我需要做什么

我有一个不了解时区的datetime对象,我需要向它添加一个时区,以便能够将它与其他了解时区的datetime对象进行比较。我不想将我的整个应用程序转换为不知道这个遗留情况的时区。

我的努力

首先,演示问题:

Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> import pytz
>>> unaware = datetime.datetime(2011,8,15,8,15,12,0)
>>> unaware
datetime.datetime(2011, 8, 15, 8, 15, 12)
>>> aware = datetime.datetime(2011,8,15,8,15,12,0,pytz.UTC)
>>> aware
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> aware == unaware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes

首先,我尝试了astimezone:

>>> unaware.astimezone(pytz.UTC)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: astimezone() cannot be applied to a naive datetime
>>>

这个失败并不奇怪,因为它实际上是在尝试进行转换。Replace似乎是一个更好的选择(如如何在Python中获得“时区感知”的datetime.today()值?):

>>> unaware.replace(tzinfo=pytz.UTC)
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> unaware == aware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes
>>> 

但正如您所看到的,replace似乎设置了tzinfo,但并没有使对象感知。我准备在解析输入字符串之前修改输入字符串以获得时区(如果有问题的话,我将使用dateutil进行解析),但这似乎非常笨拙。

此外,我在Python 2.6和Python 2.7中都尝试了这一点,得到了相同的结果。

上下文

I am writing a parser for some data files. There is an old format I need to support where the date string does not have a timezone indicator. I've already fixed the data source, but I still need to support the legacy data format. A one time conversion of the legacy data is not an option for various business BS reasons. While in general, I do not like the idea of hard-coding a default timezone, in this case it seems like the best option. I know with reasonable confidence that all the legacy data in question is in UTC, so I'm prepared to accept the risk of defaulting to that in this case.


当前回答

我在2011年写了这个Python 2脚本,但从未检查过它是否能在Python 3上运行。

我已经从dt_aware移动到dt_unconscious:

dt_unaware = dt_aware.replace(tzinfo=None)

dt_unware到dt_aware:

from pytz import timezone
localtz = timezone('Europe/Lisbon')
dt_aware = localtz.localize(dt_unware)

其他回答

一般来说,要使naive datetime具有时区感知,请使用localalize方法:

import datetime
import pytz

unaware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0)
aware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0, pytz.UTC)

now_aware = pytz.utc.localize(unaware)
assert aware == now_aware

对于UTC时区,实际上没有必要使用本地化,因为没有日光节约时间计算要处理:

now_aware = unaware.replace(tzinfo=pytz.UTC)

的工作原理。(.Replace返回一个新的日期时间;它不会无意识地修改。)

还有另一种让datetime对象不幼稚的方法:

>>> from datetime import datetime, timezone
>>> datetime.now(timezone.utc)
datetime.datetime(2021, 5, 1, 22, 51, 16, 219942, tzinfo=datetime.timezone.utc)

我在2011年写了这个Python 2脚本,但从未检查过它是否能在Python 3上运行。

我已经从dt_aware移动到dt_unconscious:

dt_unaware = dt_aware.replace(tzinfo=None)

dt_unware到dt_aware:

from pytz import timezone
localtz = timezone('Europe/Lisbon')
dt_aware = localtz.localize(dt_unware)

所有这些例子都使用了一个外部模块,但你可以只使用datetime模块来实现相同的结果,正如下面的SO答案所示:

from datetime import datetime, timezone

dt = datetime.now()
dt = dt.replace(tzinfo=timezone.utc)

print(dt.isoformat())
# '2017-01-12T22:11:31+00:00'

更少的依赖,没有pytz问题。

注意:如果你想在python3和python2中使用它,你也可以在时区导入中使用它(为UTC硬编码):

try:
    from datetime import timezone
    utc = timezone.utc
except ImportError:
    #Hi there python2 user
    class UTC(tzinfo):
        def utcoffset(self, dt):
            return timedelta(0)
        def tzname(self, dt):
            return "UTC"
        def dst(self, dt):
            return timedelta(0)
    utc = UTC()

这里有一个简单的解决方案,以尽量减少对代码的更改:

from datetime import datetime
import pytz

start_utc = datetime.utcnow()
print ("Time (UTC): %s" % start_utc.strftime("%d-%m-%Y %H:%M:%S"))

时间(UTC): 09-01-2021 03:49:03

tz = pytz.timezone('Africa/Cairo')
start_tz = datetime.now().astimezone(tz)
print ("Time (RSA): %s" % start_tz.strftime("%d-%m-%Y %H:%M:%S"))

时间(RSA): 09-01-2021 05:49:03