[BUG] fix .at for multiindexed series

Addresses: GH26989

Author: rhshadrach

doc/source/whatsnew/v1.1.0.rst

@@ -717,7 +717,7 @@ Indexing
 - Bug in :meth:`Series.__setitem__` with an :class:`IntervalIndex` and a list-like key of integers (:issue:`33473`)
 - Bug in :meth:`Series.__getitem__` allowing missing labels with ``np.ndarray``, :class:`Index`, :class:`Series` indexers but not ``list``, these now all raise ``KeyError`` (:issue:`33646`)
 - Bug in :meth:`DataFrame.truncate` and :meth:`Series.truncate` where index was assumed to be monotone increasing (:issue:`33756`)
-- Indexing with a list of strings representing datetimes failed on :class:`DatetimeIndex` or :class:`PeriodIndex`(:issue:`11278`)
+- Bug in :meth:`Series.at` when used with a :class:`MultiIndex` would raise an exception on valid inputs (:issue:`26989`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -2016,10 +2016,10 @@ def __setitem__(self, key, value):
 
         if not isinstance(key, tuple):
             key = _tuplify(self.ndim, key)
+        key = list(self._convert_key(key, is_setter=True))
         if len(key) != self.ndim:
             raise ValueError("Not enough indexers for scalar access (setting)!")
-
-        key = list(self._convert_key(key, is_setter=True))
+        
         self.obj._set_value(*key, value=value, takeable=self._takeable)
 
 
@@ -2032,6 +2032,11 @@ def _convert_key(self, key, is_setter: bool = False):
         Require they keys to be the same type as the index. (so we don't
         fallback)
         """
+        # For series, unpacking key needs to result in the label.
+        # This is already the case for len(key) == 1; e.g. (1,)
+        if isinstance(self.obj, ABCSeries) and len(key) > 1:
+            key = (key,)
+
         # allow arbitrary setting
         if is_setter:
             return list(key)

pandas/tests/indexing/test_scalar.py

@@ -351,3 +351,36 @@ def test_iat_series_with_period_index():
     expected = ser[index[0]]
     result = ser.iat[0]
     assert expected == result
+
+
+def test_tuple_indexed_series_at_get():
+    # GH 26989
+    # Series.at works with MultiIndex
+    series = Series([1, 2], index=[(1, 2), (3, 4)])
+    assert series.at[1, 2] == 1
+
+
+def test_tuple_indexed_series_at_set():
+    # GH 26989
+    # Series.at works with MultiIndex
+    series = Series([1, 2], index=[(1, 2), (3, 4)])
+    series.at[1, 2] = 3
+    assert series.at[1, 2] == 3
+
+
+def test_multiindex_series_at_get():
+    # GH 26989
+    # Series.at works with MultiIndex
+    series = Series([1, 2], index=[[1, 2], [3, 4]])
+    assert series.at[1, 3] == 1
+    assert series.loc[1, 3] == 1
+
+
+def test_multiindex_series_at_set():
+    # GH 26989
+    # Series.at works with MultiIndex
+    series = Series([1, 2], index=[[1, 2], [3, 4]])
+    series.at[1, 3] = 3
+    assert series.at[1, 3] == 3
+    series.loc[1, 3] = 4
+    assert series.loc[1, 3] == 4