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tzinfo.py

#!/usr/bin/env python
'''$Id: tzinfo.py 540 2004-09-24 15:24:50Z jdh2358 $'''

__rcs_id__  = '$Id: tzinfo.py 540 2004-09-24 15:24:50Z jdh2358 $'
__version__ = '$Revision: 540 $'[11:-2]

from datetime import datetime, timedelta, tzinfo
from bisect import bisect_right
from sets import Set

_timedelta_cache = {}
def memorized_timedelta(seconds):
    '''Create only one instance of each distinct timedelta'''
    try:
        return _timedelta_cache[seconds]
    except KeyError:
        delta = timedelta(seconds=seconds)
        _timedelta_cache[seconds] = delta
        return delta

_datetime_cache = {}
def memorized_datetime(*args):
    '''Create only one instance of each distinct datetime'''
    try:
        return _datetime_cache[args]
    except KeyError:
        dt = datetime(*args)
        _datetime_cache[args] = dt
        return dt

_ttinfo_cache = {}
def memorized_ttinfo(*args):
    '''Create only one instance of each distinct tuple'''
    try:
        return _ttinfo_cache[args]
    except KeyError:
        ttinfo = (
                memorized_timedelta(args[0]),
                memorized_timedelta(args[1]),
                args[2]
                )
        _ttinfo_cache[args] = ttinfo
        return ttinfo

_notime = memorized_timedelta(0)

class BaseTzInfo(tzinfo):
    # Overridden in subclass
    _utcoffset = None
    _tzname = None
    _zone = None

    def __str__(self):
        return self._zone
    

00057 class StaticTzInfo(BaseTzInfo):
    '''A timezone that has a constant offset from UTC

    These timezones are rare, as most regions have changed their
    offset from UTC at some point in their history

    '''
00064     def fromutc(self, dt):
        '''See datetime.tzinfo.fromutc'''
        return (dt + self._utcoffset).replace(tzinfo=self)
    
00068     def utcoffset(self,dt):
        '''See datetime.tzinfo.utcoffset'''
        return self._utcoffset

00072     def dst(self,dt):
        '''See datetime.tzinfo.dst'''
        return _notime

00076     def tzname(self,dt):
        '''See datetime.tzinfo.tzname'''
        return self._tzname

00080     def localize(self, dt, is_dst=False):
        '''Convert naive time to local time'''
        if dt.tzinfo is not None:
            raise ValueError, 'Not naive datetime (tzinfo is already set)'
        return dt.replace(tzinfo=self)

00086     def normalize(self, dt, is_dst=False):
        '''Correct the timezone information on the given datetime'''
        if dt.tzinfo is None:
            raise ValueError, 'Naive time - no tzinfo set'
        return dt.replace(tzinfo=self)

    def __repr__(self):
        return '<StaticTzInfo %r>' % (self._zone,)


00096 class DstTzInfo(BaseTzInfo):
    '''A timezone that has a variable offset from UTC
   
    The offset might change if daylight savings time comes into effect,
    or at a point in history when the region decides to change their 
    timezone definition. 

    '''
    # Overridden in subclass
    _utc_transition_times = None # Sorted list of DST transition times in UTC
    _transition_info = None # [(utcoffset, dstoffset, tzname)] corresponding
                            # to _utc_transition_times entries
    _zone = None

    # Set in __init__
    _tzinfos = None
    _dst = None # DST offset

    def __init__(self, _inf=None, _tzinfos=None):
        if _inf:
            self._tzinfos = _tzinfos
            self._utcoffset, self._dst, self._tzname = _inf
        else:
            _tzinfos = {}
            self._tzinfos = _tzinfos
            self._utcoffset, self._dst, self._tzname = self._transition_info[0]
            _tzinfos[self._transition_info[0]] = self
            for inf in self._transition_info[1:]:
                if not _tzinfos.has_key(inf):
                    _tzinfos[inf] = self.__class__(inf, _tzinfos)

00127     def fromutc(self, dt):
        '''See datetime.tzinfo.fromutc'''
        dt = dt.replace(tzinfo=None)
        idx = max(0, bisect_right(self._utc_transition_times, dt) - 1)
        inf = self._transition_info[idx]
        return (dt + inf[0]).replace(tzinfo=self._tzinfos[inf])

00134     def normalize(self, dt):
        '''Correct the timezone information on the given datetime

        If date arithmetic crosses DST boundaries, the tzinfo
        is not magically adjusted. This method normalizes the
        tzinfo to the correct one.

        To test, first we need to do some setup

        >>> from pytz import timezone
        >>> utc = timezone('UTC')
        >>> eastern = timezone('US/Eastern')
        >>> fmt = '%Y-%m-%d %H:%M:%S %Z (%z)'

        We next create a datetime right on an end-of-DST transition point,
        the instant when the wallclocks are wound back one hour.

        >>> utc_dt = datetime(2002, 10, 27, 6, 0, 0, tzinfo=utc)
        >>> loc_dt = utc_dt.astimezone(eastern)
        >>> loc_dt.strftime(fmt)
        '2002-10-27 01:00:00 EST (-0500)'

        Now, if we subtract a few minutes from it, note that the timezone
        information has not changed.

