Description
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I have checked that this issue has not already been reported.
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I have confirmed this bug exists on the latest version of pandas.
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(optional) I have confirmed this bug exists on the master branch of pandas.
Code Sample, a copy-pastable example
import decimal
from pandas.tests.extension.decimal import DecimalArray
import pandas as pd
b = decimal.Decimal("1.0")
a = decimal.Decimal("0.0")
c = decimal.Decimal("2.0")
na = decimal.Decimal("NaN")
df = pd.DataFrame({
'key': [1, 1, 2, 2, 3, 3, 1, 4],
'val': DecimalArray([b, b, na, na, a, a, b, c])
})
result = df.groupby("key").agg({'val': 'first'})
print(result['val').dtype)
# objectProblem description
When performing a groupby().agg() on extension arrays DecimalArray or JsonArray, if a NULL value is present, the resulting aggregation will be converted to an object dtype.
Expected Output
Resulting series should be the same dtype as the input array, and any NULL values should be as defined in the extension dtype.
Output of pd.show_versions()
INSTALLED VERSIONS
commit : 1c296b2
python : 3.8.6.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-58-generic
Version : #64-Ubuntu SMP Wed Dec 9 08:16:25 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : C.UTF-8
LANG : C.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.3.0.dev0+357.g1c296b20f.dirty
numpy : 1.19.4
pytz : 2020.4
dateutil : 2.8.1
pip : 20.0.2
setuptools : 49.6.0.post20201009
Cython : 0.29.21
pytest : 6.1.2
hypothesis : 5.41.5
sphinx : 3.3.1
blosc : None
feather : None
xlsxwriter : 1.3.7
lxml.etree : 4.6.2
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.19.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.4
fastparquet : 0.4.1
gcsfs : 0.7.1
matplotlib : 3.3.3
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.5
pandas_gbq : None
pyarrow : 2.0.0
pyxlsb : None
s3fs : 0.4.2
scipy : 1.5.3
sqlalchemy : 1.3.20
tables : 3.6.1
tabulate : 0.8.7
xarray : 0.16.2
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.52.0