Backwards compatibility is addressed in several Python Enhancement Proposals (PEPs): http://www.python.org/peps/pep-0005.html http://www.python.org/peps/pep-0236.html http://www.python.org/peps/pep-0004.html http://www.python.org/peps/pep-0291.html You also may want to check related threads in c.l.py, e.g. http://tinyurl.com/d3lfx. HTH, George