Which mapping methods need to be present in a Python class __dict__?

Question

From the Python 2.7 data model:

A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., C.x is translated to C.__dict__["x"] (although for new-style classes in particular there are a number of hooks which allow for other means of locating attributes).

OK, so if I do this:

class SpecialDict(object):
   ... special logic ...

class SomeClass(object):
   def _asdict(self):
     return SpecialDict(self)

   __dict__ = property(_asdict)

what mapping methods or other magic methods that mimic container types need to be present in SpecialDict? Is it just __getitem__ or are there others?

The SomeClass here is simplified from another question I had, reproduced here just so this question stands alone:

from collections import OrderedDict
from operator import itemgetter as _itemgetter

class Foo(tuple):
    __slots__ = ()
    _fields = ('foo',)
    _nargs = 1
    _repr_format = '(foo=%r)'
    foo = property(_itemgetter(0), doc='Alias for field number 0')

    def __new__(_cls, foo):
        return tuple.__new__(_cls, (foo,))
    @classmethod
    def _make(cls, iterable, new=tuple.__new__, len=len):
        'Make a new Foo object from a sequence or iterable'
        result = new(cls, iterable)
        if len(result) != cls._nargs:
            raise TypeError('Expected 1 argument, got %d' % len(result))
        return result

    def _replace(self, **kwds):
        'Return a new {typename} object replacing specified fields with new values'
        result = self._make(map(kwds.pop, self._fields, self))
        if kwds:
            raise ValueError('Got unexpected field names: %r' % kwds.keys())
        return result

    def __repr__(self):
        'Return a nicely formatted representation string'
        return self.__class__.__name__ + self._repr_format % self

    def _asdict(self):
        'Return a new OrderedDict which maps field names to their values.'
        return OrderedDict(zip(self._fields, self))

    __dict__ = property(_asdict)

    def __getstate__(self):
        'Exclude the OrderedDict from pickling'
        pass

    @property
    def antifoo(self):
        return -self.foo

class Bar(Foo):
    _fields = ('foo','bar')
    _nargs = 2
    _repr_format = '(foo=%r, bar=%r)'
    bar = property(_itemgetter(1), doc='Alias for field number 1')

    def __new__(_cls, foo, bar):
        return tuple.__new__(_cls, (foo, bar))

    @property
    def inversebar(self):
        return 1.0/self.bar