Radicale/radicale_vobject/icalendar.py

"""Definitions and behavior for iCalendar, also known as vCalendar 2.0"""

from __future__ import print_function

import datetime
import logging
import random  # for generating a UID
import socket
import string
import base64

from dateutil import rrule, tz
import six

try:
    import pytz
except ImportError:
    class Pytz:
        """fake pytz module (pytz is not required)"""

        class AmbiguousTimeError(Exception):
            """pytz error for ambiguous times
               during transition daylight->standard"""

        class NonExistentTimeError(Exception):
            """pytz error for non-existent times
               during transition standard->daylight"""

    pytz = Pytz  # keeps quantifiedcode happy

from . import behavior
from .base import (VObjectError, NativeError, ValidateError, ParseError,
                   Component, ContentLine, logger, registerBehavior,
                   backslashEscape, foldOneLine)


# ------------------------------- Constants ------------------------------------
DATENAMES = ("rdate", "exdate")
RULENAMES = ("exrule", "rrule")
DATESANDRULES = ("exrule", "rrule", "rdate", "exdate")
PRODID = u"-//PYVOBJECT//NONSGML Version 1//EN"

WEEKDAYS = "MO", "TU", "WE", "TH", "FR", "SA", "SU"
FREQUENCIES = ('YEARLY', 'MONTHLY', 'WEEKLY', 'DAILY', 'HOURLY', 'MINUTELY',
               'SECONDLY')

zeroDelta = datetime.timedelta(0)
twoHours = datetime.timedelta(hours=2)


# ---------------------------- TZID registry -----------------------------------
__tzidMap = {}


def toUnicode(s):
    """
    Take a string or unicode, turn it into unicode, decoding as utf-8
    """
    if isinstance(s, six.binary_type):
        s = s.decode('utf-8')
    return s


def registerTzid(tzid, tzinfo):
    """
    Register a tzid -> tzinfo mapping.
    """
    __tzidMap[toUnicode(tzid)] = tzinfo


def getTzid(tzid, smart=True):
    """
    Return the tzid if it exists, or None.
    """
    tz = __tzidMap.get(toUnicode(tzid), None)
    if smart and tzid and not tz:
        try:
            from pytz import timezone, UnknownTimeZoneError
            try:
                tz = timezone(tzid)
                registerTzid(toUnicode(tzid), tz)
            except UnknownTimeZoneError as e:
                logging.error(e)
        except ImportError as e:
            logging.error(e)
    return tz

utc = tz.tzutc()
registerTzid("UTC", utc)


# -------------------- Helper subclasses ---------------------------------------
class TimezoneComponent(Component):
    """
    A VTIMEZONE object.

    VTIMEZONEs are parsed by tz.tzical, the resulting datetime.tzinfo
    subclass is stored in self.tzinfo, self.tzid stores the TZID associated
    with this timezone.

    @ivar name:
        The uppercased name of the object, in this case always 'VTIMEZONE'.
    @ivar tzinfo:
        A datetime.tzinfo subclass representing this timezone.
    @ivar tzid:
        The string used to refer to this timezone.
    """
    def __init__(self, tzinfo=None, *args, **kwds):
        """
        Accept an existing Component or a tzinfo class.
        """
        super(TimezoneComponent, self).__init__(*args, **kwds)
        self.isNative = True
        # hack to make sure a behavior is assigned
        if self.behavior is None:
            self.behavior = VTimezone
        if tzinfo is not None:
            self.tzinfo = tzinfo
        if not hasattr(self, 'name') or self.name == '':
            self.name = 'VTIMEZONE'
            self.useBegin = True

    @classmethod
    def registerTzinfo(obj, tzinfo):
        """
        Register tzinfo if it's not already registered, return its tzid.
        """
        tzid = obj.pickTzid(tzinfo)
        if tzid and not getTzid(tzid, False):
            registerTzid(tzid, tzinfo)
        return tzid

    def gettzinfo(self):
        # workaround for dateutil failing to parse some experimental properties
        good_lines = ('rdate', 'rrule', 'dtstart', 'tzname', 'tzoffsetfrom',
                      'tzoffsetto', 'tzid')
        # serialize encodes as utf-8, cStringIO will leave utf-8 alone
        buffer = six.StringIO()
        # allow empty VTIMEZONEs
        if len(self.contents) == 0:
            return None

        def customSerialize(obj):
            if isinstance(obj, Component):
                foldOneLine(buffer, u"BEGIN:" + obj.name)
                for child in obj.lines():
                    if child.name.lower() in good_lines:
                        child.serialize(buffer, 75, validate=False)
                for comp in obj.components():
                    customSerialize(comp)
                foldOneLine(buffer, u"END:" + obj.name)
        customSerialize(self)
        buffer.seek(0)  # tzical wants to read a stream
        return tz.tzical(buffer).get()

    def settzinfo(self, tzinfo, start=2000, end=2030):
        """
        Create appropriate objects in self to represent tzinfo.

        Collapse DST transitions to rrules as much as possible.

        Assumptions:
        - DST <-> Standard transitions occur on the hour
        - never within a month of one another
        - twice or fewer times a year
        - never in the month of December
        - DST always moves offset exactly one hour later
        - tzinfo classes dst method always treats times that could be in either
          offset as being in the later regime
        """
        def fromLastWeek(dt):
            """
            How many weeks from the end of the month dt is, starting from 1.
            """
            weekDelta = datetime.timedelta(weeks=1)
            n = 1
            current = dt + weekDelta
            while current.month == dt.month:
                n += 1
                current += weekDelta
            return n

        # lists of dictionaries defining rules which are no longer in effect
        completed = {'daylight': [], 'standard': []}

        # dictionary defining rules which are currently in effect
        working = {'daylight': None, 'standard': None}

        # rule may be based on nth week of the month or the nth from the last
        for year in range(start, end + 1):
            newyear = datetime.datetime(year, 1, 1)
            for transitionTo in 'daylight', 'standard':
                transition = getTransition(transitionTo, year, tzinfo)
                oldrule = working[transitionTo]

                if transition == newyear:
                    # transitionTo is in effect for the whole year
                    rule = {'end'        : None,
                            'start'      : newyear,
                            'month'      : 1,
                            'weekday'    : None,
                            'hour'       : None,
                            'plus'       : None,
                            'minus'      : None,
                            'name'       : tzinfo.tzname(newyear),
                            'offset'     : tzinfo.utcoffset(newyear),
                            'offsetfrom' : tzinfo.utcoffset(newyear)}
                    if oldrule is None:
                        # transitionTo was not yet in effect
                        working[transitionTo] = rule
                    else:
                        # transitionTo was already in effect
                        if (oldrule['offset'] != tzinfo.utcoffset(newyear)):
                            # old rule was different, it shouldn't continue
                            oldrule['end'] = year - 1
                            completed[transitionTo].append(oldrule)
                            working[transitionTo] = rule
                elif transition is None:
                    # transitionTo is not in effect
                    if oldrule is not None:
                        # transitionTo used to be in effect
                        oldrule['end'] = year - 1
                        completed[transitionTo].append(oldrule)
                        working[transitionTo] = None
                else:
                    # an offset transition was found
                    try:
                        old_offset = tzinfo.utcoffset(transition - twoHours)
                        name = tzinfo.tzname(transition)
                        offset = tzinfo.utcoffset(transition)
                    except (pytz.AmbiguousTimeError, pytz.NonExistentTimeError):
                        # guaranteed that tzinfo is a pytz timezone
                        is_dst = (transitionTo == "daylight")
                        old_offset = tzinfo.utcoffset(transition - twoHours, is_dst=is_dst)
                        name = tzinfo.tzname(transition, is_dst=is_dst)
                        offset = tzinfo.utcoffset(transition, is_dst=is_dst)
                    rule = {'end'     : None,  # None, or an integer year
                            'start'   : transition,  # the datetime of transition
                            'month'   : transition.month,
                            'weekday' : transition.weekday(),
                            'hour'    : transition.hour,
                            'name'    : name,
                            'plus'    : int(
                                (transition.day - 1)/ 7 + 1),  # nth week of the month
                            'minus'   : fromLastWeek(transition),  # nth from last week
                            'offset'  : offset,
                            'offsetfrom' : old_offset}

                    if oldrule is None:
                        working[transitionTo] = rule
                    else:
                        plusMatch = rule['plus'] == oldrule['plus']
                        minusMatch = rule['minus'] == oldrule['minus']
                        truth = plusMatch or minusMatch
                        for key in 'month', 'weekday', 'hour', 'offset':
                            truth = truth and rule[key] == oldrule[key]
                        if truth:
                            # the old rule is still true, limit to plus or minus
                            if not plusMatch:
                                oldrule['plus'] = None
                            if not minusMatch:
                                oldrule['minus'] = None
                        else:
                            # the new rule did not match the old
                            oldrule['end'] = year - 1
                            completed[transitionTo].append(oldrule)
                            working[transitionTo] = rule

        for transitionTo in 'daylight', 'standard':
            if working[transitionTo] is not None:
                completed[transitionTo].append(working[transitionTo])

        self.tzid = []
        self.daylight = []
        self.standard = []

        self.add('tzid').value = self.pickTzid(tzinfo, True)

