Groupby converts None to NaN

[BayerSe]

Code Sample, a copy-pastable example if possible

import pandas as pd

df = pd.DataFrame.from_dict({
    'id': ['a'],
    'value': [None]
})

df_grouped = df.groupby('id', as_index=False).first()

print(df)
print(df_grouped)

Output:

  id value
0  a  None
  id  value
0  a  NaN

Problem description

Since at least version 1.0.2, the type of df_grouped is NaN. In Version 0.25.3, the type was None.
This breaks my code since I later check for this value using if var is None, which is False when var is NaN instead of None.

Expected Output

  id value
0  a  None
  id  value
0  a  None

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.8.1.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-88-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.0.2
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 46.0.0
Cython : None
pytest : 5.4.1
hypothesis : None
sphinx : 2.4.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : 3.2.0
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.4.1
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.15
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[hkishn]

@BayerSe
As a workaround you can "None" instead of None.
df = pd.DataFrame.from_dict({
'id': ['a'],
'value': ["None"]
})
This should give the output of None instead of Nan.
I will also check the release document of pandas 1.0.2 for this change
Hope this helps.

[BayerSe]

Thanks @hkishn but this won't help because "None" is a string and not an empty value

[BayerSe]

Gentle bump -
could anyone from the team state whether I should adjust my code or if that is something that would be fixed in a future version?

[TomAugspurger]

Not sure. Are you able to bisect to the commit that caused the change?

[simonjayhawkins]

20a84a5 is the first bad commit
commit 20a84a5
Author: Daniel Saxton [email protected]
Date: Sun Feb 23 08:58:59 2020 -0600

BUG: Avoid ambiguous condition in GroupBy.first / last (#32124)