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I somehow missed the fact that B was a dispatcher subclass. Too early in my morning, I guess.
So, I think the title of this issue should be "asyncore.dispatcher's __repr__ not being used as fallback for __str__", and your speculation looks likely to be on target, as Giampaolo says.
Unfortunately, even if forwarding everything to the socket is questionable, it would probably be a really bad idea to change it, since there is likely code in the field depending on this behavior.
The specific case of __str__ could be fixed by adding an __str__ to dispatcher, and that would be a lot less likely to break anything other than perhaps the odd doctest here and there (but that would mean it still shouldn't be changed in 2.6).