BUG: Index.sort_values fails with TypeError

[galipremsagar]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

>>> import pandas as pd
>>> pd.__version__
'1.1.0'
>>> sr = pd.Series(['a',None,'c',None,'e'])
>>> sr.sort_values()
0       a
2       c
4       e
1    None
3    None
dtype: object
>>> idx = pd.Index(['a',None,'c',None,'e'])
>>> idx
Index(['a', None, 'c', None, 'e'], dtype='object')
>>> idx.sort_values()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/indexes/base.py", line 4448, in sort_values
    _as = idx.argsort()
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/indexes/base.py", line 4563, in argsort
    return result.argsort(*args, **kwargs)
TypeError: '<' not supported between instances of 'NoneType' and 'str'

Problem description

Index.sort_values() fails when there are None values in it. However Series.sort_values performs sorting as expected in similar scenario.

Expected Output

We should be able to sort the values of an index similar to that of a series when there are None values.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : d9fff27
python : 3.7.3.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Sun Jul 5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : en_US.UTF-8
pandas : 1.1.0
numpy : 1.19.0
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 49.1.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[AlexKirko]

Checked the issue on the last commit in master. It persists.

[AlexKirko]

take

[AlexKirko]

The culprit is not separating missing values before using argsort. Will post a PR with a fix attempt this weekend.