django.utils.timezone.now()、datetime.datetime.now() 和 pytz.timezone 出现奇怪的输出

Question

我在尝试在 UTC 和特定时区之间进行转换时遇到了奇怪的行为。我希望有人能解释为什么我会看到这种行为，以及获取时区信息的更多 "correct" 方式可能是什么。

代码：

import pytz
import datetime
from django.utils import timezone

print(timezone.now())
print(pytz.utc.localize(datetime.datetime.now()))
print('\n')

def get_local_and_utc_date_ranges(days=1500, days_ago=2, local_timezone="America/Asuncion"):
    seller_timezone = pytz.timezone(local_timezone)
    utc_timezone = pytz.utc

    seller_today = timezone.now().astimezone(seller_timezone)
    seller_days_ago = seller_today - timezone.timedelta(days=days_ago)

    local_date_end = seller_days_ago.replace(hour=23, minute=59, second=59, microsecond=999999)
    local_date_start = (local_date_end - timezone.timedelta(days=days)).replace(hour=0, minute=0, second=0, microsecond=0)

    utc_date_end = local_date_end.astimezone(utc_timezone)
    utc_date_start = local_date_start.astimezone(utc_timezone)

    date_ranges = {
        "local_date_end": local_date_end,
        "local_date_start": local_date_start,
        "utc_date_end": utc_date_end,
        "utc_date_start": utc_date_start,
    }

    return date_ranges


def get_utc_and_local_date_ranges(days=1500, days_ago=2, local_timezone='America/Asuncion'):
    seller_timezone = pytz.timezone(local_timezone)
    utc_timezone = pytz.utc

    utc_today = datetime.datetime.utcnow()
    utc_days_ago = utc_today - datetime.timedelta(days=days_ago)

    local_date_end = seller_timezone.localize(utc_days_ago).replace(
        hour=23, minute=59, second=59, microsecond=999999
    )
    local_date_start = (local_date_end - datetime.timedelta(days=days)).replace(
        hour=0, minute=0, second=0, microsecond=0
    )
    utc_date_end = local_date_end.astimezone(utc_timezone)
    utc_date_start = local_date_start.astimezone(utc_timezone)

    date_ranges = {
        'local_date_end': local_date_end,
        'local_date_start': local_date_start,
        'utc_date_end': utc_date_end,
        'utc_date_start': utc_date_start,
    }

    return date_ranges



days = 1500
days_ago = 2

dates = get_local_and_utc_date_ranges(days=days, days_ago=days_ago)
dates2 = get_utc_and_local_date_ranges(days=days, days_ago=days_ago)


print('dates1:')
print('local_date_start:', dates['local_date_start'])
print('local_date_end:', dates['local_date_end'])
print('utc_date_start:', dates['utc_date_start'])
print('utc_date_end:', dates['utc_date_end'])
print('\n')

print('dates2:')
print('local_date_start:', dates2['local_date_start'])
print('local_date_end:', dates2['local_date_end'])
print('utc_date_start:', dates2['utc_date_start'])
print('utc_date_end:', dates2['utc_date_end'])
print('\n')

输出：

2019-03-25 18:57:55.929908+00:00
2019-03-25 18:57:55.930005+00:00


dates1:
local_date_start: 2015-02-12 00:00:00-04:00
local_date_end: 2019-03-23 23:59:59.999999-04:00
utc_date_start: 2015-02-12 04:00:00+00:00
utc_date_end: 2019-03-24 03:59:59.999999+00:00


dates2:
local_date_start: 2015-02-12 00:00:00-03:00
local_date_end: 2019-03-23 23:59:59.999999-03:00
utc_date_start: 2015-02-12 03:00:00+00:00
utc_date_end: 2019-03-24 02:59:59.999999+00:00

请注意不一致的 UTC 偏移量（该特定时区在 3 月 23 日切换为夏令时）。但是当我尝试使用以下代码复制问题时：

import pytz 
import datetime 
from django.utils import timezone 

now1 = timezone.now() - datetime.timedelta(days=2)
now2 = pytz.utc.localize(datetime.datetime.now()) - datetime.timedelta(days=2)

seller_timezone = pytz.timezone('America/Asuncion')

print(now1.astimezone(seller_timezone).replace(
        hour=23, minute=59, second=59, microsecond=999999
    ))
print(now2.astimezone(seller_timezone).replace(
        hour=23, minute=59, second=59, microsecond=999999
    ))

输出正确：

2019-03-23 23:59:59.999999-03:00
2019-03-23 23:59:59.999999-03:00

我希望有人能理解为什么会发生这种行为，如果是这样，我该如何避免这种不一致。

Answer 1

您的 get_local_and_utc_date_ranges() 函数产生了不正确的结果，因为它使用本地化时间进行日期时间算术运算（即减去 timedelta），这是行不通的。

seller_today = timezone.now().astimezone(seller_timezone)
seller_days_ago = seller_today - timezone.timedelta(days=days_ago)

这在 datetime module documentation 中注明：

As for addition, the result [of subtracting a timedelta] has the same tzinfo attribute as the input datetime, and no time zone adjustments are done even if the input is aware.

这在pytz documentation中也有注明：

If you perform date arithmetic on local times that cross DST boundaries, the result may be in an incorrect timezone.

pytz 提供修复：

A normalize() method is provided to correct this.

所以你可以使用：

seller_days_ago = seller_timezone.normalize(seller_today - timezone.timedelta(days=days_ago))
...
local_date_start = seller_timezone.normalize(local_date_end - timezone.timedelta(days=days)).replace(hour=0, minute=0, second=0, microsecond=0)

但是，文档 also notes that:

The preferred way of dealing with times is to always work in UTC.

所以更好的解决方案是只在 UTC 中进行算术运算：

utc_today = datetime.datetime.utcnow()
utc_date_end = utc_today - datetime.timedelta(days=days_ago)
utc_date_start = utc_date_end - datetime.timedelta(days=days)

local_date_end = seller_timezone.localize(utc_date_end).replace(hour=23, minute=59, second=59, microsecond=999999)
local_date_start = seller_timezone.localize(utc_date_start).replace(hour=0, minute=0, second=0, microsecond=0)

django.utils.timezone.now()、datetime.datetime.now() 和 pytz.timezone 出现奇怪的输出

Experiencing odd output from django.utils.timezone.now(), datetime.datetime.now() and pytz.timezone

django

python-3.x

datetime

timezone

timezone-offset