> What was the rationale for this decision?  To me it looks like a hold-
> over from the time when dicts were not subclassable.
> 
> I agree, this is a topic for python-ideas rather than bug-track, but if 
> you could point me to any prior discussions on this issue, it would help 
> me to either formulate a specific proposal or drop the issue before more 
> electrons are sacrificed.

Can't find the original discussion right now, but one statement is at

http://www.mailinglistarchive.com/python-dev@python.org/msg01728.html

In essence, inheriting from dict is intentionally unspecified: if you
only change some methods, but not all, the behavior of the
non-overridden methods is unspecified. This is necessary because
a) specifying it would be a lot of work, and
b) specifying it might unreasonable restrict future implementations, and
c) specifying late binding for some of the methods would unreasonably
    slow down their implementation, even if the common case that dict is
    not subclassed.