1 from datetime import tzinfo, timedelta, datetime

     2 

     3 ZERO = timedelta(0)

     4 HOUR = timedelta(hours=1)

     5 

     6 # A UTC class.

     7 

     8 class UTC(tzinfo):

     9     """UTC"""

    10 

    11     def utcoffset(self, dt):

    12         return ZERO

    13 

    14     def tzname(self, dt):

    15         return "UTC"

    16 

    17     def dst(self, dt):

    18         return ZERO

    19 

    20 utc = UTC()

    21 

    22 # A class building tzinfo objects for fixed-offset time zones.

    23 # Note that FixedOffset(0, "UTC") is a different way to build a

    24 # UTC tzinfo object.

    25 

    26 class FixedOffset(tzinfo):

    27     """Fixed offset in minutes east from UTC."""

    28 

    29     def __init__(self, offset, name):

    30         self.__offset = timedelta(minutes = offset)

    31         self.__name = name

    32 

    33     def utcoffset(self, dt):

    34         return self.__offset

    35 

    36     def tzname(self, dt):

    37         return self.__name

    38 

    39     def dst(self, dt):

    40         return ZERO

    41 

    42 # A class capturing the platform's idea of local time.

    43 

    44 import time as _time

    45 

    46 STDOFFSET = timedelta(seconds = -_time.timezone)

    47 if _time.daylight:

    48     DSTOFFSET = timedelta(seconds = -_time.altzone)

    49 else:

    50     DSTOFFSET = STDOFFSET

    51 

    52 DSTDIFF = DSTOFFSET - STDOFFSET

    53 

    54 class LocalTimezone(tzinfo):

    55 

    56     def utcoffset(self, dt):

    57         if self._isdst(dt):

    58             return DSTOFFSET

    59         else:

    60             return STDOFFSET

    61 

    62     def dst(self, dt):

    63         if self._isdst(dt):

    64             return DSTDIFF

    65         else:

    66             return ZERO

    67 

    68     def tzname(self, dt):

    69         return _time.tzname[self._isdst(dt)]

    70 

    71     def _isdst(self, dt):

    72         tt = (dt.year, dt.month, dt.day,

    73               dt.hour, dt.minute, dt.second,

    74               dt.weekday(), 0, -1)

    75         stamp = _time.mktime(tt)

    76         tt = _time.localtime(stamp)

    77         return tt.tm_isdst > 0

    78 

    79 Local = LocalTimezone()

    80 

    81 

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

    83 

    84 def first_sunday_on_or_after(dt):

    85     days_to_go = 6 - dt.weekday()

    86     if days_to_go:

    87         dt += timedelta(days_to_go)

    88     return dt

    89 

    90 

    91 # US DST Rules

    92 #

    93 # This is a simplified (i.e., wrong for a few cases) set of rules for US

    94 # DST start and end times. For a complete and up-to-date set of DST rules

    95 # and timezone definitions, visit the Olson Database (or try pytz):

    96 # http://www.twinsun.com/tz/tz-link.htm

    97 # http://sourceforge.net/projects/pytz/ (might not be up-to-date)

    98 #

    99 # In the US, since 2007, DST starts at 2am (standard time) on the second

   100 # Sunday in March, which is the first Sunday on or after Mar 8.

   101 DSTSTART_2007 = datetime(1, 3, 8, 2)

   102 # and ends at 2am (DST time; 1am standard time) on the first Sunday of Nov.

   103 DSTEND_2007 = datetime(1, 11, 1, 1)

   104 # From 1987 to 2006, DST used to start at 2am (standard time) on the first

   105 # Sunday in April and to end at 2am (DST time; 1am standard time) on the last

   106 # Sunday of October, which is the first Sunday on or after Oct 25.

   107 DSTSTART_1987_2006 = datetime(1, 4, 1, 2)

   108 DSTEND_1987_2006 = datetime(1, 10, 25, 1)

   109 # From 1967 to 1986, DST used to start at 2am (standard time) on the last

   110 # Sunday in April (the one on or after April 24) and to end at 2am (DST time;

   111 # 1am standard time) on the last Sunday of October, which is the first Sunday

   112 # on or after Oct 25.

   113 DSTSTART_1967_1986 = datetime(1, 4, 24, 2)

   114 DSTEND_1967_1986 = DSTEND_1987_2006

   115 

   116 class USTimeZone(tzinfo):

   117 

   118     def __init__(self, hours, reprname, stdname, dstname):

   119         self.stdoffset = timedelta(hours=hours)

   120         self.reprname = reprname

   121         self.stdname = stdname

   122         self.dstname = dstname

   123 

   124     def __repr__(self):

   125         return self.reprname

   126 

   127     def tzname(self, dt):

   128         if self.dst(dt):

   129             return self.dstname

   130         else:

   131             return self.stdname

   132 

   133     def utcoffset(self, dt):

   134         return self.stdoffset + self.dst(dt)

   135 

   136     def dst(self, dt):

   137         if dt is None or dt.tzinfo is None:

   138             # An exception may be sensible here, in one or both cases.

   139             # It depends on how you want to treat them.  The default

   140             # fromutc() implementation (called by the default astimezone()

   141             # implementation) passes a datetime with dt.tzinfo is self.

   142             return ZERO

   143         assert dt.tzinfo is self

   144 

   145         # Find start and end times for US DST. For years before 1967, return

   146         # ZERO for no DST.

   147         if 2006 < dt.year:

   148             dststart, dstend = DSTSTART_2007, DSTEND_2007

   149         elif 1986 < dt.year < 2007:

   150             dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006

   151         elif 1966 < dt.year < 1987:

   152             dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986

   153         else:

   154             return ZERO

   155 

   156         start = first_sunday_on_or_after(dststart.replace(year=dt.year))

   157         end = first_sunday_on_or_after(dstend.replace(year=dt.year))

   158 

   159         # Can't compare naive to aware objects, so strip the timezone from

   160         # dt first.

   161         if start <= dt.replace(tzinfo=None) < end:

   162             return HOUR

   163         else:

   164             return ZERO

   165 

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

   167 Central  = USTimeZone(-6, "Central",  "CST", "CDT")

   168 Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")

   169 Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")