        # old = None # unused?
        for transitionTo in 'daylight', 'standard':
            for rule in completed[transitionTo]:
                comp = self.add(transitionTo)
                dtstart = comp.add('dtstart')
                dtstart.value = rule['start']
                if rule['name'] is not None:
                    comp.add('tzname').value = rule['name']
                line = comp.add('tzoffsetto')
                line.value = deltaToOffset(rule['offset'])
                line = comp.add('tzoffsetfrom')
                line.value = deltaToOffset(rule['offsetfrom'])

                if rule['plus'] is not None:
                    num = rule['plus']
                elif rule['minus'] is not None:
                    num = -1 * rule['minus']
                else:
                    num = None
                if num is not None:
                    dayString = ";BYDAY=" + str(num) + WEEKDAYS[rule['weekday']]
                else:
                    dayString = ""
                if rule['end'] is not None:
                    if rule['hour'] is None:
                        # all year offset, with no rule
                        endDate = datetime.datetime(rule['end'], 1, 1)
                    else:
                        weekday = rrule.weekday(rule['weekday'], num)
                        du_rule = rrule.rrule(rrule.YEARLY,
                            bymonth=rule['month'], byweekday=weekday,
                            dtstart=datetime.datetime(
                               rule['end'], 1, 1, rule['hour']
                            )
                        )
                        endDate = du_rule[0]
                    endDate = endDate.replace(tzinfo=utc) - rule['offsetfrom']
                    endString = ";UNTIL=" + dateTimeToString(endDate)
                else:
                    endString = ''
                new_rule = "FREQ=YEARLY{0!s};BYMONTH={1!s}{2!s}"\
                    .format(dayString, rule['month'], endString)

                comp.add('rrule').value = new_rule

    tzinfo = property(gettzinfo, settzinfo)
    # prevent Component's __setattr__ from overriding the tzinfo property
    normal_attributes = Component.normal_attributes + ['tzinfo']

    @staticmethod
    def pickTzid(tzinfo, allowUTC=False):
        """
        Given a tzinfo class, use known APIs to determine TZID, or use tzname.
        """
        if tzinfo is None or (not allowUTC and tzinfo_eq(tzinfo, utc)):
            # If tzinfo is UTC, we don't need a TZID
            return None
        # try PyICU's tzid key
        if hasattr(tzinfo, 'tzid'):
            return toUnicode(tzinfo.tzid)

        # try pytz zone key
        if hasattr(tzinfo, 'zone'):
            return toUnicode(tzinfo.zone)

        # try tzical's tzid key
        elif hasattr(tzinfo, '_tzid'):
            return toUnicode(tzinfo._tzid)
        else:
            # return tzname for standard (non-DST) time
            notDST = datetime.timedelta(0)
            for month in range(1, 13):
                dt = datetime.datetime(2000, month, 1)
                if tzinfo.dst(dt) == notDST:
                    return toUnicode(tzinfo.tzname(dt))
        # there was no standard time in 2000!
        raise VObjectError("Unable to guess TZID for tzinfo {0!s}"
                           .format(tzinfo))

    def __str__(self):
        return "<VTIMEZONE | {0}>".format(getattr(self, 'tzid', 'No TZID'))

    def __repr__(self):
        return self.__str__()

    def prettyPrint(self, level, tabwidth):
        pre = ' ' * level * tabwidth
        print(pre, self.name)
        print(pre, "TZID:", self.tzid)
        print('')


class RecurringComponent(Component):
    """
    A vCalendar component like VEVENT or VTODO which may recur.

    Any recurring component can have one or multiple RRULE, RDATE,
    EXRULE, or EXDATE lines, and one or zero DTSTART lines.  It can also have a
    variety of children that don't have any recurrence information.

    In the example below, note that dtstart is included in the rruleset.
    This is not the default behavior for dateutil's rrule implementation unless
    dtstart would already have been a member of the recurrence rule, and as a
    result, COUNT is wrong. This can be worked around when getting rruleset by
    adjusting count down by one if an rrule has a count and dtstart isn't in its
    result set, but by default, the rruleset property doesn't do this work
    around, to access it getrruleset must be called with addRDate set True.

    @ivar rruleset:
        A U{rruleset<https://moin.conectiva.com.br/DateUtil>}.
    """
    def __init__(self, *args, **kwds):
        super(RecurringComponent, self).__init__(*args, **kwds)

        self.isNative = True

    def getrruleset(self, addRDate=False):
        """
        Get an rruleset created from self.

        If addRDate is True, add an RDATE for dtstart if it's not included in
        an RRULE or RDATE, and count is decremented if it exists.

        Note that for rules which don't match DTSTART, DTSTART may not appear
        in list(rruleset), although it should.  By default, an RDATE is not
        created in these cases, and count isn't updated, so dateutil may list
        a spurious occurrence.
        """
        rruleset = None
        for name in DATESANDRULES:
            addfunc = None
            for line in self.contents.get(name, ()):
                # don't bother creating a rruleset unless there's a rule
                if rruleset is None:
                    rruleset = rrule.rruleset()
                if addfunc is None:
                    addfunc = getattr(rruleset, name)

                try:
                    dtstart = self.dtstart.value
                except (AttributeError, KeyError):
                    # Special for VTODO - try DUE property instead
                    try:
                        if self.name == "VTODO":
                            dtstart = self.due.value
                        else:
                            # if there's no dtstart, just return None
                            logging.error('failed to get dtstart with VTODO')
                            return None
                    except (AttributeError, KeyError):
                        # if there's no due, just return None
                        logging.error('failed to find DUE at all.')
                        return None

                if name in DATENAMES:
                    if type(line.value[0]) == datetime.datetime:
                        list(map(addfunc, line.value))
                    elif type(line.value[0]) == datetime.date:
                        for dt in line.value:
                            addfunc(datetime.datetime(dt.year, dt.month, dt.day))
                    else:
                        # ignore RDATEs with PERIOD values for now
                        pass
                elif name in RULENAMES:
                    # a Ruby iCalendar library escapes semi-colons in rrules,
                    # so also remove any backslashes
                    value = line.value.replace('\\', '')
                    # If dtstart has no time zone, `until`
                    # shouldn't get one, either:
                    ignoretz = (not isinstance(dtstart, datetime.datetime) or
                                dtstart.tzinfo is None)
                    try:
                        until = rrule.rrulestr(value, ignoretz=ignoretz)._until
                    except ValueError:
                        # WORKAROUND: dateutil<=2.7.2 doesn't set the time zone
                        # of dtstart
                        if ignoretz:
                            raise
                        utc_now = datetime.datetime.now(datetime.timezone.utc)
                        until = rrule.rrulestr(value, dtstart=utc_now)._until

                    if until is not None and isinstance(dtstart,
                                                        datetime.datetime) and \
                            (until.tzinfo != dtstart.tzinfo):
                        # dateutil converts the UNTIL date to a datetime,
                        # check to see if the UNTIL parameter value was a date
                        vals = dict(pair.split('=') for pair in
                                    value.upper().split(';'))
                        if len(vals.get('UNTIL', '')) == 8:
                            until = datetime.datetime.combine(until.date(),
                                                              dtstart.time())
                        # While RFC2445 says UNTIL MUST be UTC, Chandler allows
                        # floating recurring events, and uses floating UNTIL
                        # values. Also, some odd floating UNTIL but timezoned
                        # DTSTART values have shown up in the wild, so put
                        # floating UNTIL values DTSTART's timezone
                        if until.tzinfo is None:
                            until = until.replace(tzinfo=dtstart.tzinfo)

