> It seems to me that we would need this restriction to make a reasonably
> universal frozendict that is, itself, hashable.

When people talk about frozendicts being hashable, they mean it in the
same sense that tuples are hashable.


For what it is worth, here's an incomplete, quick and dirty proof of
concept frozendict, using automatic delegation:

# Not good enough for production, not tested, buyer beware, etc.
class frozendict:
    def __new__(cls, *args, **kwargs):
        d = dict(*args, **kwargs)
        proxy = types.MappingProxyType(d)
        instance = super().__new__(cls)
        instance.__proxy = proxy
        return instance
    def __hash__(self):
        return hash(tuple(self.items()))
    def __getattr__(self, name):
        return getattr(self.__proxy, name)
    def __getitem__(self, key):
        return self.__proxy[key]
    def __repr__(self):
        return "%s(%r)" % (type(self).__name__, dict(self))