BUG: guess_datetime_format appears to be returning incomplete formats

[galipremsagar]

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Reproducible Example

>>> pd.core.tools.datetimes.guess_datetime_format("2019-07-16 00:00:00.333333333")
'%Y-%m-%d %H:%M:%S.%f'

Issue Description

The number of digits in the sub-second data appears to be missing. Ideally the format I was looking for was %Y-%m-%d %H:%M:%S.%9f'

Expected Behavior

>>> pd.core.tools.datetimes.guess_datetime_format("2019-07-16 00:00:00.333333333")
'%Y-%m-%d %H:%M:%S.%9f'
>>> pd.core.tools.datetimes.guess_datetime_format("2019-07-16 00:00:00.333333")
'%Y-%m-%d %H:%M:%S.%6f'

Installed Versions

INSTALLED VERSIONS

commit : c2a7f1a
python : 3.10.10.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-76-generic
Version : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.0.0rc1
numpy : 1.23.5
pytz : 2023.3
dateutil : 2.8.2
setuptools : 67.6.1
pip : 23.0.1
Cython : 0.29.33
pytest : 7.2.2
hypothesis : 6.70.1
sphinx : 5.3.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.12.0
pandas_datareader: None
bs4 : 4.12.0
bottleneck : None
brotli :
fastparquet : None
fsspec : 2023.3.0
gcsfs : None
matplotlib : None
numba : 0.56.4
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 11.0.0
pyreadstat : None
pyxlsb : None
s3fs : 2023.3.0
scipy : 1.10.1
snappy :
sqlalchemy : 1.4.46
tables : None
tabulate : 0.9.0
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[mroeschke]

I don't think guess_datetime_format ever supported returning the number of digits in subsecond data but I suppose that could be supported

cc @MarcoGorelli

[MarcoGorelli]

I don't think Python allows for %6f as a format: perhaps you're thinking of Rust?

In [10]: datetime.strptime('2019-07-16 00:00:00.333333', '%Y-%m-%d %H:%M:%S.%6f')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In [10], line 1
----> 1 datetime.strptime('2019-07-16 00:00:00.333333', '%Y-%m-%d %H:%M:%S.%6f')

File /usr/lib/python3.8/_strptime.py:568, in _strptime_datetime(cls, data_string, format)
    565 def _strptime_datetime(cls, data_string, format="%a %b %d %H:%M:%S %Y"):
    566     """Return a class cls instance based on the input string and the
    567     format string."""
--> 568     tt, fraction, gmtoff_fraction = _strptime(data_string, format)
    569     tzname, gmtoff = tt[-2:]
    570     args = tt[:6] + (fraction,)

File /usr/lib/python3.8/_strptime.py:341, in _strptime(data_string, format)
    339         bad_directive = "%"
    340     del err
--> 341     raise ValueError("'%s' is a bad directive in format '%s'" %
    342                         (bad_directive, format)) from None
    343 # IndexError only occurs when the format string is "%"
    344 except IndexError:

ValueError: '6' is a bad directive in format '%Y-%m-%d %H:%M:%S.%6f'

[mroeschke]

Yeah it appears that C++ can accept digits corresponding to microseconds which this method isn't intended to support, so closing as wont fix