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.
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
dst() should not return
The default implementation of utcoffset() raises NotImplementedError.
Noneif 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
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()
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)
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.
Noneif 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
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
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
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:
dt.tzinfois 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
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)).
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