Confusing error message when reducing over non-existent dimension

[mousey92]

What happened?

When reducing a DataArray after using groupby, and reducing over a dimension that does not exist, the given error is

ValueError: cannot reduce over dimensions ['<missing dimension>']. expected either '...' to reduce over all dimensions or one or more of (<existing dimension). Alternatively, install the `flox` package.

Doing the same but without using groupby gives a much more clear error

ValueError: '<missing dimension>' not found in array dimensions (<existing dimensions>)

What did you expect to happen?

I'd expect both examples to give the error

ValueError: '<missing dimension>' not found in array dimensions (<existing dimensions>)

which is a much more clear indication of what caused the error

Minimal Complete Verifiable Example

import numpy as np
import xarray as xr
ds = xr.DataArray(np.reshape(range(27), (3, 3, 3)),
                  coords=dict(lon=range(3),
                              lat=range(3),
                              time=xr.date_range('2025-10-01 00:00', '2025-10-01 02:00', freq='h')))
ds.std(dim='lon')  # OK
ds.groupby('time').std(dim='lon')  # OK
ds.std(dim='longitude')  # ValueError: 'longitude' not found in array dimensions ('lon', 'lat', 'time')
ds.groupby('time').std(dim='longitude')  # ValueError: cannot reduce over dimensions ['longitude']. expected either '...' to reduce over all dimensions or one or more of ('lon', 'lat', 'time'). Alternatively, install the `flox` package.

Steps to reproduce

No response

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.
• [x] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.14.0 | packaged by conda-forge | (main, Oct 22 2025, 23:18:03) [MSC v.1944 64 bit (AMD64)] python-bits: 64 OS: Windows OS-release: 11 machine: AMD64 processor: Intel64 Family 6 Model 189 Stepping 1, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: ('English_Netherlands', '1252') libhdf5: None libnetcdf: None

xarray: 2025.10.1 pandas: 2.3.3 numpy: 2.3.4 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None zarr: None cftime: None nc_time_axis: None iris: None bottleneck: None dask: None distributed: None matplotlib: None cartopy: None seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None flox: None numpy_groupies: None setuptools: None pip: 25.2 conda: None pytest: None mypy: None IPython: None sphinx: None

[welcome[bot]]

Thanks for opening your first issue here at xarray! Be sure to follow the issue template! If you have an idea for a solution, we would really welcome a Pull Request with proposed changes. See the Contributing Guide for more. It may take us a while to respond here, but we really value your contribution. Contributors like you help make xarray better. Thank you!

[dcherian]

The error is here: https://github.com/pydata/xarray/blob/e49cfc4f2009eb660411128eaf05fa3e714a0c83/xarray/core/groupby.py#L78

and is confusing because we check against a possibly stacked object https://github.com/pydata/xarray/blob/e49cfc4f2009eb660411128eaf05fa3e714a0c83/xarray/core/groupby.py#L679-L681

It may be simplest to reword the error message