__delitem__ "feature"

[kj]

In <mailman.302.1293387041.6505.python-list at python.org> Ian Kelly <ian.g.kelly at gmail.com> writes:

>On 12/26/2010 10:53 AM, kj wrote:
>> P.S. If you uncomment the commented-out line, and comment out the
>> last line of the __init__ method (which installs self._delitem as
>> self.__delitem__) then *all* the deletion attempts invoke the
>> __delitem__ method, and are therefore blocked.  FWIW.

>Because subclasses of builtins only check the class __dict__ for special 
>method overrides, not the instance __dict__.


How do you know this?  Is this documented?  Or is this a case of
Monday-night quarterbacking?

~kj