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prefect.server.schemas.schedules

Schedule schemas

CronSchedule

Bases: PrefectBaseModel

Cron schedule

NOTE: If the timezone is a DST-observing one, then the schedule will adjust itself appropriately. Cron's rules for DST are based on schedule times, not intervals. This means that an hourly cron schedule will fire on every new schedule hour, not every elapsed hour; for example, when clocks are set back this will result in a two-hour pause as the schedule will fire the first time 1am is reached and the first time 2am is reached, 120 minutes later. Longer schedules, such as one that fires at 9am every morning, will automatically adjust for DST.

Parameters:

Name Type Description Default
cron str

a valid cron string

 required
timezone str

a valid timezone string

 required
day_or bool

Control how croniter handles day and day_of_week entries. Defaults to True, matching cron which connects those values using OR. If the switch is set to False, the values are connected using AND. This behaves like fcron and enables you to e.g. define a job that executes each 2nd friday of a month by setting the days of month and the weekday.

 required
Source code in prefect/server/schemas/schedules.py 
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class CronSchedule(PrefectBaseModel):
    """
    Cron schedule

    NOTE: If the timezone is a DST-observing one, then the schedule will adjust
    itself appropriately. Cron's rules for DST are based on schedule times, not
    intervals. This means that an hourly cron schedule will fire on every new
    schedule hour, not every elapsed hour; for example, when clocks are set back
    this will result in a two-hour pause as the schedule will fire *the first
    time* 1am is reached and *the first time* 2am is reached, 120 minutes later.
    Longer schedules, such as one that fires at 9am every morning, will
    automatically adjust for DST.

    Args:
        cron (str): a valid cron string
        timezone (str): a valid timezone string
        day_or (bool, optional): Control how croniter handles `day` and `day_of_week`
            entries. Defaults to True, matching cron which connects those values using
            OR. If the switch is set to False, the values are connected using AND. This
            behaves like fcron and enables you to e.g. define a job that executes each
            2nd friday of a month by setting the days of month and the weekday.

    """

    class Config:
        extra = "forbid"

    cron: str = Field(default=..., example="0 0 * * *")
    timezone: Optional[str] = Field(default=None, example="America/New_York")
    day_or: bool = Field(
        default=True,
        description=(
            "Control croniter behavior for handling day and day_of_week entries."
        ),
    )

    @validator("timezone")
    def valid_timezone(cls, v):
        if v and v not in pendulum.tz.timezones:
            raise ValueError(f'Invalid timezone: "{v}"')
        return v

    @validator("cron")
    def valid_cron_string(cls, v):
        # croniter allows "random" and "hashed" expressions
        # which we do not support https://github.com/kiorky/croniter
        if not croniter.is_valid(v):
            raise ValueError(f'Invalid cron string: "{v}"')
        elif any(c for c in v.split() if c.casefold() in ["R", "H", "r", "h"]):
            raise ValueError(
                f'Random and Hashed expressions are unsupported, recieved: "{v}"'
            )
        return v

    async def get_dates(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> List[pendulum.DateTime]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to None.  If a timezone-naive datetime is
                provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): The maximum scheduled date to return. If
                a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: A list of dates
        """
        return sorted(self._get_dates_generator(n=n, start=start, end=end))

    def _get_dates_generator(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> Generator[pendulum.DateTime, None, None]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to the current date. If a timezone-naive
                datetime is provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): No returned date will exceed this date.
                If a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: a list of dates
        """
        if start is None:
            start = pendulum.now("UTC")

        start, end = _prepare_scheduling_start_and_end(start, end, self.timezone)

        if n is None:
            # if an end was supplied, we do our best to supply all matching dates (up to
            # MAX_ITERATIONS)
            if end is not None:
                n = MAX_ITERATIONS
            else:
                n = 1

        elif self.timezone:
            start = start.in_tz(self.timezone)

        # subtract one second from the start date, so that croniter returns it
        # as an event (if it meets the cron criteria)
        start = start.subtract(seconds=1)

        # croniter's DST logic interferes with all other datetime libraries except pytz
        start_localized = pytz.timezone(start.tz.name).localize(
            datetime.datetime(
                year=start.year,
                month=start.month,
                day=start.day,
                hour=start.hour,
                minute=start.minute,
                second=start.second,
                microsecond=start.microsecond,
            )
        )

        # Respect microseconds by rounding up
        if start_localized.microsecond > 0:
            start_localized += datetime.timedelta(seconds=1)

        cron = croniter(self.cron, start_localized, day_or=self.day_or)  # type: ignore
        dates = set()
        counter = 0

        while True:
            next_date = pendulum.instance(cron.get_next(datetime.datetime))
            # if the end date was exceeded, exit
            if end and next_date > end:
                break
            # ensure no duplicates; weird things can happen with DST
            if next_date not in dates:
                dates.add(next_date)
                yield next_date

            # if enough dates have been collected or enough attempts were made, exit
            if len(dates) >= n or counter > MAX_ITERATIONS:
                break

            counter += 1

get_dates async

Retrieves dates from the schedule. Up to 1,000 candidate dates are checked following the start date.

