Description
Pandas version checks
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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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I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd
c = pd.Categorical(pd.Series(["a", "b", "a"], dtype=pd.StringDtype()))
c.add_categories(pd.Series(["c", "d"], dtype=pd.StringDtype())).categories.dtype
>>> dtype('O')Issue Description
Hi,
I've noticed that when adding two different pd.Categorical objects (whose .categories.dtype == pd.StringDtype()), add_categories will downcast back to object.
Interestingly, this doesn't seem to happen with, for example, pd.Int64Dtype():
d = pd.Categorical(pd.Series([1, 2, 1], dtype=pd.Int64Dtype()))
d.add_categories(pd.Series([3, 4], dtype=pd.Int64Dtype())).categories.dtype
>>> dtype('int64')Thanks
Expected Behavior
import pandas as pd
c = pd.Categorical(pd.Series(["a", "b", "a"], dtype=pd.StringDtype()))
c.add_categories(pd.Series(["c", "d"], dtype=pd.StringDtype())).categories.dtype
>>> string[python]Installed Versions
INSTALLED VERSIONS
commit : 87cfe4e
python : 3.10.4.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19044
machine : AMD64
processor : Intel64 Family 6 Model 165 Stepping 3, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United Kingdom.1252
pandas : 1.5.0
numpy : 1.23.1
pytz : 2022.1
dateutil : 2.8.2
setuptools : 63.4.1
pip : 22.1.2
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 : 1.3.5
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : 2.8.3
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None