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BUG: Changed .at to not set values that do not exist yet in a DataFrame #48323 #48542

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BUG: Changed .at to not set values that do not exist yet in a DataFrame #48323 #48542

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10 changes: 3 additions & 7 deletions doc/source/user_guide/indexing.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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
----------------

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37 changes: 34 additions & 3 deletions doc/source/whatsnew/v1.6.0.rst
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Expand Up @@ -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:

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8 changes: 5 additions & 3 deletions pandas/core/indexing.py
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Expand Up @@ -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).
Expand Down Expand Up @@ -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):
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2 changes: 1 addition & 1 deletion pandas/tests/indexes/multi/test_get_set.py
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Expand Up @@ -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")

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23 changes: 16 additions & 7 deletions pandas/tests/indexing/test_at.py
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Expand Up @@ -6,8 +6,6 @@
import numpy as np
import pytest

from pandas.errors import InvalidIndexError

from pandas import (
CategoricalDtype,
CategoricalIndex,
Expand Down Expand Up @@ -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=""):
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Personally, I would put this into a separate test. One test for the happy path and one for the failing path. YMMV

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I don't want to split this because this isn't actually a test I created, but one I had to modify based on the code change, so I'd like to keep the change as small as I reasonably can.

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Well, fair enough.

Personally I am really reluctant to modify existing tests. So I would rather add a new one. YMMV

df.at[0, "a"] = 11

@pytest.mark.parametrize("row", (Timestamp("2019-01-01"), "2019-01-01"))
def test_at_datetime_index(self, row):
Expand All @@ -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)
Expand Down Expand Up @@ -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):
Expand All @@ -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):
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Maybe we should have two tests here, one for expansion in column axis and another one for expansion in index axis.

# GH#48323
frame = DataFrame({"a": [1, 2]})
with pytest.raises(KeyError, match="b"):
frame.at[2, "b"] = 9
6 changes: 0 additions & 6 deletions pandas/tests/indexing/test_partial.py
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Expand Up @@ -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):

Expand Down