BUG: In main, using resample().interpolate(inplace=True) raises an exception

[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 numpy as np
from pandas import date_range, DataFrame
DR = date_range("1999-1-1", periods=365, freq="D")
DF_ = DataFrame(np.random.standard_normal((365, 1)), index=DR)
S = DF_.iloc[:, 0]
DF = DataFrame({"col1": S, "col2": S})

DF.resample("ME").interpolate(inplace=True)

Issue Description

Code raises an exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Anaconda3\envs\pandasstubs\lib\site-packages\pandas\core\resample.py", line 958, in interpolate
    result_interpolated = result_interpolated.loc[final_index]
AttributeError: 'NoneType' object has no attribute 'loc'

Expected Behavior

No exception. Works fine in pandas 2.2.2

Note: Seems to be introduced by #56515 by @cbpygit , review by @MarcoGorelli and @jbrockmendel

Installed Versions

INSTALLED VERSIONS

commit : 34177d6
python : 3.9.16.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 : 3.0.0.dev0+943.g34177d6b20
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
setuptools : 68.2.2
pip : 22.3.1
Cython : None
pytest : 8.2.0
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : 3.2.0
lxml.etree : 5.2.1
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 3.1.4
IPython : None
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.8.4
numba : None
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 : None
scipy : 1.13.0
sqlalchemy : 2.0.30
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

[cbpygit]

@Dr-Irv Thank you for reporting this problem! I am able to reproduce this. The problem is purely related to the inplace=True option. The interpolation result here becomes None and internally the _update_inplace method call here does not yet possess the final result. There is still the step of index correction missing, which is only applied afterwards here.

The problem is that the context in pandas.core.generic.NDFrame.interpolate does not know about the correct final index. So far, I am unable to figure out how I can update the state of the parent data frame that led to the pandas.core.resample.Resampler.interpolate call. I tried to manipulate self._selected_obj but it does not propagate. If somebody can point me at the object that needs to be updated I can easily fix this.

[cbpygit]

I could use some input here @MarcoGorelli. As we are introducing a breaking change anyway, we could think about removing the inplace option. I think it is counter-intuitive that the second method in a chain of methods modifies the original data frame "in place". Just think about the completely valid option to split the calls like this:

df = ...
resampler = df.resample("ME")   # creates a resampler instance, does not modify df
resampler.interpolate(inplace=True)  # modifies df, returns None

[MarcoGorelli]

we could think about removing the inplace option.

yup - this looks like one of the places where inplace was a lie anyway? Agree, "in for a penny, in for a pound" - if we gonna break, let's break. It's for the better anyway

[cbpygit]

@MarcoGorelli 🙌 I'll take care of it and ping you in the PR.

[cbpygit]

@MarcoGorelli I looked into this but it is extremely convoluted 😢 The inplace option is passed on in multiple steps, partially to overloaded methods or abstract methods with many implementations. There are dozens of tests that use inplace=True or even test this in particular.

Before I spend a lot of time on this, could you indicate a safe way to approach it? Shall I really drop the inplace argument in all those methods, delete all the tests against the feature and switch to not-inplace in the tests that use it?

[MarcoGorelli]

yeah i'd be ok with that

[MarcoGorelli]

as in, remove it from interpolate. if any intermediate function (which is also used in other places) requires inplace, just pass inplace=False