support scalar cube to pandas dataframe (#6567)

* support scalar cube to pandas dataframe

* add whatsnew entry

* add comments

Author: bjlittle

docs/src/whatsnew/latest.rst

@@ -90,12 +90,16 @@ This document explains the changes made to Iris for this release
 #. `@stephenworsley`_ fixed a bug which caused :meth:`~iris.cube.CubeList.concatenate_cube`
    to fail when concatenating over multiple axes. (:pull:`6533`)
 
+#. `@bjlittle`_ fixed :func:`~iris.pandas.as_data_frame` to correctly convert a
+   scalar cube to a :class:`pandas.DataFrame`. (:issue:`6419`, :pull:`6567`)
+
 
 💣 Incompatible Changes
 =======================
 
 #. N/A
 
+
 🚀 Performance Enhancements
 ===========================
 
@@ -130,6 +134,7 @@ This document explains the changes made to Iris for this release
 #. `@tkknight`_ updated image to ensure it renders correctly using various web browsers
    on Windows and Linux. (:pull:`6560`)
 
+
 💼 Internal
 ===========
 

lib/iris/pandas.py

@@ -28,6 +28,7 @@
 from iris._deprecation import warn_deprecated
 from iris.coords import AncillaryVariable, AuxCoord, CellMeasure, DimCoord
 from iris.cube import Cube, CubeList
+from iris.util import new_axis
 from iris.warnings import IrisIgnoringWarning
 
 
@@ -827,6 +828,10 @@ def merge_metadata(meta_var_list):
                 )
         return data_frame
 
+    if getattr(cube, "ndim", None) is not None and (is_scalar := cube.ndim == 0):
+        # promote the scalar cube to a 1D cube, and convert in the same way as a 1D cube
+        cube = new_axis(cube)
+
     if iris.FUTURE.pandas_ndim:
         # Checks
         if not isinstance(cube, iris.cube.Cube):
@@ -893,4 +898,12 @@ def merge_metadata(meta_var_list):
 
         result = data_frame
 
+    if is_scalar:
+        # clear the promoted dimension name associated with any
+        # indices of the dataframe
+        if isinstance(result.index, pd.MultiIndex):
+            result.index.names = [None] * result.index.nlevels
+        else:
+            result.index.name = None
+
     return result

lib/iris/tests/unit/pandas/test_pandas.py

@@ -164,6 +164,16 @@ def test_copy_masked_false(self):
 class TestAsDataFrame(tests.IrisTest):
     """Test conversion of 2D cubes to Pandas using as_data_frame()."""
 
+    def test_scalar_cube(self):
+        cube = Cube(1.0)
+        data_frame = iris.pandas.as_data_frame(cube)
+        expected_index = [0]
+        expected_columns = [0]
+        self.assertArrayEqual(data_frame, cube.data)
+        self.assertArrayEqual(data_frame.index, expected_index)
+        self.assertArrayEqual(data_frame.columns, expected_columns)
+        self.assertIsNone(data_frame.index.name)
+
     def test_no_dim_coords(self):
         cube = Cube(np.array([[0, 1, 2, 3, 4], [5, 6, 7, 8, 9]]), long_name="foo")
         expected_index = [0, 1]
@@ -323,6 +333,15 @@ class TestAsDataFrameNDim(tests.IrisTest):
     def _activate_pandas_ndim(self, activate_pandas_ndim):
         pass
 
+    def test_scalar_cube(self):
+        cube = Cube(1.0, long_name="foo")
+        data_frame = iris.pandas.as_data_frame(cube)
+        expected_index = [0]
+        self.assertArrayEqual(data_frame.foo.values, cube.data)
+        self.assertEqual(data_frame.index.nlevels, 1)
+        self.assertIsNone(data_frame.index.names[0])
+        self.assertArrayEqual(data_frame.index.levels[0], expected_index)
+
     def test_no_dim_coords(self):
         cube = Cube(np.array([[0, 1, 2, 3, 4], [5, 6, 7, 8, 9]]), long_name="foo")
         expected_dim0 = np.repeat([0, 1], 5)