BUG: Fix Series.reindex losing values when reindexing to MultiIndex

doc/source/whatsnew/v3.0.0.rst

@@ -788,6 +788,8 @@ 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/core/series.py

@@ -4886,6 +4886,15 @@ def reindex(  # type: ignore[override]
         limit: int | None = None,
         tolerance=None,
     ) -> Series:
+        # 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
+        if (
+            level is None
+            and isinstance(index, MultiIndex)
+            and self.index.name in index.names
+        ):
+            level = self.index.name
         return super().reindex(
             index=index,
             method=method,

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

@@ -434,3 +434,49 @@ def test_reindex_expand_nonnano_nat(dtype):
         np.array([1, getattr(np, dtype)("nat", "s")], dtype=f"{dtype}[s]")
     )
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "name, expected_match_level_a",
+    [
+        # Source index has matching name - should match level "a"
+        ("a", True),
+        # Source index has no name - should not match any level
+        (None, False),
+        # Source index name doesn't match any level - should not match
+        ("x", False),
+    ],
+)
+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=Index([81, 82], name=name))
+    target = MultiIndex.from_product(
+        [[81, 82], [np.nan], ["2018-06-01", "2018-07-01"]], names=["a", "b", "c"]
+    )
+
+    result = series.reindex(target)
+
+    if expected_match_level_a:
+        # Should match behavior of explicit level="a"
+        expected = series.reindex(target, level="a")
+    else:
+        # Should contain all NaN values
+        expected = Series(np.nan, index=target, dtype=series.dtype)
+
+    tm.assert_series_equal(result, expected)
+
+
+def test_reindex_multiindex_explicit_level_overrides():
+    """
+    Test that explicit level parameter overrides automatic detection.
+    """
+    series = Series([26.73, 24.255], index=Index([81, 82], name="a"))
+    target = MultiIndex.from_product(
+        [[81, 82], [np.nan], ["2018-06-01", "2018-07-01"]], names=["a", "b", "c"]
+    )
+
+    result = series.reindex(target, level=0)
+    expected = series.reindex(target, level="a")
+    tm.assert_series_equal(result, expected)