BUG: df.where() shows no effects when inplace=True

[MemphisMeng]

• 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.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

import pandas as pd
import numpy as np

d1 = pd.DataFrame({"a": [1.0, 2.0], 'b': [3, np.nan]})
print('\nd1:')
print(d1)

print('\nd1.where(pd.notnull(d1), None, inplace=True):')
d1.where(pd.notnull(d1), None, inplace=True)

print('\nd1:')
print(d1)

d2 = pd.DataFrame({"a": [1.0, 2.0], 'b': [3, np.nan]})
print('\nd2:')
print(d2)

print('\nd2.where(pd.notnull(d2), None, inplace=True):')
d2.where(pd.notnull(d2), -1, inplace=True)

print('\nd2:')
print(d2)

Problem description

The result shows as follows. This means that if I set inplace to True, and the value of parameter "other" is None, then the manipulation isn't effective on the dataframe itself. This issue is kind of related to BUG#42295. Similarly, the bug doesn't show up if the value of other is a float/int type value.

d1:
     a    b
0  1.0  3.0
1  2.0  NaN

d1.where(pd.notnull(d1), None, inplace=True):

d1:
     a    b
0  1.0  3.0
1  2.0  NaN

d2:
     a    b
0  1.0  3.0
1  2.0  NaN

d2.where(pd.notnull(d2), None, inplace=True):

d2:
     a    b
0  1.0  3.0
1  2.0 -1.0

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS

commit : b5958ee
python : 3.7.10.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.104+
Version : #1 SMP Sat Jun 5 09:50:34 PDT 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.5
numpy : 1.19.5
pytz : 2018.9
dateutil : 2.8.1
pip : 19.3.1
setuptools : 57.0.0
Cython : 0.29.23
pytest : 3.6.4
hypothesis : None
sphinx : 1.8.5
blosc : None
feather : 0.4.1
xlsxwriter : None
lxml.etree : 4.2.6
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.7.6.1 (dt dec pq3 ext lo64)
jinja2 : 2.11.3
IPython : 5.5.0
pandas_datareader: 0.9.0
bs4 : 4.6.3
bottleneck : 1.3.2
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.7.3
odfpy : None
openpyxl : 2.5.9
pandas_gbq : 0.13.3
pyarrow : 3.0.0
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.4.18
tables : 3.4.4
tabulate : 0.8.9
xarray : 0.18.2
xlrd : 1.1.0
xlwt : 1.3.0
numba : 0.51.2