Parameters:

Name Type Description Default
n int

The number of dates to generate

 None
start datetime.datetime

The first returned date will be on or after this date. Defaults to None. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None
end datetime.datetime

The maximum scheduled date to return. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None

Returns:

Type Description
List[pendulum.DateTime]

List[pendulum.DateTime]: A list of dates
Source code in prefect/server/schemas/schedules.py 
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async def get_dates(
    self,
    n: int = None,
    start: datetime.datetime = None,
    end: datetime.datetime = None,
) -> List[pendulum.DateTime]:
    """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
    following the start date.

    Args:
        n (int): The number of dates to generate
        start (datetime.datetime, optional): The first returned date will be on or
            after this date. Defaults to None.  If a timezone-naive datetime is
            provided, it is assumed to be in the schedule's timezone.
        end (datetime.datetime, optional): The maximum scheduled date to return. If
            a timezone-naive datetime is provided, it is assumed to be in the
            schedule's timezone.

    Returns:
        List[pendulum.DateTime]: A list of dates
    """
    return sorted(self._get_dates_generator(n=n, start=start, end=end))

IntervalSchedule

Bases: PrefectBaseModel

A schedule formed by adding interval increments to an anchor_date. If no anchor_date is supplied, the current UTC time is used. If a timezone-naive datetime is provided for anchor_date, it is assumed to be in the schedule's timezone (or UTC). Even if supplied with an IANA timezone, anchor dates are always stored as UTC offsets, so a timezone can be provided to determine localization behaviors like DST boundary handling. If none is provided it will be inferred from the anchor date.

NOTE: If the IntervalSchedule anchor_date or timezone is provided in a DST-observing timezone, then the schedule will adjust itself appropriately. Intervals greater than 24 hours will follow DST conventions, while intervals of less than 24 hours will follow UTC intervals. For example, an hourly schedule will fire every UTC hour, even across DST boundaries. When clocks are set back, this will result in two runs that appear to both be scheduled for 1am local time, even though they are an hour apart in UTC time. For longer intervals, like a daily schedule, the interval schedule will adjust for DST boundaries so that the clock-hour remains constant. This means that a daily schedule that always fires at 9am will observe DST and continue to fire at 9am in the local time zone.

Parameters:

Name Type Description Default
interval datetime.timedelta

an interval to schedule on

 required
anchor_date DateTimeTZ

an anchor date to schedule increments against; if not provided, the current timestamp will be used

 required
timezone str

a valid timezone string

 required
Source code in prefect/server/schemas/schedules.py 
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class IntervalSchedule(PrefectBaseModel):
    """
    A schedule formed by adding `interval` increments to an `anchor_date`. If no
    `anchor_date` is supplied, the current UTC time is used.  If a
    timezone-naive datetime is provided for `anchor_date`, it is assumed to be
    in the schedule's timezone (or UTC). Even if supplied with an IANA timezone,
    anchor dates are always stored as UTC offsets, so a `timezone` can be
    provided to determine localization behaviors like DST boundary handling. If
    none is provided it will be inferred from the anchor date.

    NOTE: If the `IntervalSchedule` `anchor_date` or `timezone` is provided in a
    DST-observing timezone, then the schedule will adjust itself appropriately.
    Intervals greater than 24 hours will follow DST conventions, while intervals
    of less than 24 hours will follow UTC intervals. For example, an hourly
    schedule will fire every UTC hour, even across DST boundaries. When clocks
    are set back, this will result in two runs that *appear* to both be
    scheduled for 1am local time, even though they are an hour apart in UTC
    time. For longer intervals, like a daily schedule, the interval schedule
    will adjust for DST boundaries so that the clock-hour remains constant. This
    means that a daily schedule that always fires at 9am will observe DST and
    continue to fire at 9am in the local time zone.

