BUG: pd.TimeStamp & pd.Timedelta not always adhering to unit parameter in pandas-2.0

[galipremsagar]

Pandas version checks

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• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

>>> x = 340.3333333333333
>>> pd.Timestamp(x, unit='us')
Timestamp('1970-01-01 00:00:00.000340333')
>>> pd.Timestamp(x, unit='us').unit
'ns'
>>> pd.Timestamp(x, unit='us').as_unit('us')
Timestamp('1970-01-01 00:00:00.000340')
>>> pd.Timestamp(x, unit='us').as_unit('us').unit
'us'

Issue Description

When creating a pd.Timestamp & pd.Timedelta with a specific unit, the resulting Timestamp seems to be always of unit ns. We are having to perform as_unit('us') to retrieve the desired result.

Expected Behavior

>>> pd.Timestamp(x, unit='us')
Timestamp('1970-01-01 00:00:00.000340')

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 : 2023.2
dateutil : 2.8.2
setuptools : 67.6.0
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 : 8.11.0
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 : 3.1.2
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 : None
qtpy : None
pyqt5 : None

[jbrockmendel]

The "unit" keyword describes how the input is interpreted, not how the result is stored internally (which the .unit attribute represents). The overlapping naming is unfortunate.

cc @MarcoGorelli

[mroeschke]

I forgot if this was discussed, but is it possible if a user passes a numeric + unit to automatically also store the representation in that unit?

[jbrockmendel]

is it possible if a user passes a numeric + unit to automatically also store the representation in that unit?

For units that we support, yes. For minute and lower we'd default back to seconds. I'd be on board with this.

[mroeschke]

For units that we support, yes. For minute and lower we'd default back to seconds. I'd be on board with this.

Likewise, makes sense to me

[jbrockmendel]

OK with putting this in 2.0?

[mroeschke]

Yes definitely

[MarcoGorelli]

OK with putting this in 2.0?

totally - marking as blocker then if that's OK?

[jbrockmendel]

Implementing this I now remember why we didn't do it already: it introduces possibly-unwanted rounding in float cases. i.e Timestamp(1.5, unit="s) drops the half-second. We have 6 tests TestTimestamp::test_unit that this breaks.

[mroeschke]

it introduces possibly-unwanted rounding in float cases

Ah okay. Yeah I don't think specifying unit should potentially round the input. Maybe for this particular case then we should document that unit does not set the unit and users should use as_unit afterwards

[mroeschke]

totally - marking as blocker then if that's OK?

IMO I don't think this should be a strict blocker for 2.0 but a nice to fix/document

[MarcoGorelli]

sure, sounds good!

separate issue but float inputs to to_datetime feel like something we should consider deprecating anyway...removing the 'blocker' label anyway