A PEP for a mutable bytes type was composed but withdrawn two years
ago:
http://www.python.org/peps/pep-0296.html

Another was composed (status unclear) in 2004:
http://www.python.org/peps/pep-0332.html

It was mentioned on python-dev in 2004
(http://python.fyxm.net/dev/summary/2...004-08-31.html) and
2005 (http://www.python.org/dev/summary/20...005-10-15.html)
with the latter discussion mentioning that is a likely thing for 3k.

Does anyone know if a well-coded and tested implementation of such a
beast exists now? Strings make me sad.

-Mike