Improve error message on ds['x', 'y'] (#9375)

* Improve error message on `ds['x', 'y']`

While trying to help with #9372, I realize the error message for this could be much better, and so putting this PR in as some penance for my tardiness in helping there

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: max-sixty

xarray/core/dataset.py

@@ -1576,9 +1576,11 @@ def __getitem__(
             try:
                 return self._construct_dataarray(key)
             except KeyError as e:
-                raise KeyError(
-                    f"No variable named {key!r}. Variables on the dataset include {shorten_list_repr(list(self.variables.keys()), max_items=10)}"
-                ) from e
+                message = f"No variable named {key!r}. Variables on the dataset include {shorten_list_repr(list(self.variables.keys()), max_items=10)}"
+                # If someone attempts `ds['foo' , 'bar']` instead of `ds[['foo', 'bar']]`
+                if isinstance(key, tuple):
+                    message += f"\nHint: use a list to select multiple variables, for example `ds[{[d for d in key]}]`"
+                raise KeyError(message) from e
 
         if utils.iterable_of_hashable(key):
             return self._copy_listed(key)

xarray/tests/test_dataset.py

@@ -4134,6 +4134,11 @@ def test_getitem(self) -> None:
             data["notfound"]
         with pytest.raises(KeyError):
             data[["var1", "notfound"]]
+        with pytest.raises(
+            KeyError,
+            match=r"Hint: use a list to select multiple variables, for example `ds\[\['var1', 'var2'\]\]`",
+        ):
+            data["var1", "var2"]
 
         actual1 = data[["var1", "var2"]]
         expected1 = Dataset({"var1": data["var1"], "var2": data["var2"]})