BUG: Series.combine_first replaces None with NaN at index not provided by other #58977
Description
Pandas version checks
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Reproducible Example
import pandas as pd
s1 = pd.Series([None, None, None], index=['a', 'b', 'c'])
s2 = pd.Series([None, None, None], index=['b', 'c', 'd'])
result = s1.combine_first(s2)
print(result)Issue Description
The documentation of Series.combine_first states:
Update null elements with value in the same location in ‘other’.
In the above example, s2 doesn't provide any value at index 'a', so combine_first should not affect this index.
However, the result with pandas 2.2.2 is the following, which unexpectedly changes the value at 'a' from None to NaN:
a NaN
b None
c None
d None
dtype: object
Pandas 2.1.4 behaves as expected, so this looks like a regression.
The behavior was changed with this PR: #57034
Expected Behavior
The expected output is:
a None
b None
c None
d None
dtype: object
Installed Versions
pandas : 2.2.0rc0+28.g6dbeeb4009
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
setuptools : 63.2.0
pip : 24.0
Cython : 3.0.10
pytest : 8.2.0
hypothesis : 6.100.2
sphinx : 7.3.7
blosc : None
feather : None
xlsxwriter : 3.2.0
lxml.etree : 5.2.1
html5lib : 1.1
pymysql : 1.4.6
psycopg2 : 2.9.9
jinja2 : 3.1.3
IPython : 8.24.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : 1.3.8
fastparquet : 2024.2.0
fsspec : 2024.3.1
gcsfs : 2024.3.1
matplotlib : 3.8.4
numba : 0.59.1
numexpr : 2.10.0
odfpy : None
openpyxl : 3.1.2
pyarrow : 16.0.0
pyreadstat : 1.2.7
python-calamine : None
pyxlsb : 1.0.10
s3fs : 2024.3.1
scipy : 1.13.0
sqlalchemy : 2.0.29
tables : 3.9.2
tabulate : 0.9.0
xarray : 2024.3.0
xlrd : 2.0.1
zstandard : 0.22.0
tzdata : 2024.1
qtpy : None
pyqt5 : None