Support DataArray objects and nested dicts in DataTree.from_dict

[shoyer]

This PR adds three features to the DataTree.from_dict constructor:

1. It supports DataArray objects and anything that can be coerced into a DataArray via the Dataset constructor.
2. It adds a coords argument for explicitly specifying coordinates.
3. It adds support for nested dictionary values, which are automatically unflattened.

Fixes #9539, #9486

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[eni-awowale]

Thanks for adding this at @shoyer! Do you think it would be worth adding another round trip test for DataArray.from_dict and DataTree.from_dict?

Here is what I did based on DataArray.from_dict

d = {
    "coords": {
        "t": {"dims": "t", "data": [0, 1, 2], "attrs": {"units": "s"}}
    },
    "attrs": {"title": "air temperature"},
    "dims": "t",
    "data": [10, 20, 30],
    "name": "a",
}
da = xr.DataArray.from_dict(d)

dt = xr.DataTree.from_dict({'/a': da}
xr.testing.assert_identical(dt.a, da)

[eni-awowale]

Is this supposed to create a group with the name"/a"? On my end I am just seeing ('/', '/b') and the data variable with the name "a".

[shoyer]

This example was supposed to show creating a variable "a". I've replaced this with something a bit more concrete to illustrate the intended usage.

[shoyer]

Thanks for adding this at @shoyer! Do you think it would be worth adding another round trip test for DataArray.from_dict and DataTree.from_dict?

So unfortunately this doesn't work. The issue is that DataTree.from_dict works fundamentally different from Dataset.from_dict and DataArray.from_dict: #9074

• Dataset.from_dict and DataArray.from_dict parse "pure" Python dictionaries in the form you show above (e.g., {"coords": ..., "data": ..., "dims": ...})
• In contrast, DataArray.from_dict expects xarray data structures in values.

I'm not sure it makes sense to combine both in a single function. In particular, there is some ambiguity about whether dict value should be flattened (which I've added here), e.g., does {"foo": {"data": 1}} mean a DataArray at /foo/data or at /foo?

Instead, I think we should have a dedicated methods to_pure_dict()/from_pure_dict() or add pure keyword argument for controlling the output argument. Ideally we would make this consistent across DataTree/Dataset/DataArray, too. Given that pure dictionaries are relatively niche compared to this alternate constructor, I would lean towards renaming the Dataset/DataArray methods.