Pandas: Allow choosing the utc timezone class in pd.to_datetime

[vmarkovtsev]

Code Sample, a copy-pastable example if possible

from datetime import datetime, timezone
import pandas as pd

dt1 = pd.to_datetime(datetime(2020, 3, 11), utc=True)
print(repr(dt1))
print(type(dt1.tz))

dt2 = pd.to_datetime(datetime(2020, 3, 11, tzinfo=timezone.utc))
print(repr(dt2))
print(type(dt2.tz))

# dragons here
print(dt1 - dt2)

outputs

Timestamp('2020-03-11 00:00:00+0000', tz='UTC')
<class 'pytz.UTC'>
Timestamp('2020-03-11 00:00:00+0000', tz='UTC')
<class 'datetime.timezone'>

TypeError: Timestamp subtraction must have the same timezones or no timezones

Problem description

There is no ability to specify which "UTC" the Timestamp should be. I suggest extending the interface of pd.to_datetime() to specify utc_cls=pytz.UTC.

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-40-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.17.4
pytz             : 2019.2
dateutil         : 2.7.3
pip              : 19.3.1
setuptools       : 42.0.1
Cython           : 0.29.14
pytest           : 5.3.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.10.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.3.1
pyxlsb           : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.3.12
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

[TomAugspurger]

I don't think we need a separate keyword for this. I think we could accept utc=timezone.utc in addition to a boolean.

[se7entyse7en]

I think that the weird thing is that when calling tz_compare, that is called during Timestamp.__sub__, this returns False even if they're both utc. Infact both tzinfo objects described in the issue pass this check, but then tz_compare returns False given that their representation is different.

In [1]: from datetime import datetime, timezone

In [2]: import pandas as pd

In [3]: from pandas._libs.tslibs.timezones import tz_compare, is_utc

In [4]: dt1 = pd.to_datetime(datetime(2020, 3, 11), utc=True)

In [5]: dt2 = pd.to_datetime(datetime(2020, 3, 11, tzinfo=timezone.utc))

In [6]: is_utc(dt1.tzinfo), is_utc(dt2.tzinfo)
Out[6]: (True, True)

In [7]: tz_compare(dt1.tzinfo, dt2.tzinfo)
Out[7]: False

I think that it would be more correct to fix the behavior of tz_compare.

[TomAugspurger]

cc @mroeschke. I vaguely recall good reasons for them not comparing the same, but I don't recall them right now.

[mroeschke]

I believe that tz_compare has this behavior because we are trying to guard against inadvertently coercing the user's timezone object to a pytz timezone object (which is our "default" internally) after certain operations. An example is https://github.com/pandas-dev/pandas/pull/23807

Nonetheless, I think OP's example should work and just return a new Timestamp with a pytz.UTC timezone object (and better document our default timezone object behavior).

I can see modifying tz_compare to add a strict kwarg in which we can internally control cases like this.

[mroeschke]

I'd be -0.5 to allow UTC to accept non booleans as IMO it's idiomatic to use tz_convert after to_datetime

In [3]: pd.to_datetime(datetime(2020, 3, 11), utc=True).tz
Out[3]: <UTC>

In [5]: pd.to_datetime(datetime(2020, 3, 11), utc=True).tz_convert(timezone.utc).tz
Out[5]: datetime.timezone.utc

[jbrockmendel]

The bug here is in tz_compare not considering the two different UTCs as equivalent. I agree with @mroeschke that we shouldnt change the to_datetime API.

I would be +1 on changing our defaults from pytz to the stdlib tzinfo (and zoneinfo going forward)