BUG: Fix dataframe.reindex to detect matching level when source has single Index

Author: Roline Stapny Saldanha

doc/source/whatsnew/v3.0.0.rst

@@ -788,6 +788,7 @@ MultiIndex
 - Bug in :class:`DataFrame` arithmetic operations in case of unaligned MultiIndex columns (:issue:`60498`)
 - Bug in :class:`DataFrame` arithmetic operations with :class:`Series` in case of unaligned MultiIndex (:issue:`61009`)
 - Bug in :meth:`MultiIndex.from_tuples` causing wrong output with input of type tuples having NaN values (:issue:`60695`, :issue:`60988`)
+- Bug in :meth:`DataFrame.reindex` where reindexing a Dataframe with a named Index to a MultiIndex would incorrectly set all values to ``NaN``. Now correctly preserves values when the source index name matches a target level name (:issue:`60923`)
 - Bug in :meth:`Series.reindex` where reindexing a Series with a named Index to a MultiIndex would incorrectly set all values to ``NaN``. Now correctly preserves values when the source index name matches a target level name (:issue:`60923`)
 
 I/O

pandas/core/frame.py

@@ -5355,6 +5355,18 @@ def reindex(
         limit: int | None = None,
         tolerance=None,
     ) -> DataFrame:
+        # Automatically detect matching level when reindexing from Index to MultiIndex.
+        # This prevents values from being incorrectly set to NaN when the source index
+        # name matches a level name in the target MultiIndex. Only applies when source
+        # is not already a MultiIndex.
+        if (
+            level is None
+            and index is not None
+            and isinstance(index, MultiIndex)
+            and not isinstance(self.index, MultiIndex)
+            and self.index.name in index.names
+        ):
+            level = self.index.name
         return super().reindex(
             labels=labels,
             index=index,

pandas/tests/frame/methods/test_reindex.py

@@ -1258,3 +1258,55 @@ def test_invalid_method(self):
         msg = "Invalid fill method"
         with pytest.raises(ValueError, match=msg):
             df.reindex([1, 0, 2], method="asfreq")
+
+    def test_reindex_index_name_matches_multiindex_level(self):
+        """
+        Test automatic level detection when reindexing from Index to MultiIndex.
+        When source index name matches a level name in target MultiIndex and level
+        is not specified, should behave same as if level was explicitly set.
+        """
+        # Create source DataFrame with named Index
+        df = DataFrame(
+            {"value": [1, 2], "other": ["A", "B"]},
+            index=Index([10, 20], name="a"),
+        )
+
+        # Create target MultiIndex with matching level name
+        target = MultiIndex.from_product(
+            [[10, 20], ["x", "y"]],
+            names=["a", "b"],  # 'a' matches source index name
+        )
+
+        result = df.reindex(index=target)
+        expected = df.reindex(index=target, level="a")
+
+        tm.assert_frame_equal(result, expected)
+
+        # Verify values are propagated correctly
+        expected_values = {
+            (10, "x"): {"value": 1, "other": "A"},
+            (10, "y"): {"value": 1, "other": "A"},
+            (20, "x"): {"value": 2, "other": "B"},
+            (20, "y"): {"value": 2, "other": "B"},
+        }
+        for idx, expected_row in expected_values.items():
+            for col, val in expected_row.items():
+                assert result.loc[idx, col] == val
+
+    def test_reindex_index_name_no_match_multiindex_level(self):
+        """
+        Test reindexing behavior when source index name doesn't match any level
+        in target MultiIndex. Should fill with NaN since there's no level match.
+        """
+        df = DataFrame({"value": [1, 2]}, index=Index([10, 20], name="different_name"))
+
+        target = MultiIndex.from_product([[10, 20], ["x", "y"]], names=["a", "b"])
+
+        result = df.reindex(index=target)
+
+        # Should fill with NaN since no level match
+        assert result.isna().all().all()
+
+        # Verify shape is correct
+        assert result.index.equals(target)
+        assert result.columns.equals(df.columns)

pandas/tests/series/methods/test_reindex.py

@@ -437,21 +437,21 @@ def test_reindex_expand_nonnano_nat(dtype):
 
 
 @pytest.mark.parametrize(
-    "idx,expected_match_level_a",
+    "name, expected_match_level_a",
     [
         # Source index has matching name - should match level "a"
-        (Index([81, 82], name="a"), True),
+        ("a", True),
         # Source index has no name - should not match any level
-        (Index([81, 82]), False),
+        (None, False),
         # Source index name doesn't match any level - should not match
-        (Index([81, 82], name="x"), False),
+        ("x", False),
     ],
 )
-def test_reindex_multiindex_automatic_level(idx, expected_match_level_a):
+def test_reindex_multiindex_automatic_level(name, expected_match_level_a):
     """
     Test automatic level detection when reindexing from Index to MultiIndex.
     """
-    series = Series([26.73, 24.255], index=idx)
+    series = Series([26.73, 24.255], index=Index([81, 82], name=name))
     target = MultiIndex.from_product(
         [[81, 82], [np.nan], ["2018-06-01", "2018-07-01"]], names=["a", "b", "c"]
     )