        >>> before = loc_dt - timedelta(minutes=10)
        >>> before.strftime(fmt)
        '2002-10-27 00:50:00 EST (-0500)'

        But we can fix that by calling the normalize method

        >>> before = eastern.normalize(before)
        >>> before.strftime(fmt)
        '2002-10-27 01:50:00 EDT (-0400)'

        '''
        if dt.tzinfo is None:
            raise ValueError, 'Naive time - no tzinfo set'

        # Convert dt in localtime to UTC
        offset = dt.tzinfo._utcoffset
        dt = dt.replace(tzinfo=None)
        dt = dt - offset
        # convert it back, and return it
        return self.fromutc(dt)

00180     def localize(self, dt, is_dst=False):
        '''Convert naive time to local time.
        
        This method should be used to construct localtimes, rather
        than passing a tzinfo argument to a datetime constructor.

        is_dst is used to determine the correct timezone in the ambigous
        period at the end of daylight savings time.
        
        >>> from pytz import timezone
        >>> fmt = '%Y-%m-%d %H:%M:%S %Z (%z)'
        >>> amdam = timezone('Europe/Amsterdam')
        >>> dt  = datetime(2004, 10, 31, 2, 0, 0)
        >>> loc_dt1 = amdam.localize(dt, is_dst=True)
        >>> loc_dt2 = amdam.localize(dt, is_dst=False)
        >>> loc_dt1.strftime(fmt)
        '2004-10-31 02:00:00 CEST (+0200)'
        >>> loc_dt2.strftime(fmt)
        '2004-10-31 02:00:00 CET (+0100)'
        >>> str(loc_dt2 - loc_dt1)
        '1:00:00'

        Use is_dst=None to raise an AmbiguousTimeError for ambiguous
        times at the end of daylight savings

        >>> try:
        ...     loc_dt1 = amdam.localize(dt, is_dst=None)
        ... except AmbiguousTimeError:
        ...     print 'Oops'
        Oops

        >>> loc_dt1 = amdam.localize(dt, is_dst=None)
        Traceback (most recent call last):
            [...]
        AmbiguousTimeError: 2004-10-31 02:00:00

        is_dst defaults to False
        
        >>> amdam.localize(dt) == amdam.localize(dt, False)
        True

        '''
        if dt.tzinfo is not None:
            raise ValueError, 'Not naive datetime (tzinfo is already set)'

        # Find the possibly correct timezones. We probably just have one,
        # but we might end up with two if we are in the end-of-DST
        # transition period. Or possibly more in some particularly confused
        # location...
        possible_loc_dt = Set()
        for tzinfo in self._tzinfos.values():
            loc_dt = tzinfo.normalize(dt.replace(tzinfo=tzinfo))
            if loc_dt.replace(tzinfo=None) == dt:
                possible_loc_dt.add(loc_dt)

        if len(possible_loc_dt) == 1:
            return possible_loc_dt.pop()

        # If told to be strict, raise an exception since we have an
        # ambiguous case
        if is_dst is None:
            raise AmbiguousTimeError(dt)

        # Filter out the possiblilities that don't match the requested
        # is_dst
        filtered_possible_loc_dt = [
            p for p in possible_loc_dt
                if bool(p.tzinfo._dst) == is_dst
            ]

        # Hopefully we only have one possibility left. Return it.
        if len(filtered_possible_loc_dt) == 1:
            return filtered_possible_loc_dt[0]

        if len(filtered_possible_loc_dt) == 0:
            filtered_possible_loc_dt = list(possible_loc_dt)

        # If we get this far, we have in a wierd timezone transition
        # where the clocks have been wound back but is_dst is the same
        # in both (eg. Europe/Warsaw 1915 when they switched to CET).
        # At this point, we just have to guess unless we allow more
        # hints to be passed in (such as the UTC offset or abbreviation),
        # but that is just getting silly.
        #
        # Choose the earliest (by UTC) applicable timezone.
        def mycmp(a,b):
            return cmp(
                    a.replace(tzinfo=None) - a.tzinfo._utcoffset,
                    b.replace(tzinfo=None) - b.tzinfo._utcoffset,
                    )
        filtered_possible_loc_dt.sort(mycmp)
        return filtered_possible_loc_dt[0]
        
00273     def utcoffset(self, dt):
        '''See datetime.tzinfo.utcoffset'''
        return self._utcoffset

00277     def dst(self, dt):
        '''See datetime.tzinfo.dst'''
        return self._dst

00281     def tzname(self, dt):
        '''See datetime.tzinfo.tzname'''
        return self._tzname

    def __repr__(self):
        if self._dst:
            dst = 'DST'
        else:
            dst = 'STD'
        if self._utcoffset > _notime:
            return '<DstTzInfo %r %s+%s %s>' % (
                    self._zone, self._tzname, self._utcoffset, dst
                )
        else:
            return '<DstTzInfo %r %s%s %s>' % (
                    self._zone, self._tzname, self._utcoffset, dst
                )

00299 class AmbiguousTimeError(Exception):
    '''Exception raised when attempting to create an ambiguous wallclock time.

    At the end of a DST transition period, a particular wallclock time will
    occur twice (once before the clocks are set back, once after). Both
    possibilities may be correct, unless further information is supplied.

    See DstTzInfo.normalize() for more info

    '''
       


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