                        if dtstart.tzinfo is not None:
                            until = until.astimezone(dtstart.tzinfo)

                        # RFC2445 actually states that UNTIL must be a UTC
                        # value. Whilst the changes above work OK, one problem
                        # case is if DTSTART is floating but UNTIL is properly
                        # specified as UTC (or with a TZID). In that case
                        # dateutil will fail datetime comparisons. There is no
                        # easy solution to this as there is no obvious timezone
                        # (at this point) to do proper floating time offset
                        # comparisons. The best we can do is treat the UNTIL
                        # value as floating. This could mean incorrect
                        # determination of the last instance. The better
                        # solution here is to encourage clients to use COUNT
                        # rather than UNTIL when DTSTART is floating.
                        if dtstart.tzinfo is None:
                            until = until.replace(tzinfo=None)

                    value_without_until = ';'.join(
                        pair for pair in value.split(';')
                        if pair.split('=')[0].upper() != 'UNTIL')
                    rule = rrule.rrulestr(value_without_until,
                                          dtstart=dtstart, ignoretz=ignoretz)
                    rule._until = until

                    # add the rrule or exrule to the rruleset
                    addfunc(rule)

                if (name == 'rrule' or name == 'rdate') and addRDate:
                    # rlist = rruleset._rrule if name == 'rrule' else rruleset._rdate
                    try:
                        # dateutils does not work with all-day
                        # (datetime.date) items so we need to convert to a
                        # datetime.datetime (which is what dateutils
                        # does internally)
                        if not isinstance(dtstart, datetime.datetime):
                            adddtstart = datetime.datetime.fromordinal(dtstart.toordinal())
                        else:
                            adddtstart = dtstart

                        if name == 'rrule':
                            if rruleset._rrule[-1][0] != adddtstart:
                                rruleset.rdate(adddtstart)
                                added = True
                                if rruleset._rrule[-1]._count is not None:
                                    rruleset._rrule[-1]._count -= 1
                            else:
                                added = False
                        elif name == 'rdate':
                            if rruleset._rdate[0] != adddtstart:
                                rruleset.rdate(adddtstart)
                                added = True
                            else:
                                added = False
                    except IndexError:
                        # it's conceivable that an rrule has 0 datetimes
                        added = False

        return rruleset

    def setrruleset(self, rruleset):
        # Get DTSTART from component (or DUE if no DTSTART in a VTODO)
        try:
            dtstart = self.dtstart.value
        except (AttributeError, KeyError):
            if self.name == "VTODO":
                dtstart = self.due.value
            else:
                raise

        isDate = datetime.date == type(dtstart)
        if isDate:
            dtstart = datetime.datetime(dtstart.year, dtstart.month, dtstart.day)
            untilSerialize = dateToString
        else:
            # make sure to convert time zones to UTC
            untilSerialize = lambda x: dateTimeToString(x, True)

        for name in DATESANDRULES:
            if name in self.contents:
                del self.contents[name]
            setlist = getattr(rruleset, '_' + name)
            if name in DATENAMES:
                setlist = list(setlist)  # make a copy of the list
                if name == 'rdate' and dtstart in setlist:
                    setlist.remove(dtstart)
                if isDate:
                    setlist = [dt.date() for dt in setlist]
                if len(setlist) > 0:
                    self.add(name).value = setlist
            elif name in RULENAMES:
                for rule in setlist:
                    buf = six.StringIO()
                    buf.write('FREQ=')
                    buf.write(FREQUENCIES[rule._freq])

                    values = {}

                    if rule._interval != 1:
                        values['INTERVAL'] = [str(rule._interval)]
                    if rule._wkst != 0:  # wkst defaults to Monday
                        values['WKST'] = [WEEKDAYS[rule._wkst]]
                    if rule._bysetpos is not None:
                        values['BYSETPOS'] = [str(i) for i in rule._bysetpos]

                    if rule._count is not None:
                        values['COUNT'] = [str(rule._count)]
                    elif rule._until is not None:
                        values['UNTIL'] = [untilSerialize(rule._until)]

                    days = []
                    if (rule._byweekday is not None and (
                                rrule.WEEKLY != rule._freq or
                                len(rule._byweekday) != 1 or
                                rule._dtstart.weekday() != rule._byweekday[0])):
                        # ignore byweekday if freq is WEEKLY and day correlates
                        # with dtstart because it was automatically set by dateutil
                        days.extend(WEEKDAYS[n] for n in rule._byweekday)

                    if rule._bynweekday is not None:
                        days.extend(n + WEEKDAYS[day] for day, n in rule._bynweekday)

                    if len(days) > 0:
                        values['BYDAY'] = days

                    if rule._bymonthday is not None and len(rule._bymonthday) > 0:
                        if not (rule._freq <= rrule.MONTHLY and
                                len(rule._bymonthday) == 1 and
                                rule._bymonthday[0] == rule._dtstart.day):
                            # ignore bymonthday if it's generated by dateutil
                            values['BYMONTHDAY'] = [str(n) for n in rule._bymonthday]

                    if rule._bynmonthday is not None and len(rule._bynmonthday) > 0:
                        values.setdefault('BYMONTHDAY', []).extend(str(n) for n in rule._bynmonthday)

                    if rule._bymonth is not None and len(rule._bymonth) > 0:
                        if (rule._byweekday is not None or
                            len(rule._bynweekday or ()) > 0 or
                            not (rule._freq == rrule.YEARLY and
                                 len(rule._bymonth) == 1 and
                                 rule._bymonth[0] == rule._dtstart.month)):
                            # ignore bymonth if it's generated by dateutil
                            values['BYMONTH'] = [str(n) for n in rule._bymonth]

                    if rule._byyearday is not None:
                        values['BYYEARDAY'] = [str(n) for n in rule._byyearday]
                    if rule._byweekno is not None:
                        values['BYWEEKNO'] = [str(n) for n in rule._byweekno]

                    # byhour, byminute, bysecond are always ignored for now

                    for key, paramvals in values.items():
                        buf.write(';')
                        buf.write(key)
                        buf.write('=')
                        buf.write(','.join(paramvals))

                    self.add(name).value = buf.getvalue()

    rruleset = property(getrruleset, setrruleset)

    def __setattr__(self, name, value):
        """
        For convenience, make self.contents directly accessible.
        """
        if name == 'rruleset':
            self.setrruleset(value)
        else:
            super(RecurringComponent, self).__setattr__(name, value)


class TextBehavior(behavior.Behavior):
    """
    Provide backslash escape encoding/decoding for single valued properties.

    TextBehavior also deals with base64 encoding if the ENCODING parameter is
    explicitly set to BASE64.
    """
    base64string = 'BASE64'  # vCard uses B

    @classmethod
    def decode(cls, line):
        """
        Remove backslash escaping from line.value.
        """
        if line.encoded:
            encoding = getattr(line, 'encoding_param', None)
            if encoding and encoding.upper() == cls.base64string:
                line.value = base64.b64decode(line.value)
            else:
                line.value = stringToTextValues(line.value)[0]
            line.encoded = False

    @classmethod
    def encode(cls, line):
        """
        Backslash escape line.value.
        """
        if not line.encoded:
            encoding = getattr(line, 'encoding_param', None)
            if encoding and encoding.upper() == cls.base64string:
                line.value = base64.b64encode(line.value.encode('utf-8')).decode('utf-8').replace('\n', '')
            else:
                line.value = backslashEscape(line.value)
            line.encoded = True


class VCalendarComponentBehavior(behavior.Behavior):
    defaultBehavior = TextBehavior
    isComponent = True


class RecurringBehavior(VCalendarComponentBehavior):
    """
    Parent Behavior for components which should be RecurringComponents.
    """
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn a recurring Component into a RecurringComponent.
        """
        if not obj.isNative:
            object.__setattr__(obj, '__class__', RecurringComponent)
            obj.isNative = True
        return obj

    @staticmethod
    def transformFromNative(obj):
        if obj.isNative:
            object.__setattr__(obj, '__class__', Component)
            obj.isNative = False
        return obj

    @staticmethod
    def generateImplicitParameters(obj):
        """
        Generate a UID and DTSTAMP if one does not exist.

