Dataset.__setitem__ raise on being passed a Dataset (for single key)

doc/whats-new.rst

@@ -80,6 +80,9 @@ Documentation
 
 - Users are instructed to try ``use_cftime=True`` if a ``TypeError`` occurs when combining datasets and one of the types involved is a subclass of ``cftime.datetime`` (:pull:`5776`).
   By `Zeb Nicholls <https://github.com/znicholls>`_.
+- A clearer error is now raised if a user attempts to assign a Dataset to a single key of
+  another Dataset. (:pull:`5839`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Internal Changes
 ~~~~~~~~~~~~~~~~

xarray/core/dataset.py

@@ -1557,6 +1557,11 @@ def __setitem__(self, key: Union[Hashable, List[Hashable], Mapping], value) -> N
                     self.update(dict(zip(key, value)))
 
         else:
+            if isinstance(value, Dataset):
+                raise TypeError(
+                    "Cannot assign a Dataset to a single key - only a DataArray or Variable object can be stored under"
+                    "a single key."
+                )
             self.update({key: value})
 
     def _setitem_check(self, key, value):

xarray/tests/test_dataset.py

@@ -3395,6 +3395,9 @@ def test_setitem(self):
         # override an existing value
         data1["A"] = 3 * data2["A"]
         assert_equal(data1["A"], 3 * data2["A"])
+        # can't assign a dataset to a single key
+        with pytest.raises(TypeError, match="Cannot assign a Dataset to a single key"):
+            data1["D"] = xr.Dataset()
 
         # test assignment with positional and label-based indexing
         data3 = data1[["var1", "var2"]]