BUG: Changed .at to not set values that do not exist yet in a DataFrame #48323

doc/source/user_guide/indexing.rst

@@ -851,16 +851,12 @@ You can also set using these same indexers.
 
 .. ipython:: python
 
-   df.at[dates[5], 'E'] = 7
+   df.at[dates[5], 'D'] = 7
    df.iat[3, 0] = 7
-
-``at`` may enlarge the object in-place as above if the indexer is missing.
-
-.. ipython:: python
-
-   df.at[dates[-1] + pd.Timedelta('1 day'), 0] = 7
    df
 
+``at`` will not enlarge the object in-place if the indexer is missing.
+
 Boolean indexing
 ----------------
 

doc/source/whatsnew/v1.6.0.rst

@@ -39,10 +39,41 @@ 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.at_DataFrame_expand:
 
-notable_bug_fix1
-^^^^^^^^^^^^^^^^
+Using DataFrame.at to expand DataFrame
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:meth:`DataFrame.at` would allow the addition of columns and rows to a DataFrame. (:issue:`48323`)
+
+.. code-block:: ipython
+
+    In [3]: frame = pd.DataFrame({"a": [1, 2]})
+    In [4]: frame
+    Out[4]:
+       a
+    0  1
+    1  2
+
+*Old Behavior*
+
+.. code-block:: ipython
+
+    In [5]: frame.at[2, "a"] = 7
+    In [6]: frame
+    Out[6]:
+       a
+    0  1
+    1  2
+    2  7
+
+*New Behavior*
+
+.. code-block:: ipython
+
+    In [5]: frame.at[2, "a"] = 7
+    Out[5]:
+    KeyError: 2
 
 .. _whatsnew_160.notable_bug_fixes.notable_bug_fix2:
 

pandas/core/indexing.py

@@ -551,16 +551,15 @@ def at(self) -> _AtIndexer:
         Raises
         ------
         KeyError
-            * If getting a value and 'label' does not exist in a DataFrame or
-                Series.
+            * If getting or setting a value and 'label' does not exist in a
+                DataFrame or Series.
         ValueError
             * If row/column label pair is not a tuple or if any label from
                 the pair is not a scalar for DataFrame.
             * If label is list-like (*excluding* NamedTuple) for Series.
 
         See Also
         --------
-        DataFrame.at : Access a single value for a row/column pair by label.
         DataFrame.iat : Access a single value for a row/column pair by integer
             position.
         DataFrame.loc : Access a group of rows and columns by label(s).
@@ -2429,6 +2428,9 @@ def __getitem__(self, key):
         return super().__getitem__(key)
 
     def __setitem__(self, key, value):
+        # raises exception if key does not exist
+        self.__getitem__(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):

pandas/tests/indexes/multi/test_get_set.py

@@ -415,7 +415,7 @@ def test_set_value_keeps_names():
     df = df.sort_index()
     assert df._is_copy is None
     assert df.index.names == ("Name", "Number")
-    df.at[("grethe", "4"), "one"] = 99.34
+    df.loc[("grethe", "4"), "one"] = 99.34
     assert df._is_copy is None
     assert df.index.names == ("Name", "Number")
 

pandas/tests/indexing/test_at.py

@@ -6,8 +6,6 @@
 import numpy as np
 import pytest
 
-from pandas.errors import InvalidIndexError
-
 from pandas import (
     CategoricalDtype,
     CategoricalIndex,
@@ -105,12 +103,15 @@ def test_at_setitem_multiindex(self):
             np.zeros((3, 2), dtype="int64"),
             columns=MultiIndex.from_tuples([("a", 0), ("a", 1)]),
         )
-        df.at[0, "a"] = 10
+        df.at[0, ("a", 0)] = 10
+        df.at[0, ("a", 1)] = 10
         expected = DataFrame(
             [[10, 10], [0, 0], [0, 0]],
             columns=MultiIndex.from_tuples([("a", 0), ("a", 1)]),
         )
         tm.assert_frame_equal(df, expected)
+        with pytest.raises(TypeError, match=""):
+            df.at[0, "a"] = 11
 
     @pytest.mark.parametrize("row", (Timestamp("2019-01-01"), "2019-01-01"))
     def test_at_datetime_index(self, row):
@@ -126,11 +127,13 @@ def test_at_datetime_index(self, row):
         tm.assert_frame_equal(df, expected)
 
 
-class TestAtSetItemWithExpansion:
-    def test_at_setitem_expansion_series_dt64tz_value(self, tz_naive_fixture):
+class TestAtSetTzItem:
+    def test_at_setitem_series_dt64tz_value(self, tz_naive_fixture):
         # GH#25506
+        # Modified in GH#48323 due to .at change
         ts = Timestamp("2017-08-05 00:00:00+0100", tz=tz_naive_fixture)
-        result = Series(ts)
+        ts2 = Timestamp("2017-09-05 00:00:00+0100", tz=tz_naive_fixture)
+        result = Series([ts, ts2])
         result.at[1] = ts
         expected = Series([ts, ts])
         tm.assert_series_equal(result, expected)
@@ -211,7 +214,7 @@ def test_at_frame_raises_key_error2(self, indexer_al):
     def test_at_frame_multiple_columns(self):
         # GH#48296 - at shouldn't modify multiple columns
         df = DataFrame({"a": [1, 2], "b": [3, 4]})
-        with pytest.raises(InvalidIndexError, match=r"slice\(None, None, None\)"):
+        with pytest.raises(TypeError, match="col"):
             df.at[5] = [6, 7]
 
     def test_at_getitem_mixed_index_no_fallback(self):
@@ -234,3 +237,9 @@ def test_at_categorical_integers(self):
             for key in [0, 1]:
                 with pytest.raises(KeyError, match=str(key)):
                     df.at[key, key]
+
+    def test_at_does_not_expand(self):
+        # GH#48323
+        frame = DataFrame({"a": [1, 2]})
+        with pytest.raises(KeyError, match="b"):
+            frame.at[2, "b"] = 9

pandas/tests/indexing/test_partial.py

@@ -343,19 +343,13 @@ def test_partial_setting2(self):
         df = df_orig.copy()
         df.loc[dates[-1] + dates.freq, "A"] = 7
         tm.assert_frame_equal(df, expected)
-        df = df_orig.copy()
-        df.at[dates[-1] + dates.freq, "A"] = 7
-        tm.assert_frame_equal(df, expected)
 
         exp_other = DataFrame({0: 7}, index=dates[-1:] + dates.freq)
         expected = pd.concat([df_orig, exp_other], axis=1)
 
         df = df_orig.copy()
         df.loc[dates[-1] + dates.freq, 0] = 7
         tm.assert_frame_equal(df, expected)
-        df = df_orig.copy()
-        df.at[dates[-1] + dates.freq, 0] = 7
-        tm.assert_frame_equal(df, expected)
 
     def test_partial_setting_mixed_dtype(self):