Fix generator exhaustion in combine_by_coords (#11265)

* Fix generator exhaustion in combine_by_coords

`combine_by_coords` accepts `Iterable[Dataset | DataArray]`.

When a generator is passed, it is exhausted by the `DataTree` isinstance
check before `objs_are_unnamed_dataarrays` is assigned, causing subsequent
iterations to yield nothing and returning an empty Dataset.

Fix by materializing `data_objects` into a list.

* Add entry in whats-new.rst

Author: SurfyPenguin

doc/whats-new.rst

@@ -116,6 +116,8 @@ Bug Fixes
 - Fix :py:meth:`Dataset.interp` silently dropping datetime64 and timedelta64
   variables, through enabling their interpolation (:issue:`10900`, :pull:`11081`).
   By `Emmanuel Ferdman <https://github.com/emmanuel-ferdman>`_.
+- :func:`combine_by_coords` no longer returns an empty dataset when a generator is passed as ``data_objects`` (:issue:`10114`, :pull:`11265`).
+  By `Amartya Anand <https://github.com/SurfyPenguin>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/structure/combine.py

@@ -1074,6 +1074,8 @@ def combine_by_coords(
     Finally, if you attempt to combine a mix of unnamed DataArrays with either named
     DataArrays or Datasets, a ValueError will be raised (as this is an ambiguous operation).
     """
+    data_objects = list(data_objects)
+
     if any(isinstance(data_object, DataTree) for data_object in data_objects):
         raise NotImplementedError(
             "combine_by_coords() does not yet support DataTree objects."