        This is just a dummy implementation, for now.
        """
        if not hasattr(obj, 'uid'):
            rand = int(random.random() * 100000)
            now = datetime.datetime.now(utc)
            now = dateTimeToString(now)
            host = socket.gethostname()
            obj.add(ContentLine('UID', [], "{0} - {1}@{2}".format(now, rand,
                                                                  host)))

        if not hasattr(obj, 'dtstamp'):
            now = datetime.datetime.now(utc)
            obj.add('dtstamp').value = now


class DateTimeBehavior(behavior.Behavior):
    """
    Parent Behavior for ContentLines containing one DATE-TIME.
    """
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn obj.value into a datetime.

        RFC2445 allows times without time zone information, "floating times"
        in some properties.  Mostly, this isn't what you want, but when parsing
        a file, real floating times are noted by setting to 'TRUE' the
        X-VOBJ-FLOATINGTIME-ALLOWED parameter.
        """
        if obj.isNative:
            return obj
        obj.isNative = True
        if obj.value == '':
            return obj
        obj.value = obj.value
        # we're cheating a little here, parseDtstart allows DATE
        obj.value = parseDtstart(obj)
        if obj.value.tzinfo is None:
            obj.params['X-VOBJ-FLOATINGTIME-ALLOWED'] = ['TRUE']
        if obj.params.get('TZID'):
            # Keep a copy of the original TZID around
            obj.params['X-VOBJ-ORIGINAL-TZID'] = [obj.params['TZID']]
            del obj.params['TZID']
        return obj

    @classmethod
    def transformFromNative(cls, obj):
        """
        Replace the datetime in obj.value with an ISO 8601 string.
        """
        if obj.isNative:
            obj.isNative = False
            tzid = TimezoneComponent.registerTzinfo(obj.value.tzinfo)
            obj.value = dateTimeToString(obj.value, cls.forceUTC)
            if not cls.forceUTC and tzid is not None:
                obj.tzid_param = tzid
            if obj.params.get('X-VOBJ-ORIGINAL-TZID'):
                if not hasattr(obj, 'tzid_param'):
                    obj.tzid_param = obj.x_vobj_original_tzid_param
                del obj.params['X-VOBJ-ORIGINAL-TZID']

        return obj


class UTCDateTimeBehavior(DateTimeBehavior):
    """
    A value which must be specified in UTC.
    """
    forceUTC = True


class DateOrDateTimeBehavior(behavior.Behavior):
    """
    Parent Behavior for ContentLines containing one DATE or DATE-TIME.
    """
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn obj.value into a date or datetime.
        """
        if obj.isNative:
            return obj
        obj.isNative = True
        if obj.value == '':
            return obj
        obj.value = obj.value
        obj.value = parseDtstart(obj, allowSignatureMismatch=True)
        if getattr(obj, 'value_param', 'DATE-TIME').upper() == 'DATE-TIME':
            if hasattr(obj, 'tzid_param'):
                # Keep a copy of the original TZID around
                obj.params['X-VOBJ-ORIGINAL-TZID'] = [obj.tzid_param]
                del obj.tzid_param
        return obj

    @staticmethod
    def transformFromNative(obj):
        """
        Replace the date or datetime in obj.value with an ISO 8601 string.
        """
        if type(obj.value) == datetime.date:
            obj.isNative = False
            obj.value_param = 'DATE'
            obj.value = dateToString(obj.value)
            return obj
        else:
            return DateTimeBehavior.transformFromNative(obj)


class MultiDateBehavior(behavior.Behavior):
    """
    Parent Behavior for ContentLines containing one or more DATE, DATE-TIME, or
    PERIOD.
    """
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn obj.value into a list of dates, datetimes, or
        (datetime, timedelta) tuples.
        """
        if obj.isNative:
            return obj
        obj.isNative = True
        if obj.value == '':
            obj.value = []
            return obj
        tzinfo = getTzid(getattr(obj, 'tzid_param', None))
        valueParam = getattr(obj, 'value_param', "DATE-TIME").upper()
        valTexts = obj.value.split(",")
        if valueParam == "DATE":
            obj.value = [stringToDate(x) for x in valTexts]
        elif valueParam == "DATE-TIME":
            obj.value = [stringToDateTime(x, tzinfo) for x in valTexts]
        elif valueParam == "PERIOD":
            obj.value = [stringToPeriod(x, tzinfo) for x in valTexts]
        return obj

    @staticmethod
    def transformFromNative(obj):
        """
        Replace the date, datetime or period tuples in obj.value with
        appropriate strings.
        """
        if obj.value and type(obj.value[0]) == datetime.date:
            obj.isNative = False
            obj.value_param = 'DATE'
            obj.value = ','.join([dateToString(val) for val in obj.value])
            return obj
        # Fixme: handle PERIOD case
        else:
            if obj.isNative:
                obj.isNative = False
                transformed = []
                tzid = None
                for val in obj.value:
                    if tzid is None and type(val) == datetime.datetime:
                        tzid = TimezoneComponent.registerTzinfo(val.tzinfo)
                        if tzid is not None:
                            obj.tzid_param = tzid
                    transformed.append(dateTimeToString(val))
                obj.value = ','.join(transformed)
            return obj


class MultiTextBehavior(behavior.Behavior):
    """
    Provide backslash escape encoding/decoding of each of several values.

    After transformation, value is a list of strings.
    """
    listSeparator = ","

    @classmethod
    def decode(cls, line):
        """
        Remove backslash escaping from line.value, then split on commas.
        """
        if line.encoded:
            line.value = stringToTextValues(line.value,
                                            listSeparator=cls.listSeparator)
            line.encoded = False

    @classmethod
    def encode(cls, line):
        """
        Backslash escape line.value.
        """
        if not line.encoded:
            line.value = cls.listSeparator.join(backslashEscape(val)
                                                for val in line.value)
            line.encoded = True


class SemicolonMultiTextBehavior(MultiTextBehavior):
    listSeparator = ";"


# ------------------------ Registered Behavior subclasses ----------------------
class VCalendar2_0(VCalendarComponentBehavior):
    """
    vCalendar 2.0 behavior. With added VAVAILABILITY support.
    """
    name = 'VCALENDAR'
    description = 'vCalendar 2.0, also known as iCalendar.'
    versionString = '2.0'
    sortFirst = ('version', 'calscale', 'method', 'prodid', 'vtimezone')
    knownChildren = {
        'CALSCALE':      (0, 1, None),  # min, max, behaviorRegistry id
        'METHOD':        (0, 1, None),
        'VERSION':       (0, 1, None),  # required, but auto-generated
        'PRODID':        (1, 1, None),
        'VTIMEZONE':     (0, None, None),
        'VEVENT':        (0, None, None),
        'VTODO':         (0, None, None),
        'VJOURNAL':      (0, None, None),
        'VFREEBUSY':     (0, None, None),
        'VAVAILABILITY': (0, None, None),
    }

    @classmethod
    def generateImplicitParameters(cls, obj):
        """
        Create PRODID, VERSION and VTIMEZONEs if needed.

        VTIMEZONEs will need to exist whenever TZID parameters exist or when
        datetimes with tzinfo exist.
        """
        for comp in obj.components():
            if comp.behavior is not None:
                comp.behavior.generateImplicitParameters(comp)
        if not hasattr(obj, 'prodid'):
            obj.add(ContentLine('PRODID', [], PRODID))
        if not hasattr(obj, 'version'):
            obj.add(ContentLine('VERSION', [], cls.versionString))
        tzidsUsed = {}

        def findTzids(obj, table):
            if isinstance(obj, ContentLine) and (obj.behavior is None or
                                                 not obj.behavior.forceUTC):
                if getattr(obj, 'tzid_param', None):
                    table[obj.tzid_param] = 1
                else:
                    if type(obj.value) == list:
                        for item in obj.value:
                            tzinfo = getattr(obj.value, 'tzinfo', None)
                            tzid = TimezoneComponent.registerTzinfo(tzinfo)
                            if tzid:
                                table[tzid] = 1
                    else:
                        tzinfo = getattr(obj.value, 'tzinfo', None)
                        tzid = TimezoneComponent.registerTzinfo(tzinfo)
                        if tzid:
                            table[tzid] = 1
            for child in obj.getChildren():
                if obj.name != 'VTIMEZONE':
                    findTzids(child, table)

        findTzids(obj, tzidsUsed)
        oldtzids = [toUnicode(x.tzid.value) for x in getattr(obj, 'vtimezone_list', [])]
        for tzid in tzidsUsed.keys():
            tzid = toUnicode(tzid)
            if tzid != u'UTC' and tzid not in oldtzids:
                obj.add(TimezoneComponent(tzinfo=getTzid(tzid)))

    @classmethod
    def serialize(cls, obj, buf, lineLength, validate=True):
        """
        Set implicit parameters, do encoding, return unicode string.

