Replaced isinstance check, fixed comments, added asserts in tests

Author: rhshadrach

pandas/core/indexing.py

@@ -2032,9 +2032,10 @@ 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)
         """
+        # GH 26989
         # 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:
+        if self.ndim == 1 and len(key) > 1:
             key = (key,)
 
         # allow arbitrary setting

pandas/tests/indexing/test_scalar.py

@@ -355,23 +355,26 @@ def test_iat_series_with_period_index():
 
 def test_tuple_indexed_series_at_get():
     # GH 26989
-    # Series.at works with MultiIndex
+    # Series.at works with index of tuples
     series = Series([1, 2], index=[(1, 2), (3, 4)])
+    assert series.index.nlevels == 1
     assert series.at[1, 2] == 1
 
 
 def test_tuple_indexed_series_at_set():
     # GH 26989
-    # Series.at works with MultiIndex
+    # Series.at works with index of tuples
     series = Series([1, 2], index=[(1, 2), (3, 4)])
     series.at[1, 2] = 3
+    assert series.index.nlevels == 1
     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.index.nlevels == 2
     assert series.at[1, 3] == 1
     assert series.loc[1, 3] == 1
 
@@ -380,6 +383,7 @@ def test_multiindex_series_at_set():
     # GH 26989
     # Series.at works with MultiIndex
     series = Series([1, 2], index=[[1, 2], [3, 4]])
+    assert series.index.nlevels == 2
     series.at[1, 3] = 3
     assert series.at[1, 3] == 3
     series.loc[1, 3] = 4