Preserve index name in reindex (#13917)

Fixes: #13900 

This PR fixes an issue with `reindex` API, where `name` of the index being reindexed upon was lost. This PR fixes it to match pandas by using the new index name if it exists or preserving the old name.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #13917

python/cudf/cudf/core/indexed_frame.py

@@ -2582,10 +2582,12 @@ def _reindex(
 
         df = self
         if index is not None:
-            index = cudf.core.index.as_index(index)
+            index = cudf.core.index.as_index(
+                index, name=getattr(index, "name", self._index.name)
+            )
 
             idx_dtype_match = (df.index.nlevels == index.nlevels) and all(
-                left_dtype == right_dtype
+                _is_same_dtype(left_dtype, right_dtype)
                 for left_dtype, right_dtype in zip(
                     (col.dtype for col in df.index._data.columns),
                     (col.dtype for col in index._data.columns),
@@ -5405,3 +5407,24 @@ def _drop_rows_by_labels(
             res = obj.to_frame(name="tmp").join(key_df, how="leftanti")["tmp"]
             res.name = obj.name
             return res
+
+
+def _is_same_dtype(lhs_dtype, rhs_dtype):
+    # Utility specific to `_reindex` to check
+    # for matching column dtype.
+    if lhs_dtype == rhs_dtype:
+        return True
+    elif (
+        is_categorical_dtype(lhs_dtype)
+        and not is_categorical_dtype(rhs_dtype)
+        and lhs_dtype.categories.dtype == rhs_dtype
+    ):
+        return True
+    elif (
+        is_categorical_dtype(rhs_dtype)
+        and not is_categorical_dtype(lhs_dtype)
+        and rhs_dtype.categories.dtype == lhs_dtype
+    ):
+        return True
+    else:
+        return False

python/cudf/cudf/tests/test_dataframe.py

@@ -10288,3 +10288,31 @@ def test_dataframe_mixed_dtype_error(dtype):
     pdf = pd.Series([1, 2, 3], dtype=dtype).to_frame().astype(object)
     with pytest.raises(TypeError):
         cudf.from_pandas(pdf)
+
+
+@pytest.mark.parametrize(
+    "index_data,name",
+    [([10, 13], "a"), ([30, 40, 20], "b"), (["ef"], "c"), ([2, 3], "Z")],
+)
+def test_dataframe_reindex_with_index_names(index_data, name):
+    gdf = cudf.DataFrame(
+        {
+            "a": [10, 12, 13],
+            "b": [20, 30, 40],
+            "c": cudf.Series(["ab", "cd", "ef"], dtype="category"),
+        }
+    )
+    if name in gdf.columns:
+        gdf = gdf.set_index(name)
+    pdf = gdf.to_pandas()
+
+    gidx = cudf.Index(index_data, name=name)
+    actual = gdf.reindex(gidx)
+    expected = pdf.reindex(gidx.to_pandas())
+
+    assert_eq(actual, expected)
+
+    actual = gdf.reindex(index_data)
+    expected = pdf.reindex(index_data)
+
+    assert_eq(actual, expected)