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BUG: wrong future Warning on string assignment in certain condition #56503

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@xmatthias

Description

@xmatthias

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

Reproducible Example

import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randint(0, 100, size=(5, 2)), columns=list('AB'))

# Assigning string directly works
df.loc[df.index < 2, 'E'] = 'abc'

# Assigning a string and a float in the same step raises a Future warning
df.loc[df.index <= 2, ['F', 'G']] = (1, 'abc', )

# DOing the same assignment unconditionally does NOT raise the future warning
df.loc[:, ['I', 'J']] = (1, 'abc', )
df.head()

image

Issue Description

String assignment future warnings are quite flaky and seem to change and break in different ways in every recent version.

While most issues i've seen have been fixed - i just encountered a new issue, where behavior is completely non-uniform and surprising

I can reproduce this in anything after 2.1.x - including 2.1.4 - which is the latest current release
2.0.3 doesn't issue this - but i think these flaky FutureWarnings were only introduced in 2.0.3.

Expected Behavior

No future warning when creating a new string column - independant of the precence of another column in the assignment (and independent of a filter).

Installed Versions

INSTALLED VERSIONS

commit : a671b5a
python : 3.11.4.final.0
python-bits : 64
OS : Linux
OS-release : 6.6.3-arch1-1
Version : #1 SMP PREEMPT_DYNAMIC Wed, 29 Nov 2023 00:37:40 +0000
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.1.4
numpy : 1.26.2
pytz : 2023.3
dateutil : 2.8.2
setuptools : 68.2.2
pip : 23.3.1
Cython : 0.29.35
pytest : 7.4.3
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.9.3
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.14.0
pandas_datareader : None
bs4 : 4.12.2
bottleneck : None
dataframe-api-compat: None
fastparquet : None
fsspec : 2023.12.1
gcsfs : None
matplotlib : 3.7.1
numba : None
numexpr : 2.8.4
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 14.0.1
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.11.4
sqlalchemy : 2.0.23
tables : 3.9.1
tabulate : 0.9.0
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

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BugDtype ConversionsUnexpected or buggy dtype conversionsPDEP6-relatedrelated to PDEP6 (not upcasting during setitem-like Series operations)WarningsWarnings that appear or should be added to pandas

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