BUG: 'fill_value' is not a valid keyword for Series.interpolate

[kmuehlbauer]

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

s = pd.Series([np.nan, 0, 1, np.nan, 3, np.nan])
s.interpolate(method="linear", fill_value=0)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[7], line 2
      1 s = pd.Series([np.nan, 0, 1, np.nan, 3, np.nan])
----> 2 s.interpolate(method="linear", fill_value=0)

File /home/kai/miniconda/envs/xarray_311/lib/python3.11/site-packages/pandas/core/generic.py:8164, in NDFrame.interpolate(self, method, axis, limit, inplace, limit_direction, limit_area, downcast, **kwargs)
   8155             warnings.warn(
   8156                 f"{type(self).__name__}.interpolate with object dtype is "
   8157                 "deprecated and will raise in a future version. Call "
   (...)
   8160                 stacklevel=find_stack_level(),
   8161             )
   8163 if "fill_value" in kwargs:
-> 8164     raise ValueError(
   8165         "'fill_value' is not a valid keyword for "
   8166         f"{type(self).__name__}.interpolate"
   8167     )
   8169 if isinstance(obj.index, MultiIndex) and method != "linear":
   8170     raise ValueError(
   8171         "Only `method=linear` interpolation is supported on MultiIndexes."
   8172     )

ValueError: 'fill_value' is not a valid keyword for Series.interpolate

Issue Description

The issue surfaced in xarray CI builds pydata/xarray#8125, pydata/xarray#8101.

It was introduced with #53962, but guessing from that the error should only be raised for fillna-methods.

Expected Behavior

Interpolation should succeed, fill_value-kwarg should be passed on to interpolating function.

Installed Versions

INSTALLED VERSIONS

commit : ba1cccd
python : 3.11.0.final.0
python-bits : 64
OS : Linux
OS-release : 5.14.21-150500.55.19-default
Version : #1 SMP PREEMPT_DYNAMIC Tue Aug 8 22:15:01 UTC 2023 (9908c29)
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : de_DE.UTF-8
LOCALE : de_DE.UTF-8

pandas : 2.1.0
numpy : 1.24.2
pytz : 2022.7.1
dateutil : 2.8.2
setuptools : 67.6.0
pip : 23.0.1
Cython : None
pytest : 7.2.2
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.11.0
pandas_datareader : None
bs4 : 4.12.0
bottleneck : 1.3.7
dataframe-api-compat: None
fastparquet : None
fsspec : 2023.3.0
gcsfs : None
matplotlib : 3.7.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 11.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.10.1
sqlalchemy : None
tables : None
tabulate : None
xarray : 2023.8.1.dev8+g1809b913.d20230831
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[rhshadrach]

cc @Charlie-XIAO

[Charlie-XIAO]

Sorry for the confusion but I don't think it was me who introduced this error? I tried to revert all my changes in core/generic.py and see if it works, but the error still exists. What I did in that PR is moving the if block up a bit, which should (may) not cause a problem. Instead I think maybe it was introduced in #53872? Not sure but that PR is the one that adds this if block.

Anyways there seems to be already an open PR that can solve this issue.

[kmuehlbauer]

Sorry @Charlie-XIAO, I did not search further down the PR's. I'll do better next time.

[Charlie-XIAO]

Please don't apologize @kmuehlbauer, that doesn't matter at all. I'm aware that the blame showed my PR and it's kind of complicated to bisect the right commit.

[rhshadrach]

Thanks @Charlie-XIAO. I plan to run a git-bisect later today, but no problem if someone else beats me to it!

[rhshadrach]

Result of a git bisect:

CLN: unused args in pad_or_backfill (#53872)

cc @jbrockmendel