pd.Series.sum() returns nan for Int64 with pd.NA

[koizumihiroo]

Code Sample, a copy-pastable example if possible

I'm not sure this is a bug or intended behavior. If intended, I'm glad to be pointed to how API design was discussed and decided.

On 1.0.0 rc0, pd.Series.sum()returns nan when using skipna=False for 'Int64' with pd.NA.

>>> pd.Series([1, 2, pd.NA]).sum(skipna=False)
<NA>
>>> pd.Series([1, 2, pd.NA], dtype='Int64').sum(skipna=False)
nan

>>> pd.Series([1, 2, np.nan]).sum(skipna=False)
nan
>>> pd.Series([1, 2, np.nan], dtype='Int64').sum(skipna=False)
nan

>>> pd.Series([1, 2, None]).sum(skipna=False)
nan
>>> pd.Series([1, 2, None], dtype='Int64').sum(skipna=False)
nan

Problem description

I understand the second and third example returns nan for backward compatibility, but the first example of dtype='Int64' seems natural to return pd.NA.

Expected Output

>>> pd.Series([1, 2, pd.NA], dtype='Int64').sum(skipna=False)
<NA>

# When breaking backward compatibility is allowed, this is also expected (on and after pandas 2.0.0?)
>>> pd.Series([1, 2, np.nan], dtype='Int64').sum(skipna=False)
<NA>
>>> pd.Series([1, 2, None], dtype='Int64').sum(skipna=False)
<NA>

Output of pd.show_versions()

>>> pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.9.184-linuxkit
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.0rc0
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 19.3.1
setuptools : 44.0.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : 3.1.2
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

# Dockerfile
FROM python:3.7.6
WORKDIR /home
RUN pip install pandas==1.0.0rc0
CMD ["/bin/bash"]

[jorisvandenbossche]

@koizumihiroo Thanks for the report! Yes, pd.NA is expected as the result, that's an oversight (also the ones in your second and third example should return NA when using Int64 dtype, we decided to do this backwards compatibility break here).