BUG: `Index.join` works when `other` is `Series`, giving `Index`

[cmp0xff]

Pandas version checks

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Reproducible Example

import pandas as pd

pd.Index([1, 2]).join(pd.Series([1, 2, 2]))  # Index([1, 2, 2], dtype='int64')

Issue Description

Index.join(Series) is unspecified in the docs, but works at runtime.

Expected Behavior

I would expect Index.join(Series) either fails, or gives Series.

Installed Versions

INSTALLED VERSIONS

commit : f60bf95
python : 3.11.13
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.26100
machine : AMD64
processor : AMD64 Family 25 Model 116 Stepping 1, AuthenticAMD
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : Czech_Czechia.1252

pandas : 3.0.0.dev0+2338.gf60bf958da.dirty
numpy : 2.2.6
dateutil : 2.9.0.post0
pip : 25.2
Cython : 3.1.3
sphinx : 8.2.3
IPython : 9.4.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.5
bottleneck : 1.5.0
fastparquet : 2024.11.0
fsspec : 2025.7.0
html5lib : 1.1
hypothesis : 6.138.3
gcsfs : 2025.7.0
jinja2 : 3.1.6
lxml.etree : 6.0.1
matplotlib : 3.10.5
numba : 0.61.2
numexpr : 2.10.2
odfpy : None
openpyxl : 3.1.5
psycopg2 : 2.9.10
pymysql : 1.4.6
pyarrow : 19.0.1
pyiceberg : 0.9.1
pyreadstat : 1.3.1
pytest : 8.4.1
python-calamine : None
pytz : 2025.2
pyxlsb : 1.0.10
s3fs : 2025.7.0
scipy : 1.16.1
sqlalchemy : 2.0.43
tables : 3.10.2
tabulate : 0.9.0
xarray : 2025.8.0
xlrd : 2.0.1
xlsxwriter : 3.2.5
zstandard : 0.23.0
qtpy : None
pyqt5 : None

[Lakshya-Upadhyaya]

Hello, I can verify this issue on the most recent main branch (3.0.0.dev0 + 2613.g9f4c0ba7b0).

The behavior is not consistent with the documented signature that indicates other: Index. Currently, Index.join(Series) works quietly and returns an Index, but this is not documented.

I would like to work on this once it is triaged. I believe that Index.join() should produce a TypeError when called with a Series. Would you prefer that I:

1. Add type validation to raise a TypeError if the argument is not an Index?
2. Add tests to confirm this functionality?

If you have any input on how to implement this, I would appreciate it as this would be my first contribution to pandas!

[rhshadrach]

enusure_index was added to this method in #32467 which lets it be successful for Series. I'm not seeing any test fails by adding an assert isinstance(..., Index) here.

@jbrockmendel - thoughts here?

[jbrockmendel]

looks like it was already present just a bit lower in the method, so i suspect the reason was more style than anything else. im fine with requiring the argument to already be an Index

[rhshadrach]

This seems like a very straight forward deprecation, I think we should deprecate first. @Lakshya-Upadhyaya - this issue is ready to be worked on.

[Lakshya-Upadhyaya]

Thanks for clarifying, I will work on it