BUG: In development version, pd.merge with Series arguments an how="cross" fails

[Dr-Irv]

Pandas version checks

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

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

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

Reproducible Example

import pandas as pd
ls = pd.Series([1, 2, 3, 4], index=[1, 2, 3, 4], name="left")
rs = pd.Series([3, 4, 5, 6], index=[3, 4, 5, 6], name="right")
pd.merge(ls, rs, how="cross")

Issue Description

Above code produces

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Code\pandas_dev\pandas\pandas\core\reshape\merge.py", line 147, in merge
    return _cross_merge(
  File "C:\Code\pandas_dev\pandas\pandas\core\reshape\merge.py", line 210, in _cross_merge
    left = left.assign(**{cross_col: 1})
  File "C:\Code\pandas_dev\pandas\pandas\core\generic.py", line 6176, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'Series' object has no attribute 'assign'. Did you mean: 'align'?

Expected Behavior

No error is raised

Installed Versions

INSTALLED VERSIONS

commit : c126eeb
python : 3.10.11.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19045
machine : AMD64
processor : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252

pandas : 2.1.0.dev0+1159.gc126eeb392
numpy : 1.24.3
pytz : 2023.3
dateutil : 2.8.2
setuptools : 67.7.2
pip : 23.1.2
Cython : 0.29.33
pytest : 7.3.2
hypothesis : 6.79.1
sphinx : 6.2.1
blosc : None
feather : None
xlsxwriter : 3.1.2
lxml.etree : 4.9.2
html5lib : 1.1
pymysql : 1.0.3
psycopg2 : 2.9.3
jinja2 : 3.1.2
IPython : 8.14.0
pandas_datareader: None
bs4 : 4.12.2
bottleneck : 1.3.7
brotli :
fastparquet : 2023.4.0
fsspec : 2023.6.0
gcsfs : 2023.6.0
matplotlib : 3.7.1
numba : 0.57.0
numexpr : 2.8.4
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 12.0.0
pyreadstat : 1.2.2
pyxlsb : 1.0.10
s3fs : 2023.6.0
scipy : 1.10.1
snappy :
sqlalchemy : 2.0.16
tables : 3.8.0
tabulate : 0.9.0
xarray : 2023.5.0
xlrd : 2.0.1
zstandard : 0.19.0
tzdata : 2023.3
qtpy : None
pyqt5 : None

[btparrish]

I cannot reproduce this error on the most recent version of Pandas. consider closing this issue @Dr-Irv

[Dr-Irv]

I cannot reproduce this error on the most recent version of Pandas. consider closing this issue @Dr-Irv

It's not on version 2.0.3. It's on the development version on github in the main branch

[btparrish]

I cannot reproduce this error on the most recent version of Pandas. consider closing this issue @Dr-Irv

It's not on version 2.0.3. It's on the development version on github in the main branch

oh i see -- thanks for clarifying