BUG: Different behaviour for index after assign dataframe new column by series

[arybin93]

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

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

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

With pandas version 1.1.1

import pandas as pd

df = pd.DataFrame()
series = pd.Series(1.23, index=pd.RangeIndex(4, name="series_index"))
df['series'] = series
print(df)

Problem description

With pandas version 1.1.1, output:

                         series
series_index        
0                       1.23
1                       1.23
2                       1.23
3                       1.23

We perceive index from series

With pandas version 1.1.2, output:

   series
0    1.23
1    1.23
2    1.23
3    1.23

We lost the index from series

Expected Output

I am not sure about it. Could you explain is bug or feature of the latest version?
We used to perceive index from series and it worked before.

Output of pd.show_versions()

[INSTALLED VERSIONS

commit : 2a7d332
python : 3.8.5.final.0
python-bits : 64
OS : Linux
OS-release : 4.19.104-microsoft-standard
Version : #1 SMP Wed Feb 19 06:37:35 UTC 2020
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : None
LOCALE : en_US.UTF-8
pandas : 1.1.2
numpy : 1.19.2
pytz : 2020.1
dateutil : 2.8.1
pip : 20.2.3
setuptools : 50.3.0
Cython : None
pytest : 6.0.2
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.8.6 (dt dec pq3 ext lo64)
jinja2 : 2.10.3
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 1.3.19
tables : None
tabulate : 0.8.7
xarray : None
xlrd : 1.2.0
xlwt : None
numba : None
]

[jorisvandenbossche]

@arybin93 thanks for the clear report. That's indeed a regression in pandas 1.1.2 compared to 1.1.1 / 1.0

[jorisvandenbossche]

Based on the title of the PR, probably related to #36141 cc @Dr-Irv

[Dr-Irv]

@jorisvandenbossche You are right. Pushed a PR that should correct that

[arybin93]

So it looks that release notes for 1.1.2 contain fixed bug #31368

Release notes

Bug when setting empty DataFrame column to a Series in preserving name of index in frame (GH31368)

But in reality not and changes will in 1.1.3, is it so?

[Dr-Irv]

@arybin93 The fix in 1.1.2 that fixed #31368 created the issue you have reported here, which is now addressed in #36532 and is planned for 1.1.3, so that both #31368 and this issue are resolved.