error that converts nan to non-nan float value via groupby and nunique

[mnky9800n]

Code Sample, a copy-pastable example if possible

import pandas as pd
import numpy as np
print(pd.__version__)
>> '1.0.1'
df = pd.DataFrame({'a':[1,2,3], 'b':[np.nan, np.nan, np.nan], 'c':['d', 'e', 'f']}, index=[0, 1, 2])
print(df)
print(df.loc[1].b)
# >> nan
df.groupby('a').nunique()
print(df)
#    a             b  c
# 0  1 -9.223372e+18  d
# 1  2 -9.223372e+18  e
# 2  3 -9.223372e+18  f
print(df.c.unique())
print(df)
# print(df.groupby('a').nunique().c.unique())
# >> array([1])
print(df.loc[1].b)
# >> -9.223372036854776e+18

Problem description

When a groupby operation followed by an nunique operation, nans are converted to non-nan float values. According to the documentation, the nunique function should return a series but should not update the original dataframe. Also according to the documentation you can ignore nans, however this seems to still change the value nans to a non-nan number.

This does not happen if nunique is applied without the groupby function.

Expected Output

The expected output is that nan values should not be converted to any other value based on an aggregrate operation on the dataframe.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.6.10.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-45-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.1
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 45.2.0.post20200210
Cython : 0.29.15
pytest : 5.3.5
hypothesis : 5.4.1
sphinx : 2.4.0
blosc : None
feather : None
xlsxwriter : 1.2.7
lxml.etree : 4.5.0
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.12.0
pandas_datareader: None
bs4 : 4.8.2
bottleneck : 1.3.1
fastparquet : None
gcsfs : None
lxml.etree : 4.5.0
matplotlib : 3.1.3
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.3.5
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.13
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.7
numba : 0.48.0

[MarcoGorelli]

Thanks @mnky9800n - seems like a duplicate of #31950 (which is in the 1.0.2 milestone), so I'll close this for now