Forbid datatree to zarr append dim (#10156)

* new blank whatsnew

* forbid trying to append a DataTree to an existing zarr store

* release note

Author: TomNicholas

doc/whats-new.rst

@@ -26,6 +26,9 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- Explicitly forbid appending a :py:class:`~xarray.DataTree` to zarr using :py:meth:`~xarray.DataTree.to_zarr` with ``append_dim``, because the expected behaviour is currently undefined.
+  (:issue:`9858`, :pull:`10156`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Deprecations
 ~~~~~~~~~~~~

xarray/core/datatree_io.py

@@ -103,6 +103,11 @@ def _datatree_to_zarr(
             "specifying a root group for the tree has not been implemented"
         )
 
+    if "append_dim" in kwargs:
+        raise NotImplementedError(
+            "specifying ``append_dim`` with ``DataTree.to_zarr`` has not been implemented"
+        )
+
     if encoding is None:
         encoding = {}