GH4228 Clearer error on scalar to dataframe (#4533)

* GH4228 Clearer error on scalar to dataframe

* GH4228 Add change to Documentation

Author: PGijsbers

doc/whats-new.rst

@@ -54,6 +54,8 @@ Bug fixes
 Documentation
 ~~~~~~~~~~~~~
 
+- Raise a more informative error when :py:meth:`DataArray.to_dataframe` is
+  is called on a scalar (:issue:`4228`). By `Pieter Gijsbers <https://github.com/pgijsbers>`_.
 
 Internal Changes
 ~~~~~~~~~~~~~~~~

xarray/core/dataarray.py

@@ -2442,6 +2442,8 @@ def to_dataframe(
                 "cannot convert an unnamed DataArray to a "
                 "DataFrame: use the ``name`` parameter"
             )
+        if self.ndim == 0:
+            raise ValueError("cannot convert a scalar to a DataFrame")
 
         # By using a unique name, we can convert a DataArray into a DataFrame
         # even if it shares a name with one of its coordinates.

xarray/tests/test_dataarray.py

@@ -3538,6 +3538,9 @@ def test_to_dataframe(self):
         with pytest.raises(ValueError, match="does not match the set of dimensions"):
             arr.to_dataframe(dim_order=["B", "A", "C"])
 
+        with pytest.raises(ValueError, match=r"cannot convert a scalar"):
+            arr.sel(A="c", B=2).to_dataframe()
+
         arr.name = None  # unnamed
         with raises_regex(ValueError, "unnamed"):
             arr.to_dataframe()