BUG:pandas-dev#29928 Fix to_json output with 'table' orient for single level MultiIndex.

Index field name in written json was incorrect, so applying read_json resulted in NaN index values. Dataframe to_json with 'table' orient now treats single level MultiIndex like single Index.

Author: LucasG0

doc/source/whatsnew/v1.2.0.rst

@@ -503,6 +503,7 @@ I/O
 - Bug in :class:`HDFStore` was dropping timezone information when exporting :class:`Series` with ``datetime64[ns, tz]`` dtypes with a fixed HDF5 store (:issue:`20594`)
 - :func:`read_csv` was closing user-provided binary file handles when ``engine="c"`` and an ``encoding`` was requested (:issue:`36980`)
 - Bug in :meth:`DataFrame.to_hdf` was not dropping missing rows with ``dropna=True`` (:issue:`35719`)
+- Bug in :meth:`~DataFrame.to_json` with 'table' orient was writting wrong index field name for MultiIndex Dataframe with a single level (:issue:`29928`)
 
 Plotting
 ^^^^^^^^

pandas/io/json/_json.py

@@ -259,8 +259,6 @@ def __init__(
             )
             raise ValueError(msg)
 
-        self.schema = build_table_schema(obj, index=self.index)
-
         # NotImplemented on a column MultiIndex
         if obj.ndim == 2 and isinstance(obj.columns, MultiIndex):
             raise NotImplementedError(
@@ -277,10 +275,17 @@ def __init__(
             raise ValueError(msg)
 
         obj = obj.copy()
+
+        # Convert DataFrame to handled types before serializing
+        if obj.index.nlevels == 1 and isinstance(obj.index, MultiIndex):
+            obj.index = obj.index.get_level_values(0)
+
+        self.schema = build_table_schema(obj, index=self.index)
+
         timedeltas = obj.select_dtypes(include=["timedelta"]).columns
         if len(timedeltas):
             obj[timedeltas] = obj[timedeltas].applymap(lambda x: x.isoformat())
-        # Convert PeriodIndex to datetimes before serializing
+
         if is_period_dtype(obj.index.dtype):
             obj.index = obj.index.to_timestamp()
 

pandas/tests/io/json/test_json_table_schema.py

@@ -435,6 +435,23 @@ def test_to_json_categorical_index(self):
 
         assert result == expected
 
+    @pytest.mark.parametrize("name", [None, "foo"])
+    def test_multiindex_single_level(self, name):
+        # GH29928
+        index = pd.Index([1, 2, 3, 4], name=name)
+        expected = DataFrame(
+            data=[[1, 1], [2, 2], [3, 3], [4, 4]], columns=["A", "B"], index=index
+        )
+
+        index = pd.MultiIndex.from_tuples([(1,), (2,), (3,), (4,)], names=[name])
+        df = DataFrame(
+            data=[[1, 1], [2, 2], [3, 3], [4, 4]], columns=["A", "B"], index=index
+        )
+        js = df.to_json(orient="table")
+        result = pd.read_json(js, orient="table")
+
+        tm.assert_frame_equal(result, expected)
+
     @pytest.mark.filterwarnings(
         "ignore:an integer is required (got type float)*:DeprecationWarning"
     )