Pandas version checks

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Reproducible Example

import pandas as pd
idx = pd.date_range('20200101', periods=12, freq='BM')
df = pd.DataFrame(dict(a=range(12)), index=idx)
print(df.index.freq)
>> <BusinessMonthEnd>
df.stack()
print(df.index.freq()
>> None

Issue Description

Some of our CI pipeline unittests failed after upgrading to pandas 1.5.0 (linux). (I can't share this example.)

I tried to replicate the error above locally, and it seems to behave the same way. However, the same issue is only visible in ipython within my pycharm console. When I open a python instance via cmd, I cannot replicate the behaviour, which makes this bug a bit harder to trace I believe.

Expected Behavior

I would expect that the index.freq property would not be lost

Installed Versions

INSTALLED VERSIONS ------------------ commit : 87cfe4e38bafe7300a6003a1d18bd80f3f77c763 python : 3.9.7.final.0 python-bits : 64 OS : Windows OS-release : 10 Version : 10.0.19043 machine : AMD64 processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel byteorder : little LC_ALL : None LANG : None LOCALE : English_United States.1252 pandas : 1.5.0 numpy : 1.22.4 pytz : 2021.3 dateutil : 2.8.2 setuptools : 58.0.4 pip : 21.2.4 Cython : 0.29.25 pytest : 6.2.4 hypothesis : None sphinx : 4.3.1 blosc : None feather : None xlsxwriter : None lxml.etree : 4.6.4 html5lib : None pymysql : None psycopg2 : 2.9.3 jinja2 : 3.0.3 IPython : 7.29.0 pandas_datareader: 0.10.0 bs4 : None bottleneck : None brotli : None fastparquet : None fsspec : None gcsfs : None matplotlib : 3.5.0 numba : 0.55.2 numexpr : None odfpy : None openpyxl : 3.0.9 pandas_gbq : None pyarrow : 8.0.0 pyreadstat : None pyxlsb : None s3fs : None scipy : 1.7.3 snappy : None sqlalchemy : 1.4.31 tables : None tabulate : 0.8.9 xarray : None xlrd : None xlwt : None zstandard : None tzdata : None

Comment From: vamsi-verma-s

Hi @MMCMA

I think it is only happening when I do df2=df.stack() and look at df2.index . pls try yourself once.

```Python console

In [64]: idx = pd.date_range('20200101', periods=12, freq='BM')

In [65]: df = pd.DataFrame(dict(a=range(12)), index=idx)

In [66]: df.index Out[66]: DatetimeIndex(['2020-01-31', '2020-02-28', '2020-03-31', '2020-04-30', '2020-05-29', '2020-06-30', '2020-07-31', '2020-08-31', '2020-09-30', '2020-10-30', '2020-11-30', '2020-12-31'], dtype='datetime64[ns]', freq='BM')

In [67]: df2 = df.stack()

In [68]: df.index Out[68]: DatetimeIndex(['2020-01-31', '2020-02-28', '2020-03-31', '2020-04-30', '2020-05-29', '2020-06-30', '2020-07-31', '2020-08-31', '2020-09-30', '2020-10-30', '2020-11-30', '2020-12-31'], dtype='datetime64[ns]', freq='BM')

In [69]: df2.index Out[69]: MultiIndex([('2020-01-31', 'a'), ('2020-02-28', 'a'), ('2020-03-31', 'a'), ('2020-04-30', 'a'), ('2020-05-29', 'a'), ('2020-06-30', 'a'), ('2020-07-31', 'a'), ('2020-08-31', 'a'), ('2020-09-30', 'a'), ('2020-10-30', 'a'), ('2020-11-30', 'a'), ('2020-12-31', 'a')], )

In [70]: df.index Out[70]: DatetimeIndex(['2020-01-31', '2020-02-28', '2020-03-31', '2020-04-30', '2020-05-29', '2020-06-30', '2020-07-31', '2020-08-31', '2020-09-30', '2020-10-30', '2020-11-30', '2020-12-31'], dtype='datetime64[ns]', freq=None)


**Comment From: MMCMA**

Yes, I can replicate this with pandas 1.5.0, I also checked an older pandas version and the freq property in `df.index` is not lost

**Comment From: phofl**

commit fb3e3e60963ae4808ce976fc0f89864122eced15 Author: Luke Manley lukemanley@gmail.com Date: Tue Mar 15 18:43:02 2022 -0400

PERF: MultiIndex.values for MI's with DatetimeIndex, TimedeltaIndex, or ExtensionDtype levels (#46288)

cc @lukemanley #46288

Simple reproducer:

idx = pd.date_range('20200101', periods=12, freq='BM')

df = pd.DataFrame(dict(a=range(12)), index=idx)

df2 = df.stack() df2.index.repr() assert df.index.freq is not None


**Comment From: MarcoGorelli**

Hey @MMCMA 

Thanks for having reported this - there's now a release candidate for pandas 2.0.0, which you can install with

mamba install -c conda-forge/label/pandas_rc pandas==2.0.0rc0

or

python -m pip install --upgrade --pre pandas==2.0.0rc0 ``` If you try it out and report bugs, then we can fix them before the final 2.0.0 release