BUG: DataFrame.at with non-unique axes

doc/source/whatsnew/v1.1.0.rst

@@ -467,6 +467,7 @@ 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

@@ -2045,6 +2045,7 @@ def __setitem__(self, key, value):
             key = _tuplify(self.ndim, key)
         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)
 
@@ -2064,15 +2065,32 @@ 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 != 1 or not is_scalar(key):
-            # FIXME: is_scalar check is a kludge
-            return super().__getitem__(key)
 
-        # Like Index.get_value, but we do not allow positional fallback
-        obj = self.obj
-        loc = obj.index.get_loc(key)
-        return obj.index._get_values_for_loc(obj, loc, key)
+        if self.ndim == 2 and not self._axes_are_unique:
+            # GH#33041 fall back to .loc
+            if not isinstance(key, tuple) or not all(is_scalar(x) for x in key):
+                raise ValueError("Invalid call for scalar access (getting)!")
+            return self.obj.loc[key]
+
+        return super().__getitem__(key)
+
+    def __setitem__(self, key, value):
+        if self.ndim == 2 and not self._axes_are_unique:
+            # GH#33041 fall back to .loc
+            if not isinstance(key, tuple) or not all(is_scalar(x) for x in key):
+                raise ValueError("Invalid call for scalar access (setting)!")
+
+            self.obj.loc[key] = value
+            return
+
+        return super().__setitem__(key, value)
 
 
 @doc(IndexingMixin.iat)

pandas/tests/indexing/test_scalar.py

@@ -128,6 +128,46 @@ 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_frame_at_with_duplicate_axes_requires_scalar_lookup(self):
+        # GH#33041 check that falling back to loc doesn't allow non-scalar
+        #  args to slip in
+
+        arr = np.random.randn(6).reshape(3, 2)
+        df = DataFrame(arr, columns=["A", "A"])
+
+        msg = "Invalid call for scalar access"
+        with pytest.raises(ValueError, match=msg):
+            df.at[[1, 2]]
+        with pytest.raises(ValueError, match=msg):
+            df.at[1, ["A"]]
+        with pytest.raises(ValueError, match=msg):
+            df.at[:, "A"]
+
+        with pytest.raises(ValueError, match=msg):
+            df.at[[1, 2]] = 1
+        with pytest.raises(ValueError, match=msg):
+            df.at[1, ["A"]] = 1
+        with pytest.raises(ValueError, match=msg):
+            df.at[:, "A"] = 1
+
     def test_series_at_raises_type_error(self):
         # at should not fallback
         # GH 7814