I did not understand persistent until someone point me to the wikipedia article [1]: > In computing, a persistent data structure is a data structure that alway

I did not understand persistent until someone point me to the wikipedia article [1]: > In computing, a persistent data structure is a data structure that always preserves the previous version of itself when it is modified. Such data structures are effectively immutable, as their operations do not (visibly) update the structure in-place, but instead always yield a new updated structure. The way I understand it in terms of python: It's like a nametuple or dict, where each update return a new namedtuple/dict with the new values for modified attributes but still the old values for attributes that did not change. It doesn't need to deep copy all the datastructure/database. This is not persistent as in "stored on disk". Being append only is a way to achieve persistence except it is also applicable to non-stored-on-disk datastructures. [1] https://en.wikipedia.org/wiki/Persistent_data_structure https://en.wikipedia.org/wiki/Persistent_data_structure