case, it's a bug, please tell me :>
    """
    def __init__(self, *maps, unset=None):
        """Create the underlying map, filtering out the `unset` value dynamically

        The value specified by the `unset` kwarg is the value that will be treated as "unset"
        By default it is None
        """
        self._unset = unset
        # This prevents the value from being during initialization
        self.maps = [{k: v for k, v in m.items() if v != unset} for m in maps]

    def __setitem__(self, key, value):
        """Prevent the `unset` value from being set after instantiation"""
        if value != self._unset:
            self.maps[0][key] = value

This is useful when you have a dict where uninitialized/unset items have a key but a None value. Rather than having to explicitly filter out the items with a None value, you can use this as a drop-in replacement for collections.ChainMap