BUG: DataFrame.at with non-unique axes

doc/source/whatsnew/v1.1.0.rst

@@ -332,6 +332,7 @@ Indexing
 - Bug in :meth:`DatetimeIndex.get_loc` raising ``KeyError`` with converted-integer key instead of the user-passed key (:issue:`31425`)
 - Bug in :meth:`Series.xs` incorrectly returning ``Timestamp`` instead of ``datetime64`` in some object-dtype cases (:issue:`31630`)
 - Bug in :meth:`DataFrame.iat` incorrectly returning ``Timestamp`` instead of ``datetime`` in some object-dtype cases (:issue:`32809`)
+- Bug in :meth:`DataFrame.at` when either columns or index is non-unique (:issue:`33041`)
 - Bug in :meth:`Series.loc` and :meth:`DataFrame.loc` when indexing with an integer key on a object-dtype :class:`Index` that is not all-integers (:issue:`31905`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` on a :class:`DataFrame` with duplicate columns incorrectly setting values for all matching columns (:issue:`15686`, :issue:`22036`)
 - Bug in :meth:`DataFrame.loc:` and :meth:`Series.loc` with a :class:`DatetimeIndex`, :class:`TimedeltaIndex`, or :class:`PeriodIndex` incorrectly allowing lookups of non-matching datetime-like dtypes (:issue:`32650`)

pandas/core/indexing.py

@@ -2063,7 +2063,17 @@ def _convert_key(self, key, is_setter: bool = False):
 
         return key
 
+    @property
+    def _axes_are_unique(self) -> bool:
+        # Only relevant for self.ndim == 2
+        assert self.ndim == 2
+        return self.obj.index.is_unique and self.obj.columns.is_unique
+
     def __getitem__(self, key):
+        if self.ndim == 2 and not self._axes_are_unique:
+            # GH#33041 fall back to .loc
+            return self.obj.loc[key]
+
         if self.ndim != 1 or not is_scalar(key):
             # FIXME: is_scalar check is a kludge
             return super().__getitem__(key)
@@ -2073,6 +2083,14 @@ def __getitem__(self, key):
         loc = obj.index.get_loc(key)
         return obj.index._get_values_for_loc(obj, loc, key)
 
+    def __setitem__(self, key, value):
+        if self.ndim == 2 and not self._axes_are_unique:
+            # GH#33041 fall back to .loc
+            self.obj.loc[key] = value
+            return
+
+        return super().__setitem__(key, value)
+
 
 @doc(IndexingMixin.iat)
 class _iAtIndexer(_ScalarAccessIndexer):

pandas/tests/indexing/test_scalar.py

@@ -128,6 +128,24 @@ def test_imethods_with_dups(self):
         result = df.iat[2, 0]
         assert result == 2
 
+    def test_frame_at_with_duplicate_axes(self):
+        # GH#33041
+        arr = np.random.randn(6).reshape(3, 2)
+        df = DataFrame(arr, columns=["A", "A"])
+
+        result = df.at[0, "A"]
+        expected = df.iloc[0]
+
+        tm.assert_series_equal(result, expected)
+
+        result = df.T.at["A", 0]
+        tm.assert_series_equal(result, expected)
+
+        # setter
+        df.at[1, "A"] = 2
+        expected = Series([2.0, 2.0], index=["A", "A"], name=1)
+        tm.assert_series_equal(df.iloc[1], expected)
+
     def test_series_at_raises_type_error(self):
         # at should not fallback
         # GH 7814