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1from datetime import tzinfo, timedelta, datetime
2
3ZERO = timedelta(0)
4HOUR = timedelta(hours=1)
5
6# A UTC class.
7
8class 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
20utc = 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
26class 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
44import time as _time
45
46STDOFFSET = timedelta(seconds = -_time.timezone)
47if _time.daylight:
48    DSTOFFSET = timedelta(seconds = -_time.altzone)
49else:
50    DSTOFFSET = STDOFFSET
51
52DSTDIFF = DSTOFFSET - STDOFFSET
53
54class 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, 0)
75        stamp = _time.mktime(tt)
76        tt = _time.localtime(stamp)
77        return tt.tm_isdst > 0
78
79Local = LocalTimezone()
80
81
82# A complete implementation of current DST rules for major US time zones.
83
84def 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.
101DSTSTART_2007 = datetime(1, 3, 8, 2)
102# and ends at 2am (DST time; 1am standard time) on the first Sunday of Nov.
103DSTEND_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.
107DSTSTART_1987_2006 = datetime(1, 4, 1, 2)
108DSTEND_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.
113DSTSTART_1967_1986 = datetime(1, 4, 24, 2)
114DSTEND_1967_1986 = DSTEND_1987_2006
115
116class 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
166Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")
167Central  = USTimeZone(-6, "Central",  "CST", "CDT")
168Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
169Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")
170