A while ago I was chatting with two users of my Molecular Modelling Toolkit (MMTK), a library for molecular simulations written in Python. One of them asked me what I would do differently if I were to write MMTK today. That's an interesting question, but not the kind of question I can answer in a sentence or two, so I promised to write a blog post about this. Here it is. First, a bit of history.