issue_comments: 539463105

Thing is, the whole thing is undefined. What does the accessor state contain? As a xarray developer, I don't know. Is it variable names? Is it references to objects that make up the Dataset, e.g. Variables or the attrs dict? Is it objects whose contents rely on the current state of the Dataset, e.g. aggregated measures? Is it objects whose contents rely on historical events (like in your case)?

Dataset.copy() will create a copy of everything up to and excluding the numpy arrays. In order to allow you to retain accessor state, we'd need to plant a hook in it and invoke some agreed duck-type API in your object that basically states, "I called copy(), and this is the new object I created, please create a copy of yourself accordingly making extra sure you don't retain references to components of the previous object". And then there are all the other methods that currently nuke the accessor state - including many in-place ones - because they could potentially invalidate it. What should they do? Invoke a special API on the accessor? If not, why should copy() trigger special accessor API and e.g. roll() shouldn't? Planting accessor-refresher hooks in every single method that currently just wipes it away is out of question as it would need to be almost everywhere and - more importantly - it would be born broken.