        If validate is True, raise VObjectError if the line doesn't validate
        after implicit parameters are generated.

        Default is to call base.defaultSerialize.

        """

        cls.generateImplicitParameters(obj)
        if validate:
            cls.validate(obj, raiseException=True)
        if obj.isNative:
            transformed = obj.transformFromNative()
            undoTransform = True
        else:
            transformed = obj
            undoTransform = False
        out = None
        outbuf = buf or six.StringIO()
        if obj.group is None:
            groupString = ''
        else:
            groupString = obj.group + '.'
        if obj.useBegin:
            foldOneLine(outbuf, "{0}BEGIN:{1}".format(groupString, obj.name),
                        lineLength)

        try:
            first_props = [s for s in cls.sortFirst if s in obj.contents \
                                    and not isinstance(obj.contents[s][0], Component)]
            first_components = [s for s in cls.sortFirst if s in obj.contents \
                                    and isinstance(obj.contents[s][0], Component)]
        except Exception:
            first_props = first_components = []
            # first_components = []

        prop_keys = sorted(list(k for k in obj.contents.keys() if k not in first_props \
                                         and not isinstance(obj.contents[k][0], Component)))
        comp_keys = sorted(list(k for k in obj.contents.keys() if k not in first_components \
                                        and isinstance(obj.contents[k][0], Component)))

        sorted_keys = first_props + prop_keys + first_components + comp_keys
        children = [o for k in sorted_keys for o in obj.contents[k]]

        for child in children:
            # validate is recursive, we only need to validate once
            child.serialize(outbuf, lineLength, validate=False)
        if obj.useBegin:
            foldOneLine(outbuf, "{0}END:{1}".format(groupString, obj.name),
                        lineLength)
        out = buf or outbuf.getvalue()
        if undoTransform:
            obj.transformToNative()
        return out
registerBehavior(VCalendar2_0)


class VTimezone(VCalendarComponentBehavior):
    """
    Timezone behavior.
    """
    name = 'VTIMEZONE'
    hasNative = True
    description = 'A grouping of component properties that defines a time zone.'
    sortFirst = ('tzid', 'last-modified', 'tzurl', 'standard', 'daylight')
    knownChildren = {
        'TZID':          (1, 1, None),  # min, max, behaviorRegistry id
        'LAST-MODIFIED': (0, 1, None),
        'TZURL':         (0, 1, None),
        'STANDARD':      (0, None, None),  # NOTE: One of Standard or
        'DAYLIGHT':      (0, None, None)  # Daylight must appear
    }

    @classmethod
    def validate(cls, obj, raiseException, *args):
        if not hasattr(obj, 'tzid') or obj.tzid.value is None:
            if raiseException:
                m = "VTIMEZONE components must contain a valid TZID"
                raise ValidateError(m)
            return False
        if 'standard' in obj.contents or 'daylight' in obj.contents:
            return super(VTimezone, cls).validate(obj, raiseException, *args)
        else:
            if raiseException:
                m = "VTIMEZONE components must contain a STANDARD or a DAYLIGHT\
                     component"
                raise ValidateError(m)
            return False

    @staticmethod
    def transformToNative(obj):
        if not obj.isNative:
            object.__setattr__(obj, '__class__', TimezoneComponent)
            obj.isNative = True
            obj.registerTzinfo(obj.tzinfo)
        return obj

    @staticmethod
    def transformFromNative(obj):
        return obj
registerBehavior(VTimezone)


class TZID(behavior.Behavior):
    """
    Don't use TextBehavior for TZID.

    RFC2445 only allows TZID lines to be paramtext, so they shouldn't need any
    encoding or decoding.  Unfortunately, some Microsoft products use commas
    in TZIDs which should NOT be treated as a multi-valued text property, nor
    do we want to escape them.  Leaving them alone works for Microsoft's breakage,
    and doesn't affect compliant iCalendar streams.
    """
registerBehavior(TZID)


class DaylightOrStandard(VCalendarComponentBehavior):
    hasNative = False
    knownChildren = {'DTSTART':      (1, 1, None),  # min, max, behaviorRegistry id
                     'RRULE':        (0, 1, None)}

registerBehavior(DaylightOrStandard, 'STANDARD')
registerBehavior(DaylightOrStandard, 'DAYLIGHT')


class VEvent(RecurringBehavior):
    """
    Event behavior.
    """
    name = 'VEVENT'
    sortFirst = ('uid', 'recurrence-id', 'dtstart', 'duration', 'dtend')

    description = 'A grouping of component properties, and possibly including \
                   "VALARM" calendar components, that represents a scheduled \
                   amount of time on a calendar.'
    knownChildren = {
        'DTSTART':        (0, 1, None),  # min, max, behaviorRegistry id
        'CLASS':          (0, 1, None),
        'CREATED':        (0, 1, None),
        'DESCRIPTION':    (0, 1, None),
        'GEO':            (0, 1, None),
        'LAST-MODIFIED':  (0, 1, None),
        'LOCATION':       (0, 1, None),
        'ORGANIZER':      (0, 1, None),
        'PRIORITY':       (0, 1, None),
        'DTSTAMP':        (1, 1, None),  # required
        'SEQUENCE':       (0, 1, None),
        'STATUS':         (0, 1, None),
        'SUMMARY':        (0, 1, None),
        'TRANSP':         (0, 1, None),
        'UID':            (1, 1, None),
        'URL':            (0, 1, None),
        'RECURRENCE-ID':  (0, 1, None),
        'DTEND':          (0, 1, None),  # NOTE: Only one of DtEnd or
        'DURATION':       (0, 1, None),  # Duration can appear
        'ATTACH':         (0, None, None),
        'ATTENDEE':       (0, None, None),
        'CATEGORIES':     (0, None, None),
        'COMMENT':        (0, None, None),
        'CONTACT':        (0, None, None),
        'EXDATE':         (0, None, None),
        'EXRULE':         (0, None, None),
        'REQUEST-STATUS': (0, None, None),
        'RELATED-TO':     (0, None, None),
        'RESOURCES':      (0, None, None),
        'RDATE':          (0, None, None),
        'RRULE':          (0, None, None),
        'VALARM':         (0, None, None)
    }

    @classmethod
    def validate(cls, obj, raiseException, *args):
        if 'dtend' in obj.contents and 'duration' in obj.contents:
            if raiseException:
                m = "VEVENT components cannot contain both DTEND and DURATION\
                     components"
                raise ValidateError(m)
            return False
        else:
            return super(VEvent, cls).validate(obj, raiseException, *args)

registerBehavior(VEvent)


class VTodo(RecurringBehavior):
    """
    To-do behavior.
    """
    name = 'VTODO'
    description = 'A grouping of component properties and possibly "VALARM" \
                   calendar components that represent an action-item or \
                   assignment.'
    knownChildren = {
        'DTSTART':        (0, 1, None),  # min, max, behaviorRegistry id
        'CLASS':          (0, 1, None),
        'COMPLETED':      (0, 1, None),
        'CREATED':        (0, 1, None),
        'DESCRIPTION':    (0, 1, None),
        'GEO':            (0, 1, None),
        'LAST-MODIFIED':  (0, 1, None),
        'LOCATION':       (0, 1, None),
        'ORGANIZER':      (0, 1, None),
        'PERCENT':        (0, 1, None),
        'PRIORITY':       (0, 1, None),
        'DTSTAMP':        (1, 1, None),
        'SEQUENCE':       (0, 1, None),
        'STATUS':         (0, 1, None),
        'SUMMARY':        (0, 1, None),
        'UID':            (0, 1, None),
        'URL':            (0, 1, None),
        'RECURRENCE-ID':  (0, 1, None),
        'DUE':            (0, 1, None),  # NOTE: Only one of Due or
        'DURATION':       (0, 1, None),  # Duration can appear
        'ATTACH':         (0, None, None),
        'ATTENDEE':       (0, None, None),
        'CATEGORIES':     (0, None, None),
        'COMMENT':        (0, None, None),
        'CONTACT':        (0, None, None),
        'EXDATE':         (0, None, None),
        'EXRULE':         (0, None, None),
        'REQUEST-STATUS': (0, None, None),
        'RELATED-TO':     (0, None, None),
        'RESOURCES':      (0, None, None),
        'RDATE':          (0, None, None),
        'RRULE':          (0, None, None),
        'VALARM':         (0, None, None)
    }

