BUG: inconsistent returned types for pd.to_datetime(..., errors='coerce') #52094
Description
Pandas version checks
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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
>>> pd.__version__
'1.5.3'
s = ['2023-01-02', 'Mon Mar 20 11:00:00 UTC 2023', '', 'NaN', 'foo', float('NaN')]
>>> pd.to_datetime(s, errors='coerce')
Index([2023-01-02 00:00:00, 2023-03-20 11:00:00+00:00, 'NaT', 'NaT', NaT, nan], dtype='object')
>>> [type(x) for x in pd.to_datetime(s, errors='coerce')]
[datetime.datetime,
datetime.datetime,
str,
str,
pandas._libs.tslibs.nattype.NaTType,
float]Issue Description
According to the docs, errors='coerce' should convert entries to either a proper datetime, or NaT. Instead, we get a mix of:
datetime,float('NaN'),pandas._libs.tslibs.nattype.NaTType,'NaT'as string.
This makes it very difficult to have some consistent handling of the errors.
For example, instead of:
s = pd.Series(...)
res = pd.to_datetime(s, errors='coerce')
s.loc[res.isna()]to inspect the problematic input, we have to handle string 'NaT' as well as .isna(). The float NaN is also unexpected in that context.
Expected Behavior
The result should only contain NaT (as type pandas._libs.tslibs.nattype.NaTType) for entries where entries that failed to convert. All others should be valid datetime values. There should be no string 'NaN' nor float nan in the results.
A simple .isna() should indicate all the entries that were not successfully parsed and converted.
Installed Versions
/mnt/miniconda3/envs/test/lib/python3.10/site-packages/_distutils_hack/__init__.py:33: UserWarning: Setuptools is replacing distutils.
warnings.warn("Setuptools is replacing distutils.")
INSTALLED VERSIONS
------------------
commit : 2e218d10984e9919f0296931d92ea851c6a6faf5
python : 3.10.9.final.0
python-bits : 64
OS : Linux
OS-release : 4.14.281-212.502.amzn2.x86_64
Version : #1 SMP Thu May 26 09:52:17 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.5.3
numpy : 1.23.5
pytz : 2022.7
dateutil : 2.8.2
setuptools : 65.6.3
pip : 22.3.1
Cython : None
pytest : 7.1.2
hypothesis : 6.29.3
sphinx : 5.0.2
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.9.1
html5lib : 1.1
pymysql : None
psycopg2 : 2.9.3
jinja2 : 3.1.2
IPython : 8.10.0
pandas_datareader: 0.10.0
bs4 : 4.11.1
bottleneck : 1.3.5
brotli :
fastparquet : None
fsspec : 2022.11.0
gcsfs : None
matplotlib : 3.6.2
numba : 0.56.4
numexpr : 2.8.4
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : 8.0.0
pyreadstat : None
pyxlsb : None
s3fs : 2022.11.0
scipy : 1.10.0
snappy : None
sqlalchemy : 1.4.39
tables : 3.7.0
tabulate : None
xarray : 2022.11.0
xlrd : None
xlwt : None
zstandard : 0.19.0
tzdata : None