issues: 2253567622

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
2253567622 I_kwDOAMm_X86GUraG 8959 Dataset constructor always coerces 1D data variables with same name as dim to coordinates 35968931 open 0     10 2024-04-19T17:54:28Z 2024-04-28T19:57:31Z   MEMBER      

What is your issue?

Whilst xarray's data model appears to allow 1D data variables that have the same name as their dimension, it seems to be impossible to actually create this using the Dataset constructor, as they will always be converted to coordinate variables instead.

We can create a 1D data variable with the same name as it's dimension like this: ```python In [9]: ds = xr.Dataset({'x': 0})

In [10]: ds Out[10]: <xarray.Dataset> Size: 8B Dimensions: () Data variables: x int64 8B 0

In [11]: ds.expand_dims('x') Out[11]: <xarray.Dataset> Size: 8B Dimensions: (x: 1) Dimensions without coordinates: x Data variables: x (x) int64 8B 0 ``` so it seems to be a valid part of the data model.

But I can't get to that situation from the Dataset constructor. This should create the same dataset:

```python In [15]: ds = xr.Dataset(data_vars={'x': ('x', [0])})

In [16]: ds Out[16]: <xarray.Dataset> Size: 8B Dimensions: (x: 1) Coordinates: * x (x) int64 8B 0 Data variables: empty `` But actually it makesxa coordinate variable (and implicitly creates a pandas Index for it). This means that in this case there is no difference between using thedata_varsandcoords` kwargs to the constructor:

```python ds = xr.Dataset(coords={'x': ('x', [0])})

In [18]: ds Out[18]: <xarray.Dataset> Size: 8B Dimensions: (x: 1) Coordinates: * x (x) int64 8B 0 Data variables: empty ```

This all seems weird to me. I would have thought that if a 1D data variable is allowed, we shouldn't coerce to making it a coordinate variable in the constructor. If anything that's actively misleading.

Note that whilst this came up in the context of trying to avoid auto-creation of 1D indexes for coordinate variables, this issue is actually separate. (xref https://github.com/pydata/xarray/pull/8872#issuecomment-2027571714)

cc @benbovy who probably has thoughts

 
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