BUG: Series.at returning Series with one element instead of scalar (#38101)

Author: phofl

doc/source/whatsnew/v1.2.0.rst

@@ -631,6 +631,7 @@ Indexing
 - Bug in :meth:`MultiIndex.drop` does not raise if labels are partially found (:issue:`37820`)
 - Bug in :meth:`DataFrame.loc` did not raise ``KeyError`` when missing combination was given with ``slice(None)`` for remaining levels (:issue:`19556`)
 - Bug in :meth:`DataFrame.loc` raising ``TypeError`` when non-integer slice was given to select values from :class:`MultiIndex` (:issue:`25165`, :issue:`24263`)
+- Bug in :meth:`Series.at` returning :class:`Series` with one element instead of scalar when index is a :class:`MultiIndex` with one level (:issue:`38053`)
 - Bug in :meth:`DataFrame.loc` returning and assigning elements in wrong order when indexer is differently ordered than the :class:`MultiIndex` to filter (:issue:`31330`, :issue:`34603`)
 - Bug in :meth:`DataFrame.loc` and :meth:`DataFrame.__getitem__`  raising ``KeyError`` when columns were :class:`MultiIndex` with only one level (:issue:`29749`)
 - Bug in :meth:`Series.__getitem__` and :meth:`DataFrame.__getitem__` raising blank ``KeyError`` without missing keys for :class:`IntervalIndex` (:issue:`27365`)

pandas/core/indexes/multi.py

@@ -2530,6 +2530,10 @@ def _get_values_for_loc(self, series: "Series", loc, key):
         if is_scalar(loc):
             return new_values
 
+        if len(new_values) == 1 and not self.nlevels > 1:
+            # If more than one level left, we can not return a scalar
+            return new_values[0]
+
         new_index = self[loc]
         new_index = maybe_droplevels(new_index, key)
         new_ser = series._constructor(new_values, index=new_index, name=series.name)

pandas/tests/indexing/test_scalar.py

@@ -268,35 +268,41 @@ def test_at_with_tuple_index_set():
     assert series.at[1, 2] == 3
 
 
-def test_multiindex_at_get():
-    # GH 26989
-    # DataFrame.at and DataFrame.loc getter works with MultiIndex
-    df = DataFrame({"a": [1, 2]}, index=[[1, 2], [3, 4]])
-    assert df.index.nlevels == 2
-    assert df.at[(1, 3), "a"] == 1
-    assert df.loc[(1, 3), "a"] == 1
-
-    # Series.at and Series.loc getter works with MultiIndex
-    series = df["a"]
-    assert series.index.nlevels == 2
-    assert series.at[1, 3] == 1
-    assert series.loc[1, 3] == 1
-
-
-def test_multiindex_at_set():
-    # GH 26989
-    # DataFrame.at and DataFrame.loc setter works with MultiIndex
-    df = DataFrame({"a": [1, 2]}, index=[[1, 2], [3, 4]])
-    assert df.index.nlevels == 2
-    df.at[(1, 3), "a"] = 3
-    assert df.at[(1, 3), "a"] == 3
-    df.loc[(1, 3), "a"] = 4
-    assert df.loc[(1, 3), "a"] == 4
-
-    # Series.at and Series.loc setter works with MultiIndex
-    series = df["a"]
-    assert series.index.nlevels == 2
-    series.at[1, 3] = 5
-    assert series.at[1, 3] == 5
-    series.loc[1, 3] = 6
-    assert series.loc[1, 3] == 6
+class TestMultiIndexScalar:
+    def test_multiindex_at_get(self):
+        # GH 26989
+        # DataFrame.at and DataFrame.loc getter works with MultiIndex
+        df = DataFrame({"a": [1, 2]}, index=[[1, 2], [3, 4]])
+        assert df.index.nlevels == 2
+        assert df.at[(1, 3), "a"] == 1
+        assert df.loc[(1, 3), "a"] == 1
+
+        # Series.at and Series.loc getter works with MultiIndex
+        series = df["a"]
+        assert series.index.nlevels == 2
+        assert series.at[1, 3] == 1
+        assert series.loc[1, 3] == 1
+
+    def test_multiindex_at_set(self):
+        # GH 26989
+        # DataFrame.at and DataFrame.loc setter works with MultiIndex
+        df = DataFrame({"a": [1, 2]}, index=[[1, 2], [3, 4]])
+        assert df.index.nlevels == 2
+        df.at[(1, 3), "a"] = 3
+        assert df.at[(1, 3), "a"] == 3
+        df.loc[(1, 3), "a"] = 4
+        assert df.loc[(1, 3), "a"] == 4
+
+        # Series.at and Series.loc setter works with MultiIndex
+        series = df["a"]
+        assert series.index.nlevels == 2
+        series.at[1, 3] = 5
+        assert series.at[1, 3] == 5
+        series.loc[1, 3] = 6
+        assert series.loc[1, 3] == 6
+
+    def test_multiindex_at_get_one_level(self):
+        # GH#38053
+        s2 = Series((0, 1), index=[[False, True]])
+        result = s2.at[False]
+        assert result == 0