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

Author: Roline Stapny Saldanha

doc/source/whatsnew/v3.0.0.rst

@@ -787,6 +787,7 @@ 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:`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/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/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(
+    "idx,expected_match_level_a",
+    [
+        # Source index has matching name - should match level "a"
+        (Index([81, 82], name="a"), True),
+        # Source index has no name - should not match any level
+        (Index([81, 82]), False),
+        # Source index name doesn't match any level - should not match
+        (Index([81, 82], name="x"), False),
+    ],
+)
+def test_reindex_multiindex_automatic_level(idx, expected_match_level_a):
+    """
+    Test automatic level detection when reindexing from Index to MultiIndex.
+    """
+    series = Series([26.73, 24.255], index=idx)
+    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)