From ef4b0f9c6ae052e96eba327353c643a845dd236c Mon Sep 17 00:00:00 2001
From: "Liang, Jingyi" <619631993@qq.com>
Date: Fri, 23 Oct 2026 22:14:48 +0800
Subject: [PATCH] Fix query on coordinate-only datasets (GH6449)

Include coordinate names in the pandas resolver scope while retaining the
Dataset as the first resolver. Extend query coverage to coordinate-only
datasets and explicit and virtual dimension coordinates.

Co-authored-by: Codex <noreply@openai.com>
---
 doc/whats-new.rst            |  3 +++
 xarray/core/dataset.py       |  7 +++++--
 xarray/tests/test_dataset.py | 25 +++++++++++++++++++++++--
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 21f3671188a..bf50d2c4ba2 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -165,6 +165,9 @@ Deprecations
 
 Bug Fixes
 ~~~~~~~~~
+- :py:meth:`Dataset.query` now resolves coordinates in datasets with no data
+  variables (:issue:`6449`).
+  By `Liang, Jingyi <https://github.com/LJimmy007>`_.
 - Results of the string accessor methods ``strip``, ``pad``, ``center``,
   ``ljust``, ``rjust``, ``zfill`` and ``replace`` on dask arrays are no longer
   truncated to a single character, and ``repeat`` no longer fails on dask arrays
diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 7976a8b5f62..2e23a5dfaab 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -10021,9 +10021,12 @@ def query(
                 msg = f"expr for dim {dim} must be a string to be evaluated, {type(expr)} given"
                 raise ValueError(msg)
 
-        # evaluate the queries to create the indexers
+        # Include coords so pandas considers the resolvers nonempty even when
+        # there are no data variables.
         indexers = {
-            dim: pd.eval(expr, resolvers=[self], parser=parser, engine=engine)
+            dim: pd.eval(
+                expr, resolvers=[self, self.coords], parser=parser, engine=engine
+            )
             for dim, expr in queries.items()
         }
 
diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index a0887c5b9cb..4daa9a06741 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -37,7 +37,7 @@
 from xarray.core.common import duck_array_ops, full_like
 from xarray.core.coordinates import Coordinates, DatasetCoordinates
 from xarray.core.indexes import Index, PandasIndex
-from xarray.core.types import ArrayLike
+from xarray.core.types import ArrayLike, QueryEngineOptions, QueryParserOptions
 from xarray.core.utils import is_scalar
 from xarray.groupers import SeasonResampler, TimeResampler
 from xarray.namedarray.pycompat import array_type, integer_types
@@ -7926,11 +7926,18 @@ def test_astype_attrs(self) -> None:
         assert not data.astype(float, keep_attrs=False).attrs
         assert not data.astype(float, keep_attrs=False).var1.attrs
 
+    @pytest.mark.parametrize("coords_only", [False, True])
     @pytest.mark.parametrize("parser", ["pandas", "python"])
     @pytest.mark.parametrize(
         "engine", ["python", None, pytest.param("numexpr", marks=[requires_numexpr])]
     )
-    def test_query(self, use_dask: bool, engine, parser) -> None:
+    def test_query(
+        self,
+        use_dask: bool,
+        engine: QueryEngineOptions,
+        parser: QueryParserOptions,
+        coords_only: bool,
+    ) -> None:
         """Test querying a dataset."""
         # setup test data
         np.random.seed(42)
@@ -7985,6 +7992,9 @@ def test_query(self, use_dask: bool, engine, parser) -> None:
                 },
             )
 
+        if coords_only:
+            ds = ds.set_coords(list(ds.data_vars))
+
         # query single dim, single variable
         with raise_if_dask_computes():
             actual = ds.query(x="a2 > 5", engine=engine, parser=parser)
@@ -8077,6 +8087,17 @@ def test_query(self, use_dask: bool, engine, parser) -> None:
             expect = ds.isel(dict(x=(a > 5), y=(c < 0.5), z=(d == "bar")))
             assert_identical(expect, actual)
 
+        # query a dimension without a coordinate
+        with raise_if_dask_computes():
+            actual = ds.query(x="x > 5", engine=engine, parser=parser)
+        assert_identical(ds.isel(x=slice(6, None)), actual)
+
+        # query a dimension coordinate
+        ds = ds.assign_coords(x=a + 10)
+        with raise_if_dask_computes():
+            actual = ds.query(x="x > 15", engine=engine, parser=parser)
+        assert_identical(ds.isel(x=slice(6, None)), actual)
+
         # test error handling
         with pytest.raises(ValueError):
             ds.query("a > 5")  # type: ignore[arg-type] # must be dict or kwargs