##
5.1.6 `tzinfo` Objects

`tzinfo` is an abstract base clase, meaning that this class should not be instantiated directly. You need to derive a concrete subclass, and (at least) supply implementations of the standard `tzinfo` methods needed by the `datetime` methods you use. The `datetime` module does not supply any concrete subclasses of `tzinfo`.

An instance of (a concrete subclass of) `tzinfo` can be passed to the constructors for `datetime` and `time` objects. The latter objects view their members as being in local time, and the `tzinfo` object supports methods revealing offset of local time from UTC, the name of the time zone, and DST offset, all relative to a date or time object passed to them.

Special requirement for pickling: A `tzinfo` subclass must have an `__init__` method that can be called with no arguments, else it can be pickled but possibly not unpickled again. This is a technical requirement that may be relaxed in the future.

A concrete subclass of `tzinfo` may need to implement the following methods. Exactly which methods are needed depends on the uses made of aware `datetime` objects. If in doubt, simply implement all of them.

`utcoffset`( | self, dt)

  Return offset of local time from UTC, in minutes east of UTC. If local time is west of UTC, this should be negative. Note that this is intended to be the total offset from UTC; for example, if a `tzinfo` object represents both time zone and DST adjustments, `utcoffset()` should return their sum. If the UTC offset isn't known, return `None`. Else the value returned must be a `timedelta` object specifying a whole number of minutes in the range -1439 to 1439 inclusive (1440 = 24*60; the magnitude of the offset must be less than one day). Most implementations of `utcoffset()` will probably look like one of these two:

          return CONSTANT                 # fixed-offset class
          return CONSTANT + self.dst(dt)  # daylight-aware class

  If `utcoffset()` does not return `None`, `dst()` should not return `None` either.

  The default implementation of `utcoffset()` raises `NotImplementedError`.

`dst`( | self, dt)

  Return the daylight saving time (DST) adjustment, in minutes east of UTC, or `None` if DST information isn't known. Return `timedelta(0)` if DST is not in effect. If DST is in effect, return the offset as a `timedelta` object (see `utcoffset()` for details). Note that DST offset, if applicable, has already been added to the UTC offset returned by `utcoffset()`, so there's no need to consult `dst()` unless you're interested in obtaining DST info separately. For example, `datetime.timetuple()` calls its `tzinfo` member's `dst()` method to determine how the `tm_isdst` flag should be set, and `tzinfo.fromutc()` calls `dst()` to account for DST changes when crossing time zones.

  An instance tz of a `tzinfo` subclass that models both standard and daylight times must be consistent in this sense:

  `tz.utcoffset(dt) - tz.dst(dt)`

  must return the same result for every `datetime` dt with `dt.tzinfo == tz` For sane `tzinfo` subclasses, this expression yields the time zone's "standard offset", which should not depend on the date or the time, but only on geographic location. The implementation of `datetime.astimezone()` relies on this, but cannot detect violations; it's the programmer's responsibility to ensure it. If a `tzinfo` subclass cannot guarantee this, it may be able to override the default implementation of `tzinfo.fromutc()` to work correctly with `astimezone()` regardless.

  Most implementations of `dst()` will probably look like one of these two:

          def dst(self):
              # a fixed-offset class:  doesn't account for DST
              return timedelta(0)

  or

          def dst(self):
              # Code to set dston and dstoff to the time zone's DST
              # transition times based on the input dt.year, and expressed
              # in standard local time.  Then

              if dston <= dt.replace(tzinfo=None) < dstoff:
                  return timedelta(hours=1)
              else:
                  return timedelta(0)

  The default implementation of `dst()` raises `NotImplementedError`.

`tzname`( | self, dt)

  Return the time zone name corresponding to the `datetime` object dt, as a string. Nothing about string names is defined by the `datetime` module, and there's no requirement that it mean anything in particular. For example, "GMT", "UTC", "-500", "-5:00", "EDT", "US/Eastern", "America/New York" are all valid replies. Return `None` if a string name isn't known. Note that this is a method rather than a fixed string primarily because some `tzinfo` subclasses will wish to return different names depending on the specific value of dt passed, especially if the `tzinfo` class is accounting for daylight time.

  The default implementation of `tzname()` raises `NotImplementedError`.

These methods are called by a `datetime` or `time` object, in response to their methods of the same names. A `datetime` object passes itself as the argument, and a `time` object passes `None` as the argument. A `tzinfo` subclass's methods should therefore be prepared to accept a dt argument of `None`, or of class `datetime`.

When `None` is passed, it's up to the class designer to decide the best response. For example, returning `None` is appropriate if the class wishes to say that time objects don't participate in the `tzinfo` protocols. It may be more useful for `utcoffset(None)` to return the standard UTC offset, as there is no other convention for discovering the standard offset.

When a `datetime` object is passed in response to a `datetime` method, `dt.tzinfo` is the same object as self. `tzinfo` methods can rely on this, unless user code calls `tzinfo` methods directly. The intent is that the `tzinfo` methods interpret dt as being in local time, and not need worry about objects in other timezones.

There is one more `tzinfo` method that a subclass may wish to override:

`fromutc`( | self, dt)

  This is called from the default `datetime.astimezone()` implementation. When called from that, `dt.tzinfo` is self, and dt's date and time members are to be viewed as expressing a UTC time. The purpose of `fromutc()` is to adjust the date and time members, returning an equivalent datetime in self's local time.

