Excellent introduction to Python with respect to bioinformatics! As someone with experience in bioinformatics, coming from perl+bioperl, I can say programming in Python was a great relief. Concise, consistent and very powerfull; I managed to write a distributed genome analysis program in weeks.
One note (also from experience); many DNA strings contain lowercase characters, eg to distinguish G from C or to denote handedited bases. The example class will fail miserably when it encounters such a character whem making a complement.
Either convert the input to all upper or lowercase or change the functions so that it can handle both variants.
If the program outputs DNA strings, which are intended to be used further, many users prefer a system that keeps the case.