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The documentation (at least the obvious places, see Doc/reference/datamodel.rst) says classes and class instances have the '__dict__' attribute, but nothing is said about what happens when assigning to it (like obj.__dict__ = something).
As far as I understand that's undefined behavior and other implementations may not work the same as CPython (and CPython itself behaves differently between versions).
I'd submit a documentation patch if I knew how to specify this matter. Maybe just say the behavior is not defined?

What kind of behavior are you referring to?
Are there really differences between versions?

Amaury, I don't honestly know, I would have proposed something otherwise.
I have been advised on #python (Freenode) not to assign to obj.__dict__ because its behaviour changes between versions and implementations, but I wouldn't know what has changed between CPython version.
One obscure thing is what can be assigned to __dict__. For class dicts no assignment is allowed. Object dicts can be assigned dict objects. If you try to assign a non-dict mapping it will complain.
However you can assign a dict-derived object, something like:
class SillyDict(dict):
 def __getitem__(self, key):
 return "hello"
obj.__dict__ = SillyDict()
Unfortunately accessing the attributes of obj will still use PyDict_* functions, thus ignoring any magic methods.
PyPy's behavior is slightly different. A class __dict__ attribute still is read only, but you can assign any mapping object to an object __dict__.

I think assigning to __dict__ is an implementation detail; we shouldn't document it before it is clearly agreed what it should do (which probably deserves asking on python-dev).