Setting node name keeps tree linkage

[etienneschalk]

Small bugfix. I added a test reproducing the example in #309, and tests are not broken

Note: In _post_attach, I set the private _name instead of setting the name. Indeed, it can lead to infinite recursions when a setter is used inside of a class. I assume the _post_attach method is like a "runtime assertion"

• Closes setting node name breaks tree linkage #309
• Tests added
• Passes pre-commit run --all-files
• New functions/methods are listed in api.rst
• Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

Instead of this orphaning then un-orphaning logic, couldn't we just change the key in .children directly via some logic more like in _post_attach

[etienneschalk]

This does seem immediate to me right now (I tried some things but not successfully until now)
I still need to remove then add again the node in the parent's children at some point 🤔

In all cases I get more messy code than the current solution

__delitem__ itself uses orphan under the hood for instance

[etienneschalk]

Finally, I went for a .children = assignment. I ensured the order of children is preserved by re-creating an OrderedDict from a generator on the existing parent's children. Code is more verbose, but I think this is unavoidable to ensure the order is preserved. Indeed, orphaning then re-attaching would have lost the ordering information.

Note: _post_attach solely won't rename the node _name, so two renaming still occur:

• One on the parent's children (preserving order)
• One on the node _name itself

Edit: if the node had a parent, there is no need to manually set the ._name. Reassigning the updated list of children to its parents renames the node along the way

[etienneschalk]

I remained conservative and forbid the renaming of a node to None if it has a parent

Edit: according to a comment in the datatree design document mentioned in pydata/xarray#8747, section 4) Are nodes named? in practice, only the root node would remain anonmyous. Hence it makes sense to only authorize a node without a parent (root) to be renamed to None?

See my comment #309 (comment)

Edit: build is failing because of the new change in printing sizes of DataArrays: pydata/xarray#8702

Whether build or dev-build pass, they are mutually exclusive. What is the one that should be favoured? Current version of xarray of next version?

[marcel-goldschen-ohm]

I see I got to this pretty late, so it may be solved already (if so, please ignore this). But I couldn't quite tell from this conversation if everything was working or not. If not, I have a solution which passes all pytests (except for a format error that was not passing prior to my changes) at https://github.com/marcel-goldschen-ohm/datatree/tree/namelinkbugfix. Changes are in datatree.py in name.setter and _attach methods, and I added a test_namechange method to test_datatree.py. Again, if this is already solved, please ignore.