如何使用 Pandas Timestamp fold 参数？

Question

在处理时区转换和夏令时影响时，我很难弄清楚 Pandas 构造函数的 fold 参数的实现。 documentation 提到：

Due to daylight saving time, one wall clock time can occur twice when shifting from summer to winter time; fold describes whether the datetime-like corresponds to the first (0) or the second time (1) the wall clock hits the ambiguous time.

到目前为止没有惊喜，但是当我运行下面的代码时：

import pandas as pd
from datetime import datetime

pre_fold = pd.Timestamp(datetime(2022,10,30,1,30,0), tz="CET")
in_fold_fold0 = pd.Timestamp(datetime(2022,10,30,2,30,0), tz="CET")
in_fold_fold1 = pd.Timestamp(datetime(2022,10,30,2,30,0), tz="CET", fold=1)
post_fold = pd.Timestamp(datetime(2022,10,30,3,30,0), tz="CET")

print(f"fold0: {in_fold_fold0.fold}")
print(f"fold1: {in_fold_fold1.fold}")

print(f"Pre CET fold:       {pre_fold}  ->  UTC {pre_fold.tz_convert(tz='UTC')}")
print(f"In CET fold, fold0: {in_fold_fold0}  ->  UTC {in_fold_fold0.tz_convert(tz='UTC')}")
print(f"In CET fold, fold1: {in_fold_fold1}  ->  UTC {in_fold_fold1.tz_convert(tz='UTC')}")
print(f"Post CET fold:      {post_fold}  ->  UTC {post_fold.tz_convert(tz='UTC')}")

输出不符合预期：

fold0: 0
fold1: 1
Pre CET fold:       2022-10-30 01:30:00+02:00  ->  UTC 2022-10-29 23:30:00+00:00
In CET fold, fold0: 2022-10-30 02:30:00+01:00  ->  UTC 2022-10-30 01:30:00+00:00
In CET fold, fold1: 2022-10-30 02:30:00+01:00  ->  UTC 2022-10-30 01:30:00+00:00
Post CET fold:      2022-10-30 03:30:00+01:00  ->  UTC 2022-10-30 02:30:00+00:00

第 4 行应该是：

In CET fold, fold0: 2022-10-30 02:30:00+02:00  ->  UTC 2022-10-30 00:30:00+00:00

我在这里错过了什么？

PS: 使用 python 的 datetime 对象会产生预期的输出：

from datetime import datetime
from dateutil import tz

dt_pre_fold = datetime(2022,10,30,1,30,0, tzinfo=tz.gettz("CET"))
dt_in_fold_fold0 = datetime(2022,10,30,2,30,0, tzinfo=tz.gettz("CET"))
dt_in_fold_fold1 = datetime(2022,10,30,2,30,0, tzinfo=tz.gettz("CET"), fold=1)
dt_post_fold = datetime(2022,10,30,3,30,0, tzinfo=tz.gettz("CET"))

print(f"Pre CET fold:       {dt_pre_fold}  ->  UTC {dt_pre_fold.astimezone(tz.gettz('UTC'))}")
print(f"In CET fold, fold0: {dt_in_fold_fold0}  ->  UTC {dt_in_fold_fold0.astimezone(tz.gettz('UTC'))}")
print(f"In CET fold, fold1: {dt_in_fold_fold1}  ->  UTC {dt_in_fold_fold1.astimezone(tz.gettz('UTC'))}")
print(f"Post CET fold:      {dt_post_fold}  ->  UTC {dt_post_fold.astimezone(tz.gettz('UTC'))}")

输出：

Pre CET fold:       2022-10-30 01:30:00+02:00  ->  UTC 2022-10-29 23:30:00+00:00
In CET fold, fold0: 2022-10-30 02:30:00+02:00  ->  UTC 2022-10-30 00:30:00+00:00
In CET fold, fold1: 2022-10-30 02:30:00+01:00  ->  UTC 2022-10-30 01:30:00+00:00
Post CET fold:      2022-10-30 03:30:00+01:00  ->  UTC 2022-10-30 02:30:00+00:00

Answer 1

似乎没有正确指定时区信息：

# using your code
x = pd.Timestamp(datetime(2022,10,30,2,30,0), fold = 0, tz="CET")
x.tz_convert('UTC')
# Timestamp('2022-10-30 01:30:00+0000', tz='UTC')

但是如果你使用from dateutil import tz

x = pd.Timestamp(datetime(2022,10,30,2,30,0), fold = 0, tz=tz.gettz("CET"))
x.tz_convert('UTC')
# Timestamp('2022-10-30 00:30:00+0000', tz='UTC')

它returns正确的值

Answer 2

这种绕过问题但不是使用 'fold'，您可以将时间戳本地化到某个时区并使用 ambiguous 关键字来指定它应该是 DST 还是non-DST次，从docs:

ambiguous [...] bool-ndarray where True signifies a DST time, False signifies a non-DST time (note that this flag is only applicable for ambiguous times)

所以你可以做你想做的事

import pandas as pd

f0 = pd.Timestamp("2022-10-30 02:30:00").tz_localize("Europe/Berlin", ambiguous=True)
f1 = pd.Timestamp("2022-10-30 02:30:00").tz_localize("Europe/Berlin", ambiguous=False)

print(f0.tz_convert('UTC'))
print(f1.tz_convert('UTC'))
# 2022-10-30 00:30:00+00:00 # was DST, UTC+2
# 2022-10-30 01:30:00+00:00 # was non-DST, UTC+1

至于为什么 'fold' 不能正常工作，我认为这可以追溯到 pandas 仍在内部使用 pytz 进行时区处理的事实。 pytz 不支持'fold'，而是使用关键字'is_dst'。您可以找到更多信息，例如在 this blog post by Paul Ganssle. There's also some hints deep in the pandas src。相比之下，dateutil 的时区确实支持 'fold'，这可能是 @ZLi 的解决方案有效的原因。

旁注：

最好使用实际的 IANA time zone names 以避免缩写可能产生的歧义
不要混合原生 Python 日期时间和 pandas' 日期时间，以避免原生 Python 日期时间

如何使用 Pandas Timestamp fold 参数？

How to use Pandas Timestamp fold argument?

python

datetime

timestamp

fold

pandas