first version of some basic timezone utils which came in handy for a train scheduling application I wrote (etavta)

boltons/tzutils.py

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+# -*- coding: utf-8 -*-
+
+import time
+from datetime import tzinfo, timedelta, datetime
+
+from timeutils import total_seconds
+
+# Basic timezones cribbed from etavta Python docs. Beyond this, look
+# to pytz.
+
+ZERO = timedelta(0)
+HOUR = timedelta(hours=1)
+
+
+class ConstantTZ(tzinfo):
+    def __init__(self, name="ConstantTZ", offset=ZERO):
+        self.name = name
+        self.offset = offset
+
+    @property
+    def utcoffset_hours(self):
+        return total_seconds(self.offset) / (60 * 60)
+
+    def utcoffset(self, dt):
+        return self.offset
+
+    def tzname(self, dt):
+        return self.name
+
+    def dst(self, dt):
+        return ZERO
+
+    def __repr__(self):
+        cn = self.__class__.__name__
+        return '%s(name=%r, offset=%r)' % (cn, self.name, self.offset)
+
+
+UTC = ConstantTZ('UTC')
+
+
+class LocalTZ(tzinfo):
+    _std_offset = timedelta(seconds=-time.timezone)
+    _dst_offset = _std_offset
+    if time.daylight:
+        _dst_offset = timedelta(seconds=-time.altzone)
+
+    def is_dst(self, dt):
+        dt_t = dt.timetuple()[:-2] + (0, -1)
+        local_t = time.localtime(time.mktime(dt_t))
+        return local_t.tm_isdst > 0
+
+    def utcoffset(self, dt):
+        if self.is_dst(dt):
+            return self._dst_offset
+        return self._std_offset
+
+    def dst(self, dt):
+        if self.is_dst(dt):
+            return self._dst_offset - self._std_offset
+        return ZERO
+
+    def tzname(self, dt):
+        return time.tzname[self.is_dst(dt)]
+
+
+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")
+Central = USTimeZone(-6, "Central",  "CST", "CDT")
+Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
+Pacific = USTimeZone(-8, "Pacific",  "PST", "PDT")