BUG: reset_index() looses the frequency of a DatetimeIndex

[annika-rudolph]

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

>>> index = pd.DatetimeIndex(pd.date_range(start="2000", freq = 'YS', periods = 10), name = 'Date')
>>> df = pd.DataFrame(data=list(range(10)), index = index)
>>> print(df.index.freq)
<YearBegin: month=1>
>>> print(df.reset_index()['Date']._values.freq)
None
>>> df = df.reset_index().set_index('Date')
>>> print(df.index.freq)
None

Issue Description

When doing reset_index() on a DatetimeIndex this leads to the frequency being lost. Although the newly created column is a DatetimeArray, it does not seem to carry the freq attribute. As a result, when doing reset_index() -> set_index() I cannot restore the original index which potentially creates issues.

Expected Behavior

I would expect that reset_index().set_index() let's me recover the original index :)

Installed Versions

INSTALLED VERSIONS

commit : bfe5be0
python : 3.10.12
python-bits : 64
OS : Linux
OS-release : 5.15.153.1-microsoft-standard-WSL2
Version : #1 SMP Fri Mar 29 23:14:13 UTC 2024
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 0+untagged.34794.gbfe5be0
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
pip : 22.0.2
Cython : 3.0.10
sphinx : 7.3.7
IPython : 8.23.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
blosc : None
bottleneck : 1.3.8
fastparquet : 2024.2.0
fsspec : 2024.3.1
html5lib : 1.1
hypothesis : 6.100.1
gcsfs : 2024.3.1
jinja2 : 3.1.3
lxml.etree : 5.2.1
matplotlib : 3.8.4
numba : 0.59.1
numexpr : 2.10.0
odfpy : None
openpyxl : 3.1.2
psycopg2 : 2.9.9
pymysql : 1.4.6
pyarrow : 16.0.0
pyreadstat : 1.2.7
pytest : 8.1.1
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
xlsxwriter : 3.2.0
zstandard : 0.22.0
tzdata : 2024.1
qtpy : None
pyqt5 : None

[aram-cinnamon]

take

[aram-cinnamon]

I did some digging, and it seems it's intended that freq becomes None in a column:

pandas/pandas/core/internals/blocks.py

Lines 2158 to 2160 in 18a3eec

	if isinstance(values, (DatetimeArray, TimedeltaArray)) and values.freq is not None:
	# freq is only stored in DatetimeIndex/TimedeltaIndex, not in Series/DataFrame
	values = values._with_freq(None)

The above was added in this PR #41425, which mentions that "The long-term behavior is definitely going to always drop the freq (more specifically, DTA/TDA won't have freq, xref #31218). So this PR standardizes always-dropping."

@annika-rudolph What do you think?
Also @jbrockmendel @jreback @mroeschke @jorisvandenbossche you created/reviewed/were mentioned in the PR. What are your thoughts on this issue?

[annika-rudolph]

Thanks for digging into this! It is what I suspected :)

From a user perspective I can say that frequencies in DatetimeIndices are quite important, even more so since some functionality (like businessday and resample) will be dropped for Periodindices -- which for us means that we recently moved everything to DatetimeIndices. Thus, it would be nice if they could cover the same functionality as Periodindices and specifically, the frequency attribute could be retained in all transformations.
Reset_index() -> set_index() is a common pattern that I see a lot when working with MultiIndices, which is also very relevant in many of my projects.

It seems to me that the decision on always dropping the frequency was taken some time ago (before deciding to drop PeriodIndex functionality?), so maybe it could be reconsidered?

[yuanx749]

I encountered this issue and did some debugging. It is the reshape below that leads to loss of freq.

pandas/pandas/core/internals/blocks.py

Line 2313 in 1fa5025

values = values.reshape(1, -1)

But as mentioned by @aram-cinnamon , I think this behaviour is expected.

[TorstenPietrek]

With the deprecation of PeriodIndex functionality progressing and the current recommendation to use a DateTimeIndex in place, I think the frequency property of a DateTimeIndex should not be dropped. If you drop the frequency property the object no longer holds information on the periods, so it could not be used to replace the PeriodIndex. @jbrockmendel it would be great to hear your opinion on that