ERR: Raise ValueError when non-default index is given for orc format

doc/source/whatsnew/v2.1.0.rst

@@ -174,7 +174,7 @@ MultiIndex
 
 I/O
 ^^^
--
+- :meth:`DataFrame.to_orc` now raising ``ValueError`` when non-default :class:`Index` is given (:issue:`0`)
 -
 
 Period

pandas/io/orc.py

@@ -29,6 +29,7 @@
     is_unsigned_integer_dtype,
 )
 
+from pandas import RangeIndex
 from pandas.core.arrays import ArrowExtensionArray
 from pandas.core.frame import DataFrame
 
@@ -209,6 +210,21 @@ def to_orc(
     if engine_kwargs is None:
         engine_kwargs = {}
 
+    # validate index
+    # --------------
+
+    # validate that we have only a default index
+    # raise on anything else as we don't serialize the index
+
+    if not df.index.equals(RangeIndex.from_range(range(len(df)))):
+        raise ValueError(
+            "orc does not support serializing a non-default index for the index; "
+            "you can .reset_index() to make the index into column(s)"
+        )
+
+    if df.index.name is not None:
+        raise ValueError("orc does not serialize index meta-data on a default index")
+
     # If unsupported dtypes are found raise NotImplementedError
     # In Pyarrow 8.0.0 this check will no longer be needed
     if pa_version_under8p0:

pandas/tests/io/test_orc.py

@@ -406,3 +406,21 @@ def test_orc_uri_path():
         uri = pathlib.Path(path).as_uri()
         result = read_orc(uri)
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "index",
+    [
+        pd.RangeIndex(start=2, stop=5, step=1),
+        pd.RangeIndex(start=0, stop=3, step=1, name="non-default"),
+        pd.Index([1, 2, 3]),
+    ],
+)
+def test_to_orc_non_default_index(index):
+    df = pd.DataFrame({"a": [1, 2, 3]}, index=index)
+    msg = (
+        "orc does not support serializing a non-default index|"
+        "orc does not serialize index meta-data"
+    )
+    with pytest.raises(ValueError, match=msg):
+        df.to_orc()