Fix 48224: DataFrame.at must fail when adding more than one value

The problem occurred because when adding a new column to a DataFrame using
DataFrame.at, DataFrame._set_value falls back to DataFrame.loc before the
InvalidIndexError is raised.

The proposed solution makes sure that both, the column index and the row index
are determined so that both have a chance to raise.

Author: johannes-mueller

doc/source/whatsnew/v1.6.0.rst

@@ -39,10 +39,44 @@ Notable bug fixes
 
 These are bug fixes that might have notable behavior changes.
 
-.. _whatsnew_160.notable_bug_fixes.notable_bug_fix1:
+.. _whatsnew_160.notable_bug_fixes.notable_bug_frame_at_new_column_with_slice:
+
+Using :meth:`DataFrame.at` raises when trying to add a new column with multiple values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+*Old behavior*
+
+.. code-block:: ipython
+    In [1]: import pandas as pd
+
+    In [2]: df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+    In [3]: # should raise InvalidIndexError
+            df.at[slice(0, 1, None), 'c'] = 7
+
+    In [4]: df
+    Out[4]:
+       a  b    c
+    0  1  4  7.0
+    1  2  5  7.0
+    2  3  6  NaN
+
+
+*New behavior*
+
+.. code-block:: ipython
+
+    In [1]: import pandas as pd
+
+    In [2]: df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+    In [3]: df.at[slice(0, 1, None), 'c'] = 7
+
+    ---------------------------------------------------------------------------
+    InvalidIndexError                         Traceback (most recent call last)
+    <ipython-input-3-a89a95224c2b> in <module>
+    ----> 1 df.at[slice(0, 1, None), 'c'] = 7
 
-notable_bug_fix1
-^^^^^^^^^^^^^^^^
 
 .. _whatsnew_160.notable_bug_fixes.notable_bug_fix2:
 

pandas/core/frame.py

@@ -4206,8 +4206,20 @@ def _set_value(
                 icol = col
                 iindex = cast(int, index)
             else:
-                icol = self.columns.get_loc(col)
-                iindex = self.index.get_loc(index)
+                exception = None
+                try:
+                    icol = self.columns.get_loc(col)
+                except (KeyError, TypeError, ValueError) as e:
+                    exception = e
+
+                try:
+                    iindex = self.index.get_loc(index)
+                except (KeyError, TypeError, ValueError) as e:
+                    exception = e
+
+                if exception is not None:
+                    raise exception
+
             self._mgr.column_setitem(icol, iindex, value)
             self._clear_item_cache()
 

pandas/tests/indexing/test_at.py

@@ -214,6 +214,13 @@ def test_at_frame_multiple_columns(self):
         with pytest.raises(InvalidIndexError, match=r"slice\(None, None, None\)"):
             df.at[5] = [6, 7]
 
+    def test_at_frame_new_column_when_index_is_slice(self):
+        # GH-48224
+        df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+        with pytest.raises(InvalidIndexError, match=r"slice\(0, 1, None\)"):
+            df.at[slice(0, 1, None), "c"] = 7
+
     def test_at_getitem_mixed_index_no_fallback(self):
         # GH#19860
         ser = Series([1, 2, 3, 4, 5], index=["a", "b", "c", 1, 2])