  Most `tzinfo` subclasses should be able to inherit the default `fromutc()` implementation without problems. It's strong enough to handle fixed-offset time zones, and time zones accounting for both standard and daylight time, and the latter even if the DST transition times differ in different years. An example of a time zone the default `fromutc()` implementation may not handle correctly in all cases is one where the standard offset (from UTC) depends on the specific date and time passed, which can happen for political reasons. The default implementations of `astimezone()` and `fromutc()` may not produce the result you want if the result is one of the hours straddling the moment the standard offset changes.

  Skipping code for error cases, the default `fromutc()` implementation acts like:

        def fromutc(self, dt):
            # raise ValueError error if dt.tzinfo is not self
            dtoff = dt.utcoffset()
            dtdst = dt.dst()
            # raise ValueError if dtoff is None or dtdst is None
            delta = dtoff - dtdst  # this is self's standard offset
            if delta:
                dt += delta   # convert to standard local time
                dtdst = dt.dst()
                # raise ValueError if dtdst is None
            if dtdst:
                return dt + dtdst
            else:
                return dt

Example `tzinfo` classes:

    from datetime import tzinfo, timedelta, datetime

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

    # A UTC class.

    class UTC(tzinfo):
        """UTC"""

        def utcoffset(self, dt):
            return ZERO

        def tzname(self, dt):
            return "UTC"

        def dst(self, dt):
            return ZERO

    utc = UTC()

    # A class building tzinfo objects for fixed-offset time zones.
    # Note that FixedOffset(0, "UTC") is a different way to build a
    # UTC tzinfo object.

    class FixedOffset(tzinfo):
        """Fixed offset in minutes east from UTC."""

        def __init__(self, offset, name):
            self.__offset = timedelta(minutes = offset)
            self.__name = name

        def utcoffset(self, dt):
            return self.__offset

        def tzname(self, dt):
            return self.__name

        def dst(self, dt):
            return ZERO

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

    import time as _time

    STDOFFSET = timedelta(seconds = -_time.timezone)
    if _time.daylight:
        DSTOFFSET = timedelta(seconds = -_time.altzone)
    else:
        DSTOFFSET = STDOFFSET

    DSTDIFF = DSTOFFSET - STDOFFSET

    class LocalTimezone(tzinfo):

        def utcoffset(self, dt):
            if self._isdst(dt):
                return DSTOFFSET
            else:
                return STDOFFSET

        def dst(self, dt):
            if self._isdst(dt):
                return DSTDIFF
            else:
                return ZERO

        def tzname(self, dt):
            return _time.tzname[self._isdst(dt)]

        def _isdst(self, dt):
            tt = (dt.year, dt.month, dt.day,
                  dt.hour, dt.minute, dt.second,
                  dt.weekday(), 0, -1)
            stamp = _time.mktime(tt)
            tt = _time.localtime(stamp)
            return tt.tm_isdst > 0

    Local = LocalTimezone()

    # 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

    # In the US, DST starts at 2am (standard time) on the first Sunday in April.
    DSTSTART = datetime(1, 4, 1, 2)
    # and ends at 2am (DST time; 1am standard time) on the last Sunday of Oct.
    # which is the first Sunday on or after Oct 25.
    DSTEND = datetime(1, 10, 25, 1)

    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 first Sunday in April & the last in October.
            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")

Note that there are unavoidable subtleties twice per year in a `tzinfo` subclass accounting for both standard and daylight time, at the DST transition points. For concreteness, consider US Eastern (UTC -0500), where EDT begins the minute after 1:59 (EST) on the first Sunday in April, and ends the minute after 1:59 (EDT) on the last Sunday in October:

        UTC   3:MM  4:MM  5:MM  6:MM  7:MM  8:MM
        EST  22:MM 23:MM  0:MM  1:MM  2:MM  3:MM
        EDT  23:MM  0:MM  1:MM  2:MM  3:MM  4:MM

      start  22:MM 23:MM  0:MM  1:MM  3:MM  4:MM

        end  23:MM  0:MM  1:MM  1:MM  2:MM  3:MM

When DST starts (the "start" line), the local wall clock leaps from 1:59 to 3:00. A wall time of the form 2:MM doesn't really make sense on that day, so `astimezone(Eastern)` won't deliver a result with `hour == 2` on the day DST begins. In order for `astimezone()` to make this guarantee, the `rzinfo.dst()` method must consider times in the "missing hour" (2:MM for Eastern) to be in daylight time.

When DST ends (the "end" line), there's a potentially worse problem: there's an hour that can't be spelled unambiguously in local wall time: the last hour of daylight time. In Eastern, that's times of the form 5:MM UTC on the day daylight time ends. The local wall clock leaps from 1:59 (daylight time) back to 1:00 (standard time) again. Local times of the form 1:MM are ambiguous. `astimezone()` mimics the local clock's behavior by mapping two adjacent UTC hours into the same local hour then. In the Eastern example, UTC times of the form 5:MM and 6:MM both map to 1:MM when converted to Eastern. In order for `astimezone()` to make this guarantee, the `tzinfo.dst()` method must consider times in the "repeated hour" to be in standard time. This is easily arranged, as in the example, by expressing DST switch times in the time zone's standard local time.

Applications that can't bear such ambiguities should avoid using hybrid `tzinfo` subclasses; there are no ambiguities when using UTC, or any other fixed-offset `tzinfo` subclass (such as a class representing only EST (fixed offset -5 hours), or only EDT (fixed offset -4 hours)).