assigning a custom mapping type to __dict__

[Steven Bethard]

I tried to Google for past discussion on this topic, but without much 
luck.  If this has been discussed before, I'd be grateful for a pointer.

Does anyone know why you can't assign a custom mapping type to an 
object's __dict__?

py> class M(object):
...     def __getitem__(self, key):
...         return 42
...     def __setitem__(self, key, value):
...         pass
...
py> class C(object):
...     pass
...
py> c = C()
py> c.__dict__ = M()
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
TypeError: __dict__ must be set to a dictionary

I looked at the source in typeobject.c (where this error originates), 
but I'm not fluent enough in CPython yet to be able to tell why a true 
dict type is preferred here over just a mapping type...

STeVe