BUG: when I assign value of 1-dim np.array holding single instance, it results in 0-dim array instance

[kcerniauskas3]

Pandas version checks

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Reproducible Example

arr = np.array(['one'], dtype="object")
df = pd.DataFrame({'col1': [None]}, index=[100])
df.at[100, 'col1'] = arr

Issue Description

when I assign value of 1-dim np.array holding single instance, it results in 0-dim array instance:

df.at[100, 'col1']
array('one', dtype=object)

Expected Behavior

df.at[100, 'col1']
array(['one'], dtype=object)

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.12.7
python-bits : 64
OS : Windows
OS-release : 11
Version : 10.0.26100
machine : AMD64
processor : AMD64 Family 25 Model 33 Stepping 0, AuthenticAMD
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : English_United States.1252

pandas : 2.2.3
numpy : 2.2.0
pytz : 2024.2
dateutil : 2.9.0.post0
pip : 24.3.1
Cython : None
sphinx : None
IPython : 8.28.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : 2024.6.1
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.4
lxml.etree : 5.3.0
matplotlib : 3.9.2
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 17.0.0
pyreadstat : None
pytest : 8.3.3
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.14.1
sqlalchemy : None
tables : None
tabulate : 0.9.0
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2024.2
qtpy : None
pyqt5 : None

[QuentinFuxa]

The single-element array is being “unboxed” by pandas when assigned to a single cell. This is a known quirk rather than a newly-introduced bug I think : Pandas stores single-element object arrays as scalar objects internally

If preserving the shape is critical, you may need to store arrays-of-arrays (e.g. 2D arrays or lists containing your 1D arrays, like df.at[100, 'col1'] = [arr]) or assign these arrays in a way that the entire column ends up typed as “object of array.”

[kcerniauskas3]

Thanks.
I ended up converting to list, saving in .parquet, and reloading dataset. Parquet saves list as np.array, and loads back correctly in 1-dim.
Although df.at[100, 'col1'] = [arr] worked for this case, it did not work for arrays with more than 1 element. Also, assigning entire column should have worked, but my case was update by index.

[kcerniauskas3]

Seems to be a known quirk.