    @classmethod
    def validate(cls, obj, raiseException, *args):
        if 'due' in obj.contents and 'duration' in obj.contents:
            if raiseException:
                m = "VTODO components cannot contain both DUE and DURATION\
                     components"
                raise ValidateError(m)
            return False
        else:
            return super(VTodo, cls).validate(obj, raiseException, *args)

registerBehavior(VTodo)


class VJournal(RecurringBehavior):
    """
    Journal entry behavior.
    """
    name = 'VJOURNAL'
    knownChildren = {
        'DTSTART':        (0, 1, None),  # min, max, behaviorRegistry id
        'CLASS':          (0, 1, None),
        'CREATED':        (0, 1, None),
        'DESCRIPTION':    (0, 1, None),
        'LAST-MODIFIED':  (0, 1, None),
        'ORGANIZER':      (0, 1, None),
        'DTSTAMP':        (1, 1, None),
        'SEQUENCE':       (0, 1, None),
        'STATUS':         (0, 1, None),
        'SUMMARY':        (0, 1, None),
        'UID':            (0, 1, None),
        'URL':            (0, 1, None),
        'RECURRENCE-ID':  (0, 1, None),
        'ATTACH':         (0, None, None),
        'ATTENDEE':       (0, None, None),
        'CATEGORIES':     (0, None, None),
        'COMMENT':        (0, None, None),
        'CONTACT':        (0, None, None),
        'EXDATE':         (0, None, None),
        'EXRULE':         (0, None, None),
        'REQUEST-STATUS': (0, None, None),
        'RELATED-TO':     (0, None, None),
        'RDATE':          (0, None, None),
        'RRULE':          (0, None, None)
    }
registerBehavior(VJournal)


class VFreeBusy(VCalendarComponentBehavior):
    """
    Free/busy state behavior.
    """
    name = 'VFREEBUSY'
    description = 'A grouping of component properties that describe either a \
                   request for free/busy time, describe a response to a request \
                   for free/busy time or describe a published set of busy time.'
    sortFirst = ('uid', 'dtstart', 'duration', 'dtend')
    knownChildren = {
        'DTSTART':        (0, 1, None),  # min, max, behaviorRegistry id
        'CONTACT':        (0, 1, None),
        'DTEND':          (0, 1, None),
        'DURATION':       (0, 1, None),
        'ORGANIZER':      (0, 1, None),
        'DTSTAMP':        (1, 1, None),
        'UID':            (0, 1, None),
        'URL':            (0, 1, None),
        'ATTENDEE':       (0, None, None),
        'COMMENT':        (0, None, None),
        'FREEBUSY':       (0, None, None),
        'REQUEST-STATUS': (0, None, None)
    }

registerBehavior(VFreeBusy)


class VAlarm(VCalendarComponentBehavior):
    """
    Alarm behavior.
    """
    name = 'VALARM'
    description = 'Alarms describe when and how to provide alerts about events \
                   and to-dos.'
    knownChildren = {
        'ACTION':       (1, 1, None),  # min, max, behaviorRegistry id
        'TRIGGER':      (1, 1, None),
        'DURATION':     (0, 1, None),
        'REPEAT':       (0, 1, None),
        'DESCRIPTION':  (0, 1, None)
    }

    @staticmethod
    def generateImplicitParameters(obj):
        """
        Create default ACTION and TRIGGER if they're not set.
        """
        try:
            obj.action
        except AttributeError:
            obj.add('action').value = 'AUDIO'
        try:
            obj.trigger
        except AttributeError:
            obj.add('trigger').value = datetime.timedelta(0)

    @classmethod
    def validate(cls, obj, raiseException, *args):
        """
        # TODO
        if obj.contents.has_key('dtend') and obj.contents.has_key('duration'):
            if raiseException:
                m = "VEVENT components cannot contain both DTEND and DURATION\
                     components"
                raise ValidateError(m)
            return False
        else:
            return super(VEvent, cls).validate(obj, raiseException, *args)
        """
        return True

registerBehavior(VAlarm)


class VAvailability(VCalendarComponentBehavior):
    """
    Availability state behavior.

    Used to represent user's available time slots.
    """
    name = 'VAVAILABILITY'
    description = 'A component used to represent a user\'s available time slots.'
    sortFirst = ('uid', 'dtstart', 'duration', 'dtend')
    knownChildren = {
        'UID':           (1, 1, None),  # min, max, behaviorRegistry id
        'DTSTAMP':       (1, 1, None),
        'BUSYTYPE':      (0, 1, None),
        'CREATED':       (0, 1, None),
        'DTSTART':       (0, 1, None),
        'LAST-MODIFIED': (0, 1, None),
        'ORGANIZER':     (0, 1, None),
        'SEQUENCE':      (0, 1, None),
        'SUMMARY':       (0, 1, None),
        'URL':           (0, 1, None),
        'DTEND':         (0, 1, None),
        'DURATION':      (0, 1, None),
        'CATEGORIES':    (0, None, None),
        'COMMENT':       (0, None, None),
        'CONTACT':       (0, None, None),
        'AVAILABLE':     (0, None, None),
    }

    @classmethod
    def validate(cls, obj, raiseException, *args):
        if 'dtend' in obj.contents and 'duration' in obj.contents:
            if raiseException:
                m = "VAVAILABILITY components cannot contain both DTEND and DURATION components"
                raise ValidateError(m)
            return False
        else:
            return super(VAvailability, cls).validate(obj, raiseException, *args)

registerBehavior(VAvailability)


class Available(RecurringBehavior):
    """
    Event behavior.
    """
    name = 'AVAILABLE'
    sortFirst = ('uid', 'recurrence-id', 'dtstart', 'duration', 'dtend')
    description = 'Defines a period of time in which a user is normally available.'
    knownChildren = {
        'DTSTAMP':       (1, 1, None),  # min, max, behaviorRegistry id
        'DTSTART':       (1, 1, None),
        'UID':           (1, 1, None),
        'DTEND':         (0, 1, None),  # NOTE: One of DtEnd or
        'DURATION':      (0, 1, None),  # Duration must appear, but not both
        'CREATED':       (0, 1, None),
        'LAST-MODIFIED': (0, 1, None),
        'RECURRENCE-ID': (0, 1, None),
        'RRULE':         (0, 1, None),
        'SUMMARY':       (0, 1, None),
        'CATEGORIES':    (0, None, None),
        'COMMENT':       (0, None, None),
        'CONTACT':       (0, None, None),
        'EXDATE':        (0, None, None),
        'RDATE':         (0, None, None),
    }

    @classmethod
    def validate(cls, obj, raiseException, *args):
        has_dtend = 'dtend' in obj.contents
        has_duration = 'duration' in obj.contents
        if has_dtend and has_duration:
            if raiseException:
                m = "AVAILABLE components cannot contain both DTEND and DURATION\
                     properties"
                raise ValidateError(m)
            return False
        elif not (has_dtend or has_duration):
            if raiseException:
                m = "AVAILABLE components must contain one of DTEND or DURATION\
                     properties"
                raise ValidateError(m)
            return False
        else:
            return super(Available, cls).validate(obj, raiseException, *args)

registerBehavior(Available)


class Duration(behavior.Behavior):
    """
    Behavior for Duration ContentLines.  Transform to datetime.timedelta.
    """
    name = 'DURATION'
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn obj.value into a datetime.timedelta.
        """
        if obj.isNative:
            return obj
        obj.isNative = True
        obj.value = obj.value
        if obj.value == '':
            return obj
        else:
            deltalist = stringToDurations(obj.value)
            # When can DURATION have multiple durations?  For now:
            if len(deltalist) == 1:
                obj.value = deltalist[0]
                return obj
            else:
                raise ParseError("DURATION must have a single duration string.")

