BUG: .drop with MultiIndex does not drop indices from MultiIndexlevels.

[ahalev]

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 pandas as pd

 
import pandas as pd
>>> df = pd.DataFrame(index=pd.MultiIndex.from_product([[1,2], [3, 4]]), columns=['a', 'b'])
>>> df
       a    b
1 3  NaN  NaN
  4  NaN  NaN
2 3  NaN  NaN
  4  NaN  NaN

>>> df_2 = df.drop((1,))
>>> df_2
       a    b
2 3  NaN  NaN
  4  NaN  NaN
>>> df_2.index
MultiIndex([(2, 3),
            (2, 4)],
           )
>>> df_2.index.levels[0]
Int64Index([1, 2], dtype='int64')

Issue Description

When dropping rows from a DataFrame with a Pandas MultiIndex, it correctly drops the corresponding indices from the overall MultiIndex (see above) but the corresponding keys are not dropped from MultiIndex.levels, as they should be.

Expected Behavior

In the example above, df_2.index.levels[0] should be Int64Index([2], dtype='int64').

Installed Versions

NSTALLED VERSIONS

commit : 66e3805
python : 3.7.12.final.0
python-bits : 64
OS : Darwin
OS-release : 21.1.0
Version : Darwin Kernel Version 21.1.0: Wed Oct 13 17:33:01 PDT 2021; root:xnu-8019.41.5~1/RELEASE_ARM64_T6000
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : en_US.UTF-8
pandas : 1.3.5
numpy : 1.21.5
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3.1
setuptools : 60.2.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.3
IPython : 7.31.1
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : None
odfpy : None
openpyxl : 3.0.9
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.7.3
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[purna135]

HI, I would like to work on this issue.

[phofl]

This works as it should. They are kept on purpose.

We had a few issues about this in the past, they should be on the issue tracker

[phofl]

#36227 and https://pandas.pydata.org/pandas-docs/stable/user_guide/advanced.html#defined-levels