• [X] Closes #7919
• [X] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

What happened?

Since #7561 with xarray-2023.5.0, the new grouper object raises an unexpected exception with an IndexVariable.

What did you expect to happen?

With xarray-2023.3.0 there was no issue, the grouper operation returned a new DataArray object.

Minimal Complete Verifiable Example

```Python import numpy as np import pandas as pd import xarray as xr

da = xr.DataArray( np.linspace(0, 1826, num=1827), coords=[pd.date_range("2000-01-01", "2004-12-31", freq="D")], dims="time", ) iv = xr.IndexVariable(dims=("time",), data=pd.Index(da.time.dt.year))

This is where the exception is raised

m = da.groupby(iv).mean() print(m) ```

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

```Python

UnboundLocalError Traceback (most recent call last) Cell In[1], line 13 10 iv = xr.IndexVariable(dims=("time",), data=pd.Index(da.time.dt.year)) 12 # This is where the exception is raised ---> 13 m = da.groupby(iv).mean() 14 print(m)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/dataarray.py:6503, in DataArray.groupby(self, group, squeeze, restore_coord_dims) 6495 from xarray.core.groupby import ( 6496 DataArrayGroupBy, 6497 ResolvedUniqueGrouper, 6498 UniqueGrouper, 6499 _validate_groupby_squeeze, 6500 ) 6502 _validate_groupby_squeeze(squeeze) -> 6503 rgrouper = ResolvedUniqueGrouper(UniqueGrouper(), group, self) 6504 return DataArrayGroupBy( 6505 self, 6506 (rgrouper,), 6507 squeeze=squeeze, 6508 restore_coord_dims=restore_coord_dims, 6509 )

File <string>:6, in init(self, grouper, group, obj)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/groupby.py:335, in ResolvedGrouper.post_init(self) 334 def post_init(self) -> None: --> 335 self.group: T_Group = _resolve_group(self.obj, self.group) 337 ( 338 self.group1d, 339 self.stacked_obj, 340 self.stacked_dim, 341 self.inserted_dims, 342 ) = _ensure_1d(group=self.group, obj=self.obj)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/groupby.py:640, in _resolve_group(obj, group) 637 else: 638 newgroup = group --> 640 if newgroup.size == 0: 641 raise ValueError(f"{newgroup.name} must not be empty") 643 return newgroup

UnboundLocalError: local variable 'newgroup' referenced before assignment ```

Anything else we need to know?

With xarray-2023.3.0 the output for the example is: <xarray.DataArray (time: 5)> array([ 182.5, 548. , 913. , 1278. , 1643.5]) Coordinates: * time (time) int64 2000 2001 2002 2003 2004

Environment

Since June 2022, setuptools_scm_git_archive is obsolete since "setuptools_scm >= 7.0.0 supports Git archives by itself".

This PR follows the migration guide to update .git_archival.txt, and establish setuptools-scm>=7 in pyproject.toml.

This PR does a few related things:

• Remove wheel from pyproject.toml, since it is no longer needed/documented (e.g. here), and rename other dependencies to their names in PyPI
• Remove pip install build dependencies already specified in pyproject.toml
• Remove check-manifest, since it isn't used
• The command python -m build has defaults to build both sdist and wheel outputs
• Use twine check --strict to return non-zero status on warnings

This PR improves CITATION.cff to add abstract, license and repository-code fields, and to modify url to https://xarray.dev/

Also, update the metadata licence from "Apache", to "Apache-2.0", which is the short identifier defined by SPDX.

This PR generally changes (e.g.) isinstance(filename, pathlib.Path) to isinstance(filename, os.PathLike), and uses os.fspath to convert it to (usually) str type.

(If it is vital these are always str, then should os.fsdecode be considered? bytes paths are not common, and only possible on some platforms).

If other path-like objects are used e.g. py.path used by the tmpdir pytest fixture, an error message is shown:

ValueError: can only read bytes or file-like objects with engine='scipy' or 'h5netcdf'

This PR allows other path-like objects to be used.

A few typing objects are also adjusted too.

Be aware there are file-like and path-like object terms used in the core Python glossary. In light of this, some "file-like" wordings may need to be adjusted, such as the error message described above. This can be done in this PR if anyone aggrees.

IOError has been an alias of OSError since Python 3.3 (see PEP 3151).