BUG: View on DataFrame with a Multiindex using .loc doesn't give proper results for df.index.levels or df.index.levshape

[scmartin]

If I create a view on a subset of a dataframe with a Multiindex using .loc, if I print the index of the subset, it get the expected subset of the Multiindex. However index.levels and index.levshape return the values for the original dataframe, not the subset.

import pandas as pd
import numpy as np

index = [[1,1,1,1,2,2,2,2,3,3,3,3],
             [1,2,3,4,1,2,3,4,1,2,3,4]]
data = pd.DataFrame(np.random.randn(12,2),index=index)
smalldata = data.loc[(slice(None),slice(1,2)),:]
print(data.index)
print(smalldata.index)
print(smalldata.index.levels)
print(smalldata.index.levshape)

Problem description

If the index of the created view is only the subset of values which appear in the view, the index methods levels and levshape should reflect this subset of values, not the whole index of the original dataframe

Expected Output

>>> print(smalldata.index.levels)
[[1, 2, 3], [1, 2]]

Output of pd.show_versions()

INSTALLED VERSIONS

commit : d9fff27
python : 3.8.5.final.0
python-bits : 64
OS : Linux
OS-release : 3.10.0-957.1.3.el7.x86_64
Version : #1 SMP Thu Nov 29 14:49:43 UTC 2018
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : None.None

pandas : 1.1.0
numpy : 1.19.1
pytz : 2020.1
dateutil : 2.8.1
pip : 20.2
setuptools : 49.2.0.post20200712
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.17.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.3.0
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.5.2
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[phofl]

Hey, thanks for your report.

This has nothing to do with the view. MultiIndex levels are not updated, when values are deleted from the Index, see for example:

x = data.drop(index=[(1, 4), (2, 4), (3, 4)])

still has

[[1, 2, 3], [1, 2, 3, 4]]

There was discussion about this in the past

[phofl]

See #36227 for example

[phofl]

Closing as this is expected

[rambo-yuanbo]

but still, maybe this should be mentioned in MultiIndex.levshape Documentation

[phofl]

This is mentioned in multiple laces in the documentation