    @staticmethod
    def transformFromNative(obj):
        """
        Replace the datetime.timedelta in obj.value with an RFC2445 string.
        """
        if not obj.isNative:
            return obj
        obj.isNative = False
        obj.value = timedeltaToString(obj.value)
        return obj

registerBehavior(Duration)


class Trigger(behavior.Behavior):
    """
    DATE-TIME or DURATION
    """
    name = 'TRIGGER'
    description = 'This property specifies when an alarm will trigger.'
    hasNative = True
    forceUTC = True

    @staticmethod
    def transformToNative(obj):
        """
        Turn obj.value into a timedelta or datetime.
        """
        if obj.isNative:
            return obj
        value = getattr(obj, 'value_param', 'DURATION').upper()
        if hasattr(obj, 'value_param'):
            del obj.value_param
        if obj.value == '':
            obj.isNative = True
            return obj
        elif value == 'DURATION':
            try:
                return Duration.transformToNative(obj)
            except ParseError:
                logger.warning("TRIGGER not recognized as DURATION, trying "
                               "DATE-TIME, because iCal sometimes exports "
                               "DATE-TIMEs without setting VALUE=DATE-TIME")
                try:
                    obj.isNative = False
                    dt = DateTimeBehavior.transformToNative(obj)
                    return dt
                except:
                    msg = "TRIGGER with no VALUE not recognized as DURATION " \
                          "or as DATE-TIME"
                    raise ParseError(msg)
        elif value == 'DATE-TIME':
            # TRIGGERs with DATE-TIME values must be in UTC, we could validate
            # that fact, for now we take it on faith.
            return DateTimeBehavior.transformToNative(obj)
        else:
            raise ParseError("VALUE must be DURATION or DATE-TIME")

    @staticmethod
    def transformFromNative(obj):
        if type(obj.value) == datetime.datetime:
            obj.value_param = 'DATE-TIME'
            return UTCDateTimeBehavior.transformFromNative(obj)
        elif type(obj.value) == datetime.timedelta:
            return Duration.transformFromNative(obj)
        else:
            raise NativeError("Native TRIGGER values must be timedelta or "
                              "datetime")
registerBehavior(Trigger)


class PeriodBehavior(behavior.Behavior):
    """
    A list of (date-time, timedelta) tuples.
    """
    hasNative = True

    @staticmethod
    def transformToNative(obj):
        """
        Convert comma separated periods into tuples.
        """
        if obj.isNative:
            return obj
        obj.isNative = True
        if obj.value == '':
            obj.value = []
            return obj
        tzinfo = getTzid(getattr(obj, 'tzid_param', None))
        obj.value = [stringToPeriod(x, tzinfo) for x in obj.value.split(",")]
        return obj

    @classmethod
    def transformFromNative(cls, obj):
        """
        Convert the list of tuples in obj.value to strings.
        """
        if obj.isNative:
            obj.isNative = False
            transformed = []
            for tup in obj.value:
                transformed.append(periodToString(tup, cls.forceUTC))
            if len(transformed) > 0:
                tzid = TimezoneComponent.registerTzinfo(tup[0].tzinfo)
                if not cls.forceUTC and tzid is not None:
                    obj.tzid_param = tzid

            obj.value = ','.join(transformed)

        return obj


class FreeBusy(PeriodBehavior):
    """
    Free or busy period of time, must be specified in UTC.
    """
    name = 'FREEBUSY'
    forceUTC = True
registerBehavior(FreeBusy, 'FREEBUSY')


class RRule(behavior.Behavior):
    """
    Dummy behavior to avoid having RRULEs being treated as text lines (and thus
    having semi-colons inaccurately escaped).
    """
registerBehavior(RRule, 'RRULE')
registerBehavior(RRule, 'EXRULE')


# ------------------------ Registration of common classes ----------------------
utcDateTimeList = ['LAST-MODIFIED', 'CREATED', 'COMPLETED', 'DTSTAMP']
list(map(lambda x: registerBehavior(UTCDateTimeBehavior, x), utcDateTimeList))

dateTimeOrDateList = ['DTEND', 'DTSTART', 'DUE', 'RECURRENCE-ID']
list(map(lambda x: registerBehavior(DateOrDateTimeBehavior, x),
         dateTimeOrDateList))

registerBehavior(MultiDateBehavior, 'RDATE')
registerBehavior(MultiDateBehavior, 'EXDATE')


textList = ['CALSCALE', 'METHOD', 'PRODID', 'CLASS', 'COMMENT', 'DESCRIPTION',
            'LOCATION', 'STATUS', 'SUMMARY', 'TRANSP', 'CONTACT', 'RELATED-TO',
            'UID', 'ACTION', 'BUSYTYPE']
list(map(lambda x: registerBehavior(TextBehavior, x), textList))

list(map(lambda x: registerBehavior(MultiTextBehavior, x), ['CATEGORIES',
                                                            'RESOURCES']))
registerBehavior(SemicolonMultiTextBehavior, 'REQUEST-STATUS')


# ------------------------ Serializing helper functions ------------------------
def numToDigits(num, places):
    """
    Helper, for converting numbers to textual digits.
    """
    s = str(num)
    if len(s) < places:
        return ("0" * (places - len(s))) + s
    elif len(s) > places:
        return s[len(s)-places:]
    else:
        return s


def timedeltaToString(delta):
    """
    Convert timedelta to an ical DURATION.
    """
    if delta.days == 0:
        sign = 1
    else:
        sign = delta.days / abs(delta.days)
    delta = abs(delta)
    days = delta.days
    hours = int(delta.seconds / 3600)
    minutes = int((delta.seconds % 3600) / 60)
    seconds = int(delta.seconds % 60)

    output = ''
    if sign == -1:
        output += '-'
    output += 'P'
    if days:
        output += '{}D'.format(days)
    if hours or minutes or seconds:
        output += 'T'
    elif not days:  # Deal with zero duration
        output += 'T0S'
    if hours:
        output += '{}H'.format(hours)
    if minutes:
        output += '{}M'.format(minutes)
    if seconds:
        output += '{}S'.format(seconds)
    return output


def timeToString(dateOrDateTime):
    """
    Wraps dateToString and dateTimeToString, returning the results
    of either based on the type of the argument
    """
    if hasattr(dateOrDateTime, 'hour'):
        return dateTimeToString(dateOrDateTime)
    return dateToString(dateOrDateTime)


def dateToString(date):
    year = numToDigits(date.year,  4)
    month = numToDigits(date.month, 2)
    day = numToDigits(date.day,   2)
    return year + month + day


def dateTimeToString(dateTime, convertToUTC=False):
    """
    Ignore tzinfo unless convertToUTC.  Output string.
    """
    if dateTime.tzinfo and convertToUTC:
        dateTime = dateTime.astimezone(utc)

    datestr = "{0}{1}{2}T{3}{4}{5}".format(
        numToDigits(dateTime.year, 4),
        numToDigits(dateTime.month, 2),
        numToDigits(dateTime.day, 2),
        numToDigits(dateTime.hour, 2),
        numToDigits(dateTime.minute, 2),
        numToDigits(dateTime.second, 2),
    )
    if tzinfo_eq(dateTime.tzinfo, utc):
        datestr += "Z"
    return datestr


def deltaToOffset(delta):
    absDelta = abs(delta)
    hours = int(absDelta.seconds / 3600)
    hoursString = numToDigits(hours, 2)
    minutesString = '00'
    if absDelta == delta:
        signString = "+"
    else:
        signString = "-"
    return signString + hoursString + minutesString


def periodToString(period, convertToUTC=False):
    txtstart = dateTimeToString(period[0], convertToUTC)
    if isinstance(period[1], datetime.timedelta):
        txtend = timedeltaToString(period[1])
    else:
        txtend = dateTimeToString(period[1], convertToUTC)
    return txtstart + "/" + txtend


# ----------------------- Parsing functions ------------------------------------
def isDuration(s):
    s = s.upper()
    return (s.find("P") != -1) and (s.find("P") < 2)


def stringToDate(s):
    year = int(s[0:4])
    month = int(s[4:6])
    day = int(s[6:8])
    return datetime.date(year, month, day)


def stringToDateTime(s, tzinfo=None):
    """
    Returns datetime.datetime object.
    """
    try:
        year = int(s[0:4])
        month = int(s[4:6])
        day = int(s[6:8])
        hour = int(s[9:11])
        minute = int(s[11:13])
        second = int(s[13:15])
        if len(s) > 15:
            if s[15] == 'Z':
                tzinfo = getTzid('UTC')
    except:
        raise ParseError("'{0!s}' is not a valid DATE-TIME".format(s))
    year = year and year or 2000
    if tzinfo is not None and hasattr(tzinfo,'localize'):  # PyTZ case
        return tzinfo.localize(datetime.datetime(year, month, day, hour, minute, second))
    return datetime.datetime(year, month, day, hour, minute, second, 0, tzinfo)


