State which variables not present in drop vars error message (#7518)

* test

* improve error message

* whatsnew

* order the names of missing variables before raising

* escape regex so it matches

Author: TomNicholas

doc/whats-new.rst

@@ -45,6 +45,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+
+- Improve error message when using in :py:meth:`Dataset.drop_vars` to state which variables can't be dropped. (:pull:`7518`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Require to explicitly defining optional dimensions such as hue
   and markersize for scatter plots. (:issue:`7314`, :pull:`7277`).
   By `Jimmy Westling <https://github.com/illviljan>`_.

xarray/core/dataset.py

@@ -5051,9 +5051,9 @@ def _assert_all_in_dataset(
         if virtual_okay:
             bad_names -= self.virtual_variables
         if bad_names:
+            ordered_bad_names = [name for name in names if name in bad_names]
             raise ValueError(
-                "One or more of the specified variables "
-                "cannot be found in this dataset"
+                f"These variables cannot be found in this dataset: {ordered_bad_names}"
             )
 
     def drop_vars(

xarray/tests/test_dataset.py

@@ -2503,7 +2503,12 @@ def test_drop_variables(self) -> None:
         actual = data.drop_vars(["time"])
         assert_identical(expected, actual)
 
-        with pytest.raises(ValueError, match=r"cannot be found"):
+        with pytest.raises(
+            ValueError,
+            match=re.escape(
+                "These variables cannot be found in this dataset: ['not_found_here']"
+            ),
+        ):
             data.drop_vars("not_found_here")
 
         actual = data.drop_vars("not_found_here", errors="ignore")