This isn't a bug because the MappingProxy doesn't promise to do early detection and because it can't really be accomplished cleanly.

Perfect detection of non-mappings isn't possible.  Some easy, minimal checks are made for early detection using PyMapping_Check, PyList_Check, and PyTuple_Check.  Anything that gets past those checks will just need to be duck-typed downstream when it is used.

We could add a few other specific exclusions but that isn't a general solution and it slows down instantiation.  Further it could break code that is intended to work (subclasses of str that mapping methods) or that just happen to work:

    >>> from types import MappingProxyType
    >>> mp = MappingProxyType('abc')
    >>> mp[0]
    'a'