I believe that delegating to the actual underlying mapping without exposing it is a bit of a lost cause, since for some type m we still need these to work:

>>> types.MappingProxyType(m({"a": 0)) | types.MappingProxyType(m({"b": 1}))
m({'a': 0, 'b': 1}) 
>>> types.MappingProxyType(m({"a": 0)) == types.MappingProxyType(m({"a": 0}))
True

(Note that because both sides are proxies, it's impossible for any resolution to happen without m explicitly knowing how to handle them unless both mappings are unwrapped simultaneously.)

Instead, the attached PR delegates to a *copy* of the underlying mapping for these operations instead. I think this is the easiest way to approximate the current behavior while maintaining proper encapsulation.