In python 2, you could use the gc module to get a list of every reference to a certain object. Using this, you could replace all references to that object with

In python 2, you could use the gc module to get a list of every reference to a certain object. Using this, you could replace all references to that object with references to some other object. For references occurring in immutable things like tuples, you would need to replace the immutable thing recursively. I assume this still works in python 3, but I have not tried it. https://docs.python.org/3/library/gc.html#gc.get_referrers https://docs.python.org/3/library/gc.html#gc.get_referrers While the docs say that this shouldn't be used for any purpose other than debugging, it never caused any trouble IME. Far more trouble was caused by classes from python's stdlib just blatantly not following the pickle protocol, not calling __init__, calling __init__ more than once, etc. I assume this is why people advised me to use composition rather than inheritance when making classes that were a list with extra behavior, a dict with extra behavior, and so on. Edit: Well, the above primitive is not enough on its own, but if one gets the old module and the new module and explores outwardly from those to find the subset of the object graph that differs, then one can replace all references to the old things with references to the new things. This is still not enough in general, because you'll have e.g. a bunch of objects that never had a certain member set during __init__ because __init__ was different when they were created. You'll probably meet an additional can of worms if you want to use __slots__ and add or remove class members.