# DQUOTE included to work around iCal's penchant for backslash escaping it,
# although it isn't actually supposed to be escaped according to rfc2445 TEXT
escapableCharList = '\\;,Nn"'


def stringToTextValues(s, listSeparator=',', charList=None, strict=False):
    """
    Returns list of strings.
    """
    if charList is None:
        charList = escapableCharList

    def escapableChar(c):
        return c in charList

    def error(msg):
        if strict:
            raise ParseError(msg)
        else:
            logging.error(msg)

    # vars which control state machine
    charIterator = enumerate(s)
    state = "read normal"

    current = []
    results = []

    while True:
        try:
            charIndex, char = next(charIterator)
        except:
            char = "eof"

        if state == "read normal":
            if char == '\\':
                state = "read escaped char"
            elif char == listSeparator:
                state = "read normal"
                current = "".join(current)
                results.append(current)
                current = []
            elif char == "eof":
                state = "end"
            else:
                state = "read normal"
                current.append(char)

        elif state == "read escaped char":
            if escapableChar(char):
                state = "read normal"
                if char in 'nN':
                    current.append('\n')
                else:
                    current.append(char)
            else:
                state = "read normal"
                # leave unrecognized escaped characters for later passes
                current.append('\\' + char)

        elif state == "end":  # an end state
            if len(current) or len(results) == 0:
                current = "".join(current)
                results.append(current)
            return results

        elif state == "error":  # an end state
            return results

        else:
            state = "error"
            error("unknown state: '{0!s}' reached in {1!s}".format(state, s))


def stringToDurations(s, strict=False):
    """
    Returns list of timedelta objects.
    """
    def makeTimedelta(sign, week, day, hour, minute, sec):
        if sign == "-":
            sign = -1
        else:
            sign = 1
        week = int(week)
        day = int(day)
        hour = int(hour)
        minute = int(minute)
        sec = int(sec)
        return sign * datetime.timedelta(weeks=week, days=day, hours=hour,
                                         minutes=minute, seconds=sec)

    def error(msg):
        if strict:
            raise ParseError(msg)
        else:
            raise ParseError(msg)

    # vars which control state machine
    charIterator = enumerate(s)
    state = "start"

    durations = []
    current = ""
    sign = None
    week = 0
    day = 0
    hour = 0
    minute = 0
    sec = 0

    while True:
        try:
            charIndex, char = next(charIterator)
        except:
            char = "eof"

        if state == "start":
            if char == '+':
                state = "start"
                sign = char
            elif char == '-':
                state = "start"
                sign = char
            elif char.upper() == 'P':
                state = "read field"
            elif char == "eof":
                state = "error"
                error("got end-of-line while reading in duration: " + s)
            elif char in string.digits:
                state = "read field"
                current = current + char  # update this part when updating "read field"
            else:
                state = "error"
                error("got unexpected character {0} reading in duration: {1}"
                      .format(char, s))

        elif state == "read field":
            if (char in string.digits):
                state = "read field"
                current = current + char  # update part above when updating "read field"
            elif char.upper() == 'T':
                state = "read field"
            elif char.upper() == 'W':
                state = "read field"
                week = current
                current = ""
            elif char.upper() == 'D':
                state = "read field"
                day = current
                current = ""
            elif char.upper() == 'H':
                state = "read field"
                hour = current
                current = ""
            elif char.upper() == 'M':
                state = "read field"
                minute = current
                current = ""
            elif char.upper() == 'S':
                state = "read field"
                sec = current
                current = ""
            elif char == ",":
                state = "start"
                durations.append(makeTimedelta(sign, week, day, hour, minute,
                                               sec))
                current = ""
                sign = None
                week = None
                day = None
                hour = None
                minute = None
                sec = None
            elif char == "eof":
                state = "end"
            else:
                state = "error"
                error("got unexpected character reading in duration: " + s)

        elif state == "end":  # an end state
            if (sign or week or day or hour or minute or sec):
                durations.append(makeTimedelta(sign, week, day, hour, minute,
                                               sec))
            return durations

        elif state == "error":  # an end state
            error("in error state")
            return durations

        else:
            state = "error"
            error("unknown state: '{0!s}' reached in {1!s}".format(state, s))


def parseDtstart(contentline, allowSignatureMismatch=False):
    """
    Convert a contentline's value into a date or date-time.

    A variety of clients don't serialize dates with the appropriate VALUE
    parameter, so rather than failing on these (technically invalid) lines,
    if allowSignatureMismatch is True, try to parse both varieties.
    """
    tzinfo = getTzid(getattr(contentline, 'tzid_param', None))
    valueParam = getattr(contentline, 'value_param', 'DATE-TIME').upper()
    if valueParam == "DATE":
        return stringToDate(contentline.value)
    elif valueParam == "DATE-TIME":
        try:
            return stringToDateTime(contentline.value, tzinfo)
        except:
            if allowSignatureMismatch:
                return stringToDate(contentline.value)
            else:
                raise


def stringToPeriod(s, tzinfo=None):
    values = s.split("/")
    start = stringToDateTime(values[0], tzinfo)
    valEnd = values[1]
    if isDuration(valEnd):  # period-start = date-time "/" dur-value
        delta = stringToDurations(valEnd)[0]
        return (start, delta)
    else:
        return (start, stringToDateTime(valEnd, tzinfo))


def getTransition(transitionTo, year, tzinfo):
    """
    Return the datetime of the transition to/from DST, or None.
    """
    def firstTransition(iterDates, test):
        """
        Return the last date not matching test, or None if all tests matched.
        """
        success = None
        for dt in iterDates:
            if not test(dt):
                success = dt
            else:
                if success is not None:
                    return success
        return success  # may be None

    def generateDates(year, month=None, day=None):
        """
        Iterate over possible dates with unspecified values.
        """
        months = range(1, 13)
        days = range(1, 32)
        hours = range(0, 24)
        if month is None:
            for month in months:
                yield datetime.datetime(year, month, 1)
        elif day is None:
            for day in days:
                try:
                    yield datetime.datetime(year, month, day)
                except ValueError:
                    pass
        else:
            for hour in hours:
                yield datetime.datetime(year, month, day, hour)

    assert transitionTo in ('daylight', 'standard')
    if transitionTo == 'daylight':
        def test(dt):
            try:
                return tzinfo.dst(dt) != zeroDelta
            except pytz.NonExistentTimeError:
                return True  # entering daylight time
            except pytz.AmbiguousTimeError:
                return False  # entering standard time
    elif transitionTo == 'standard':
        def test(dt):
            try:
                return tzinfo.dst(dt) == zeroDelta
            except pytz.NonExistentTimeError:
                return False  # entering daylight time
            except pytz.AmbiguousTimeError:
                return True  # entering standard time
    newyear = datetime.datetime(year, 1, 1)
    monthDt = firstTransition(generateDates(year), test)
    if monthDt is None:
        return newyear
    elif monthDt.month == 12:
        return None
    else:
        # there was a good transition somewhere in a non-December month
        month = monthDt.month
        day = firstTransition(generateDates(year, month), test).day
        uncorrected = firstTransition(generateDates(year, month, day), test)
        if transitionTo == 'standard':
            # assuming tzinfo.dst returns a new offset for the first
            # possible hour, we need to add one hour for the offset change
            # and another hour because firstTransition returns the hour
            # before the transition
            return uncorrected + datetime.timedelta(hours=2)
        else:
            return uncorrected + datetime.timedelta(hours=1)


def tzinfo_eq(tzinfo1, tzinfo2, startYear=2000, endYear=2020):
    """
    Compare offsets and DST transitions from startYear to endYear.
    """
    if tzinfo1 == tzinfo2:
        return True
    elif tzinfo1 is None or tzinfo2 is None:
        return False

    def dt_test(dt):
        if dt is None:
            return True
        return tzinfo1.utcoffset(dt) == tzinfo2.utcoffset(dt)

    if not dt_test(datetime.datetime(startYear, 1, 1)):
        return False
    for year in range(startYear, endYear):
        for transitionTo in 'daylight', 'standard':
            t1 = getTransition(transitionTo, year, tzinfo1)
            t2 = getTransition(transitionTo, year, tzinfo2)
            if t1 != t2 or not dt_test(t1):
                return False
    return True


# ------------------- Testing and running functions ----------------------------
if __name__ == '__main__':
    import tests
    tests._test()