BUG: row filtering by index slicing does not always work

[yohplala]

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

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

---

Code Sample, a copy-pastable example

import pandas as pd
df = pd.DataFrame(columns=['a', 'b'], index=range(6))
# Slicings that work
df[::2] = 1
df['a'][::2] = 'a'
df[1::2] = 2
df['a'][1::2] = 'c'
# Slicing that does not work
df[['a','b']][1::2] = 'aha'

Problem description

The last slicing does not work (Dataframe is not modified), and no error message is issued.
I could check that it works with pd.IndexSlice.
At least an error message should be raised.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f2c8480
python : 3.8.5.final.0
python-bits : 64
OS : Linux
OS-release : 5.8.0-48-generic
Version : #54~20.04.1-Ubuntu SMP Sat Mar 20 13:40:25 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : fr_FR.UTF-8
LOCALE : fr_FR.UTF-8

pandas : 1.2.3
numpy : 1.18.5
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 52.0.0.post20210125
Cython : 0.29.22
pytest : 6.2.2
hypothesis : None
sphinx : 3.5.3
blosc : None
feather : None
xlsxwriter : 1.3.7
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : 7.21.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.7
fastparquet : 0.5.0
gcsfs : None
matplotlib : 3.3.4
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 2.0.0
pyxlsb : None
s3fs : None
scipy : 1.6.2
sqlalchemy : 1.3.23
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 2.0.1
xlwt : 1.3.0
numba : 0.51.2

[mzeitlin11]

Thanks for the report @yohplala. I don't think this is row filtering specific, but rather a case of chained assignment (see https://pandas.pydata.org/docs/user_guide/indexing.html#indexing-view-versus-copy). Column setting also fails in this case, eg: df[['a','b']][1::2] = 5 also does not modify df. The right way to slice with your final statement would be

df.loc[1::2, ['a', 'b']] = 'aha'

[yohplala]

Thanks @mzeitlin11
The .loc does the trick indeed, a lot of thanks.
Bests
(leaving the ticket open - I think issuing an error in above mentionned case would be the answer)

[attack68]

I think this is already covered here: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#indexing-view-versus-copy

[phofl]

This should probably raise a SettingWithCopyWarning?

[jreback]

yeah ideally this would show the warning

[mzeitlin11]

Will close in favor of #9767