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I am not at all unwilling (and this isn't a problem for me
personally - the issue here is whether this is good for
Python in general) to subclass.  Deciding to subclass does
*not* mean that you should have to replace all functions in
the parent class.  If you did, then there would be little
point in subclassing at all.  Sensible default behaviour
should be provided as methods of classes.  The current
behaviour of the handle_expt() method is not sensible.  It
essentially forces the user to override that method, even if
they have no interest in handling errors (e.g. and would
normally just override handle_read and handle_write).

This is *not* rare.  You haven't seen any in years, because
this was a change introduced in Python 2.4, which hasn't
been released for even one year yet.  I agree that the
desired behaviour will be application specific.  But what is
the point of having default behaviour that will essentially
crash the program/system running it?  Having default
behaviour be "pass" would be more useful.  At the very
least, this is a problem that many people (compared to the
number that will use asyncore) will come across and should
be reflected as such in the documentation.

If you haven't replicated this problem on your system so
that you understand it, please do.  I am happy to provide a
simple script to demonstrate, if necessary.