Question

您好我能找到这个问题的答案，但没有找到时区（http://stackoverflow.com/questions/1712116/formatting-yesterdays-date-in-python）

这对我来说很好：

>>> import time
>>> time.strftime('/%Z/%Y/%m/%d')
'/EST/2013/01/18'

但有没有办法获得昨天的约会？当我们从EST切换到EDT，从EDT切换到EST

时，我需要处理时区变化

datetime模块允许使用timedelta，但是天真的对象默认不支持时区，我不知道如何处理它。

Answer 1

自己做的方法，但我不知道是否有更好的方法

>>> import time
>>> from datetime import date, timedelta
>>> yesterday = date.today() - timedelta(1)
>>> yesterday = yesterday.strftime('%Y/%m/%d')
>>> yesterday = "/%s/%s" % ( time.tzname[0], yesterday )
>>> print yesterday
'/CET/2013/01/17'

Answer 2

对于会计时区和夏令时更改，datetime.datetime对象需要时区识别，即tzinfo属性不应为None。这可以通过继承tzinfo抽象类来完成。 ustimezone模块就是这样做的。

>>> import ustimezone
>>> from datetime import datetime, tzinfo, timedelta
>>> yesterday = datetime.now() - timedelta(days=1)
>>> yesterday
datetime.datetime(2013, 1, 17, 18, 24, 22, 106445)
>>> yesterday_aware = yesterday.replace(tzinfo=ustimezone.Eastern)
>>> yesterday.strftime('%Z/%Y/%m/%d')
'EST/2013/01/17'    # Will show EDT/EST depending on dst being present or not.

dst信息由timetuple的tm_isdst值捕获。

>>> yesterday_aware.timetuple()
time.struct_time(tm_year=2013, tm_mon=1, tm_mday=17, tm_hour=18, tm_min=24, tm_sec=22, tm_wday=3, tm_yday=17, tm_isdst=0)
>>> _.tm_isdst
>>> 0    # the tm_isdst flag of the time tuple can be used for checking if DST is present or not.

这里的ustimezone模块看起来像这样（借用Python datetime参考中的示例）

from datetime import datetime, tzinfo, timedelta

ZERO = timedelta(0)
HOUR = timedelta(hours=1)

# A complete implementation of current DST rules for major US time zones.

def first_sunday_on_or_after(dt):
    days_to_go = 6 - dt.weekday()
    if days_to_go:
        dt += timedelta(days_to_go)
    return dt


# US DST Rules
#
# This is a simplified (i.e., wrong for a few cases) set of rules for US
# DST start and end times. For a complete and up-to-date set of DST rules
# and timezone definitions, visit the Olson Database (or try pytz):
# http://www.twinsun.com/tz/tz-link.htm
# http://sourceforge.net/projects/pytz/ (might not be up-to-date)
#
# In the US, since 2007, DST starts at 2am (standard time) on the second
# Sunday in March, which is the first Sunday on or after Mar 8.
DSTSTART_2007 = datetime(1, 3, 8, 2)
# and ends at 2am (DST time; 1am standard time) on the first Sunday of Nov.
DSTEND_2007 = datetime(1, 11, 1, 1)
# From 1987 to 2006, DST used to start at 2am (standard time) on the first
# Sunday in April and to end at 2am (DST time; 1am standard time) on the last
# Sunday of October, which is the first Sunday on or after Oct 25.
DSTSTART_1987_2006 = datetime(1, 4, 1, 2)
DSTEND_1987_2006 = datetime(1, 10, 25, 1)
# From 1967 to 1986, DST used to start at 2am (standard time) on the last
# Sunday in April (the one on or after April 24) and to end at 2am (DST time;
# 1am standard time) on the last Sunday of October, which is the first Sunday
# on or after Oct 25.
DSTSTART_1967_1986 = datetime(1, 4, 24, 2)
DSTEND_1967_1986 = DSTEND_1987_2006

class USTimeZone(tzinfo):

    def __init__(self, hours, reprname, stdname, dstname):
        self.stdoffset = timedelta(hours=hours)
        self.reprname = reprname
        self.stdname = stdname
        self.dstname = dstname

    def __repr__(self):
        return self.reprname

    def tzname(self, dt):
        if self.dst(dt):
            return self.dstname
        else:
            return self.stdname

    def utcoffset(self, dt):
        return self.stdoffset + self.dst(dt)

    def dst(self, dt):
        if dt is None or dt.tzinfo is None:
            # An exception may be sensible here, in one or both cases.
            # It depends on how you want to treat them.  The default
            # fromutc() implementation (called by the default astimezone()
            # implementation) passes a datetime with dt.tzinfo is self.
            return ZERO
        assert dt.tzinfo is self

        # Find start and end times for US DST. For years before 1967, return
        # ZERO for no DST.
        if 2006 < dt.year:
            dststart, dstend = DSTSTART_2007, DSTEND_2007
        elif 1986 < dt.year < 2007:
            dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006
        elif 1966 < dt.year < 1987:
            dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986
        else:
            return ZERO

        start = first_sunday_on_or_after(dststart.replace(year=dt.year))
        end = first_sunday_on_or_after(dstend.replace(year=dt.year))

        # Can't compare naive to aware objects, so strip the timezone from
        # dt first.
        if start <= dt.replace(tzinfo=None) < end:
            return HOUR
        else:
            return ZERO

Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")

将昨天的日期与时区联系起来

2 个答案: