What happened?

I tried to reduce dataset using scipy.stats.skew, but it raised an error.

What did you expect to happen?

I expected it to succed, especially because the same operation with dataarray succeeds.

Minimal Complete Verifiable Example

```Python

dataset = xr.Dataset( ... { ... "math_scores": ( ... ["student", "test"], ... [[90, 85, 92], [78, 80, 85], [95, 92, 98]], ... ), ... "english_scores": ( ... ["student", "test"], ... [[88, 90, 92], [75, 82, 79], [93, 96, 91]], ... ), ... }, ... coords={ ... "student": ["Alice", "Bob", "Charlie"], ... "test": ["Test 1", "Test 2", "Test 3"], ... }, ... )

from scipy.stats import skew

Calling DataArray.reduce on each variable individually succeeds

In [14]: dataset['math_scores'].reduce(skew, dim=["test", 'student']) Out[14]: <xarray.DataArray 'math_scores' ()> array(-0.19423043)

In [15]: dataset['english_scores'].reduce(skew, dim=["test", 'student']) Out[15]: <xarray.DataArray 'english_scores' ()> array(-0.60125)

But calling Dataset.reduce fails

dataset.reduce(skew, dim=["test", 'student'])

---

ValueError Traceback (most recent call last) Cell In[16], line 1 ----> 1 dataset.reduce(skew, dim=["test", 'student'])

File ~/Documents/Work/Code/xarray/xarray/core/dataset.py:5972, in Dataset.reduce(self, func, dim, keep_attrs, keepdims, numeric_only, kwargs) 5955 if ( 5956 # Some reduction functions (e.g. std, var) need to run on variables 5957 # that don't have the reduce dims: PR5393 (...) 5965 # the former is often more efficient 5966 # keep single-element dims as list, to support Hashables 5967 reduce_maybe_single = ( 5968 None 5969 if len(reduce_dims) == var.ndim and var.ndim != 1 5970 else reduce_dims 5971 ) -> 5972 variables[name] = var.reduce( 5973 func, 5974 dim=reduce_maybe_single, 5975 keep_attrs=keep_attrs, 5976 keepdims=keepdims, 5977 kwargs, 5978 ) 5980 coord_names = {k for k in self.coords if k in variables} 5981 indexes = {k: v for k, v in self._indexes.items() if k in variables}

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:2045, in Variable.reduce(self, func, dim, axis, keep_attrs, keepdims, **kwargs) 2042 keep_attrs = _get_keep_attrs(default=False) 2043 attrs = self._attrs if keep_attrs else None -> 2045 return Variable(dims, data, attrs=attrs)

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:367, in Variable.init(self, dims, data, attrs, encoding, fastpath) 347 """ 348 Parameters 349 ---------- (...) 364 unrecognized encoding items. 365 """ 366 self._data = as_compatible_data(data, fastpath=fastpath) --> 367 self._dims = self._parse_dimensions(dims) 368 self._attrs = None 369 self._encoding = None

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:683, in Variable._parse_dimensions(self, dims) 681 dims = tuple(dims) 682 if len(dims) != self.ndim: --> 683 raise ValueError( 684 f"dimensions {dims} must have the same length as the " 685 f"number of data dimensions, ndim={self.ndim}" 686 ) 687 return dims

ValueError: dimensions () must have the same length as the number of data dimensions, ndim=1 ```

MVCE confirmation

• [x] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [x] Complete example — the example is self-contained, including all data and the text of any traceback.
• [x] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

xarray main branch