BUG: setting item to iterable with .at fails when column doesn't exist or has wrong dtype

[jbogar]

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• I have confirmed this bug exists on the latest version of pandas.

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Reproducible Example

import pandas as pd

df=pd.DataFrame(data=[[1,2],[3,4]],index=["a","b"],columns=["A","B"])
df.at["a","C"]=[1,2,3]

Issue Description

When using .at to set cell value to an iterable, it fails if it has to create a new column.

It works fine if setting the cell value to a scalar (like df.at["a","C"]=1).

It also fails if the column exists but is of the wrong dtype.

This fails:
df.at["a","A"]=[1,2,3]

But this works:

df.loc[:,"A"]=df.A.astype(object)
df.at["a","A"]=[1,2,3]

The error trace:

KeyError                                  Traceback (most recent call last)
File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexes/base.py:3805, in Index.get_loc(self, key)
   3804 try:
-> 3805     return self._engine.get_loc(casted_key)
   3806 except KeyError as err:

File index.pyx:167, in pandas._libs.index.IndexEngine.get_loc()

File index.pyx:196, in pandas._libs.index.IndexEngine.get_loc()

File pandas/_libs/hashtable_class_helper.pxi:7081, in pandas._libs.hashtable.PyObjectHashTable.get_item()

File pandas/_libs/hashtable_class_helper.pxi:7089, in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 'C'

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
File ~/miniconda3/lib/python3.12/site-packages/pandas/core/frame.py:4561, in DataFrame._set_value(self, index, col, value, takeable)
   4560 else:
-> 4561     icol = self.columns.get_loc(col)
   4562     iindex = self.index.get_loc(index)

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexes/base.py:3812, in Index.get_loc(self, key)
   3811         raise InvalidIndexError(key)
-> 3812     raise KeyError(key) from err
   3813 except TypeError:
   3814     # If we have a listlike key, _check_indexing_error will raise
   3815     #  InvalidIndexError. Otherwise we fall through and re-raise
   3816     #  the TypeError.

KeyError: 'C'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
Cell In[1], line 4
      1 import pandas as pd
      3 df=pd.DataFrame(data=[[1,2],[3,4]],index=["a","b"],columns=["A","B"])
----> 4 df.at["a","C"]=[1,2,3]

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:2586, in _AtIndexer.__setitem__(self, key, value)
   2583     self.obj.loc[key] = value
   2584     return
-> 2586 return super().__setitem__(key, value)

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:2542, in _ScalarAccessIndexer.__setitem__(self, key, value)
   2539 if len(key) != self.ndim:
   2540     raise ValueError("Not enough indexers for scalar access (setting)!")
-> 2542 self.obj._set_value(*key, value=value, takeable=self._takeable)

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/frame.py:4575, in DataFrame._set_value(self, index, col, value, takeable)
   4573         self.iloc[index, col] = value
   4574     else:
-> 4575         self.loc[index, col] = value
   4576     self._item_cache.pop(col, None)
   4578 except InvalidIndexError as ii_err:
   4579     # GH48729: Seems like you are trying to assign a value to a
   4580     # row when only scalar options are permitted

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:911, in _LocationIndexer.__setitem__(self, key, value)
    908 self._has_valid_setitem_indexer(key)
    910 iloc = self if self.name == "iloc" else self.obj.iloc
--> 911 iloc._setitem_with_indexer(indexer, value, self.name)

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:1890, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1885         self.obj[key] = infer_fill_value(value)
   1887     new_indexer = convert_from_missing_indexer_tuple(
   1888         indexer, self.obj.axes
   1889     )
-> 1890     self._setitem_with_indexer(new_indexer, value, name)
   1892     return
   1894 # reindex the axis
   1895 # make sure to clear the cache because we are
   1896 # just replacing the block manager here
   1897 # so the object is the same

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:1942, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1939 # align and set the values
   1940 if take_split_path:
   1941     # We have to operate column-wise
-> 1942     self._setitem_with_indexer_split_path(indexer, value, name)
   1943 else:
   1944     self._setitem_single_block(indexer, value, name)

File ~/miniconda3/lib/python3.12/site-packages/pandas/core/indexing.py:1998, in _iLocIndexer._setitem_with_indexer_split_path(self, indexer, value, name)
   1993     if len(value) == 1 and not is_integer(info_axis):
   1994         # This is a case like df.iloc[:3, [1]] = [0]
   1995         #  where we treat as df.iloc[:3, 1] = 0
   1996         return self._setitem_with_indexer((pi, info_axis[0]), value[0])
-> 1998     raise ValueError(
   1999         "Must have equal len keys and value "
   2000         "when setting with an iterable"
   2001     )
   2003 elif lplane_indexer == 0 and len(value) == len(self.obj.index):
   2004     # We get here in one case via .loc with a all-False mask
   2005     pass

ValueError: Must have equal len keys and value when setting with an iterable

Expected Behavior

The cell value is set without errors.

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.9.21
python-bits : 64
OS : Darwin
OS-release : 24.3.0
Version : Darwin Kernel Version 24.3.0: Thu Jan 2 20:24:23 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6020
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 2.2.3
numpy : 2.0.2
pytz : 2025.1
dateutil : 2.8.2
pip : 25.0.1
Cython : None
sphinx : None
IPython : 8.18.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.3
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : 2023.6.0
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.6
lxml.etree : 5.3.1
matplotlib : 3.9.4
numba : None
numexpr : 2.10.2
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 18.1.0
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : 2023.6.0
scipy : 1.13.1
sqlalchemy : None
tables : N/A
tabulate : 0.9.0
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[yuanx749]

The same errors happen using .loc.

A workaround is specifying the column first:
df["C"] = pd.Series()