BUG: expanding dataframe row-wise with single-element-list is no-op, but two-element-list copies

[MarcoGorelli]

Pandas version checks

• I have checked that this issue has not already been reported.

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

df = pd.DataFrame(columns=['x', 'y'])
df.loc[:, 'x'] = [1]
print(df)  # empty dataframe

df = pd.DataFrame(columns=['x', 'y'])
df.loc[:, 'x'] = [1, 2]
print(df)  # 2-row dataframe

Issue Description

I get the output

Empty DataFrame
Columns: [x, y]
Index: []
   x    y
0  1  NaN
1  2  NaN

Expected Behavior

probably

   x    y
0  1  NaN
   x    y
0  1  NaN
1  2  NaN

Installed Versions

INSTALLED VERSIONS

commit : 1d56672
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.102.1-microsoft-standard-WSL2
Version : #1 SMP Wed Mar 2 00:30:59 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 2.2.0.dev0+612.g1d56672407
numpy : 1.26.2
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 59.6.0
pip : 23.3.1
Cython : 3.0.5
pytest : 7.4.3
hypothesis : 6.89.0
sphinx : 6.2.1
blosc : 1.11.1
feather : None
xlsxwriter : 3.1.9
lxml.etree : 4.9.3
html5lib : 1.1
pymysql : 1.4.6
psycopg2 : 2.9.9
jinja2 : 3.1.2
IPython : 8.17.2
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.2
bottleneck : 1.3.7
dataframe-api-compat : None
fastparquet : 2023.10.1
fsspec : 2023.10.0
gcsfs : 2023.10.0
matplotlib : 3.8.1
numba : 0.58.1
numexpr : 2.8.7
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 14.0.1
pyreadstat : 1.2.4
python-calamine : None
pyxlsb : 1.0.10
s3fs : 2023.10.0
scipy : 1.11.3
sqlalchemy : 2.0.23
tables : 3.9.1
tabulate : 0.9.0
xarray : 2023.10.1
xlrd : 2.0.1
zstandard : 0.22.0
tzdata : 2023.3
qtpy : None
pyqt5 : None

[sleepallday]

Take

[sleepallday]

When execute the code piece,

df = pd.DataFrame(columns=['x', 'y'])
df.loc[:, 'x'] = [1, 2]

it raises error

> /pandas/pandas/core/internals/managers.py(2237)setitem_inplace()
-> arr[indexer] = value
ValueError: could not broadcast input array from shape (2,) into shape (0,)

which does not happen to the 1-row case, can we verify that if this kind of expansion usage is correct?

[MarcoGorelli]

can't reproduce, sorry, I don't get any error running that (neither on main, nor on the latest release)

[sleepallday]

Do you set breakpoint and execute line by line or you just run df.loc[:, 'x'] = [1, 2]? In the latter case, the error will be caught so i guess that's why you don't see it?

[MarcoGorelli]

ah I see what you mean now, sorry