BUG: Documentation not clear or inconsistent with code behaviour: pct_change(fill_method=None)

[MMCMA]

Pandas version checks

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Reproducible Example

import pandas as pd
a = pd.DataFrame(dict(a=[1, None, 2]))
a.pct_change()
     a
0  NaN
1  0.0
2  1.0

a.pct_change(fill_method=None)
    a
0 NaN
1 NaN
2 NaN

Issue Description

I was attempting to ensure our code complies with version 2.1 and noticed that the documentation for pct_change() indicates that the fill_method argument has been deprecated since version 2.1. However, the code still executes without a warning or error. Could someone please provide clarification on whether the documentation is incorrect, or if I am misunderstanding something? In the event that this argument will be deprecated in the future, how can I replicate the behavior of .pct_change(fill_method=None) in previous versions? The current default behavior simply carries forward the previous values and calculates percentage changes. Wouldn't this be a scenario where users should use ffill() beforehand?

This relates to #53491, not sure if is a duplicate.

Expected Behavior

NA

Installed Versions

INSTALLED VERSIONS

commit : e86ed37
python : 3.10.12.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19045
machine : AMD64
processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252
pandas : 2.1.1
numpy : 1.24.4
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 68.1.2
pip : 23.2.1
Cython : 3.0.2
pytest : 7.4.2
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.9.3
html5lib : 1.1
pymysql : None
psycopg2 : 2.9.6
jinja2 : 3.1.2
IPython : 8.15.0
pandas_datareader : 0.10.0
bs4 : 4.12.2
bottleneck : None
dataframe-api-compat: None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.7.2
numba : 0.57.1
numexpr : None
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 13.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.10.1
sqlalchemy : 2.0.20
tables : None
tabulate : 0.9.0
xarray : None
xlrd : 2.0.1
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[rhshadrach]

However, the code still executes without a warning or error.

On 2.1.x I get two warnings, one for each line in the OP:

FutureWarning: The default fill_method='pad' in DataFrame.pct_change is deprecated and will be removed in a future version. Call ffill before calling pct_change to retain current behavior and silence this warning.
a.pct_change()

FutureWarning: The 'fill_method' and 'limit' keywords in DataFrame.pct_change are deprecated and will be removed in a future version. Call ffill before calling pct_change instead.
a.pct_change(fill_method=None)

Are you sure you're running with pandas 2.1.1 when you don't see warnings?

[MMCMA]

It seems the warning got supressed due to some test settings in my local enviroment - I see the warning on our servers, sorry for the confusion. However, I would still expect an error according to the documentation.

Generally, I am still a bit confused. The argument will eventually be deprecated but currently I still need to (as of 2.1.1) use it with an explizit a.pct_change(fill_method=None) to mimick the future default behaviour and to avoid NA fills. In the future, I will need to eventually drop fill_method=None and no NA fills will be applied, unless I use ffill() prior to calling .pct_change() . In short, currently I cannot write code that will work once fill_method is dropped for good and that does not ffill() the data.

[rhshadrach]

Thanks @MMCMA - I believe your 2nd paragraph makes this a duplicate of the discussion here:

#53491 (comment)

Let me know if this is not the case and we can reopen!

[MMCMA]

Thanks @rhshadrach - yes you can close it - the documentation could still be a bit more clear here in my opinion but I guess it might be resolved within the duplicate issue.