@tmr232 @nieldk @cs @tshirtman Thanks for the responses, but my diagnosis was wrong - although I still don't quite get it: it turns out that the object I'm writing out (json.dump) contains a list that gets extended by the script (this sort of explains why I'm writing out more and more data), BUT the list is part of an object that I reinstantiate on every run, which should empty the list (I call super() with an empty list). Problem is the (super)class is generated code and I suspect the list is not in fact an object member but a static class variable that may cause this??