[BUG] fix .at for multiindexed series

Addresses: GH26989

Author: rhshadrach

pandas/core/indexing.py

@@ -7,6 +7,7 @@
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import doc
 
+
 from pandas.core.dtypes.common import (
     is_hashable,
     is_integer,
@@ -2031,6 +2032,10 @@ def __getitem__(self, key):
                 raise ValueError("Invalid call for scalar access (getting)!")
 
         key = self._convert_key(key)
+
+        if isinstance(self.obj, ABCSeries) and len(key) > 1:
+            key = [key]
+
         return self.obj._get_value(*key, takeable=self._takeable)
 
     def __setitem__(self, key, value):
@@ -2042,6 +2047,10 @@ def __setitem__(self, key, value):
 
         if not isinstance(key, tuple):
             key = _tuplify(self.ndim, key)
+
+        if isinstance(self.obj, ABCSeries) and len(key) > 1:
+            key = [key]
+
         if len(key) != self.ndim:
             raise ValueError("Not enough indexers for scalar access (setting)!")
         key = list(self._convert_key(key, is_setter=True))

pandas/tests/series/indexing/test_indexing.py

@@ -923,3 +923,14 @@ def test_getitem_2d_no_warning():
     series = pd.Series([1, 2, 3], index=[1, 2, 3])
     with tm.assert_produces_warning(None):
         series[:, None]
+
+
+def test_multiindex_at_get():
+    series = pd.Series([1, 2], index=[[1, 2], [3, 4]])
+    assert series.at[1, 3] == 1
+
+
+def test_multiindex_at_set():
+    series = pd.Series([1, 2], index=[[1, 2], [3, 4]])
+    series.at[1, 3] = 3
+    assert series.at[1, 3] == 3