DataTree + Zarr-Python 3

[jhamman]

What is your issue?

In order to limit the scope of #9552, we opted to delay complete DataTree compatibility zarr-python 3. It would be nice to get this working now that zarr 3.0 is out. This issue tracks what is left to do to make this integration work:

• Go through the datatree test suite and remove any skips for zarr-python 3, e.g.

  xarray/xarray/tests/test_backends_datatree.py

  Lines 376 to 378 in 1c7ee65

  	@pytest.mark.skipif(
  	have_zarr_v3, reason="datatree support for zarr 3 is not implemented yet"
  	)

• DataTree passes path with a leading slash, despite fix/normalize storage paths zarr-developers/zarr-python#2384, this still seems to break things in zarr 3, an upstream fix may be required

xrefs:

• Zarr Python 3 tracking issue #9515
• zarr.core.group.Group does not allow to access nested groups using file path-like syntax (?) zarr-developers/zarr-python#2765
• Supporting Zarr V3 sharding with `.to_zarr` #9938
• Open multiple groups (e.g. as DataTree) with zarr-python v3 #9733
• Paths with leading slashes do bad things zarr-developers/zarr-python#2357
• Missing data variables on roundtripped Xarray.DataTree earth-mover/icechunk#624
• DataTree roundtrip fails on None group lookup #9960

cc @dcherian, @d-v-b, @maxrjones

@TomAugspurger - if you have recollections of other things that need to be fixed to make this work, please add to this list.

[TomNicholas]

Thanks - new methods in zarr-developers/zarr-python#2665 for creating multiple groups at once are also relevant.

cc @eni-awowale @aladinor

[maxrjones]

For anyone following, I think this will be fixed by #10020. Thanks for working on the issue @aladinor!

[meridionaljet]

Should #9858 be on this list also? This seems to be a barrier for datatrees with empty groups in the hierarchy (e.g., hollow trees).