    Args:
        interval (datetime.timedelta): an interval to schedule on
        anchor_date (DateTimeTZ, optional): an anchor date to schedule increments against;
            if not provided, the current timestamp will be used
        timezone (str, optional): a valid timezone string
    """

    class Config:
        extra = "forbid"
        exclude_none = True

    interval: datetime.timedelta
    anchor_date: DateTimeTZ = None
    timezone: Optional[str] = Field(default=None, example="America/New_York")

    @validator("interval")
    def interval_must_be_positive(cls, v):
        if v.total_seconds() <= 0:
            raise ValueError("The interval must be positive")
        return v

    @validator("anchor_date", always=True)
    def default_anchor_date(cls, v):
        if v is None:
            return pendulum.now("UTC")
        return pendulum.instance(v)

    @validator("timezone", always=True)
    def default_timezone(cls, v, *, values, **kwargs):
        # if was provided, make sure its a valid IANA string
        if v and v not in pendulum.tz.timezones:
            raise ValueError(f'Invalid timezone: "{v}"')

        # otherwise infer the timezone from the anchor date
        elif v is None and values.get("anchor_date"):
            tz = values["anchor_date"].tz.name
            if tz in pendulum.tz.timezones:
                return tz
            # sometimes anchor dates have "timezones" that are UTC offsets
            # like "-04:00". This happens when parsing ISO8601 strings.
            # In this case we, the correct inferred localization is "UTC".
            else:
                return "UTC"

        return v

    async def get_dates(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> List[pendulum.DateTime]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to None.  If a timezone-naive datetime is
                provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): The maximum scheduled date to return. If
                a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: A list of dates
        """
        return sorted(self._get_dates_generator(n=n, start=start, end=end))

    def _get_dates_generator(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> Generator[pendulum.DateTime, None, None]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to None.  If a timezone-naive datetime is
                provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): The maximum scheduled date to return. If
                a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: a list of dates
        """
        if n is None:
            # if an end was supplied, we do our best to supply all matching dates (up to
            # MAX_ITERATIONS)
            if end is not None:
                n = MAX_ITERATIONS
            else:
                n = 1

        if start is None:
            start = pendulum.now("UTC")

        anchor_tz = self.anchor_date.in_tz(self.timezone)
        start, end = _prepare_scheduling_start_and_end(start, end, self.timezone)

        # compute the offset between the anchor date and the start date to jump to the
        # next date
        offset = (start - anchor_tz).total_seconds() / self.interval.total_seconds()
        next_date = anchor_tz.add(seconds=self.interval.total_seconds() * int(offset))

        # break the interval into `days` and `seconds` because pendulum
        # will handle DST boundaries properly if days are provided, but not
        # if we add `total seconds`. Therefore, `next_date + self.interval`
        # fails while `next_date.add(days=days, seconds=seconds)` works.
        interval_days = self.interval.days
        interval_seconds = self.interval.total_seconds() - (
            interval_days * 24 * 60 * 60
        )

        # daylight saving time boundaries can create a situation where the next date is
        # before the start date, so we advance it if necessary
        while next_date < start:
            next_date = next_date.add(days=interval_days, seconds=interval_seconds)

        counter = 0
        dates = set()

        while True:
            # if the end date was exceeded, exit
            if end and next_date > end:
                break

            # ensure no duplicates; weird things can happen with DST
            if next_date not in dates:
                dates.add(next_date)
                yield next_date

            # if enough dates have been collected or enough attempts were made, exit
            if len(dates) >= n or counter > MAX_ITERATIONS:
                break

            counter += 1

            next_date = next_date.add(days=interval_days, seconds=interval_seconds)

get_dates async

Retrieves dates from the schedule. Up to 1,000 candidate dates are checked following the start date.

Parameters:

Name Type Description Default
n int

The number of dates to generate

 None
start datetime.datetime

The first returned date will be on or after this date. Defaults to None. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None
end datetime.datetime

The maximum scheduled date to return. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None

Returns:

Type Description
List[pendulum.DateTime]

List[pendulum.DateTime]: A list of dates
Source code in prefect/server/schemas/schedules.py 
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async def get_dates(
    self,
    n: int = None,
    start: datetime.datetime = None,
    end: datetime.datetime = None,
) -> List[pendulum.DateTime]:
    """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
    following the start date.

    Args:
        n (int): The number of dates to generate
        start (datetime.datetime, optional): The first returned date will be on or
            after this date. Defaults to None.  If a timezone-naive datetime is
            provided, it is assumed to be in the schedule's timezone.
        end (datetime.datetime, optional): The maximum scheduled date to return. If
            a timezone-naive datetime is provided, it is assumed to be in the
            schedule's timezone.

    Returns:
        List[pendulum.DateTime]: A list of dates
    """
    return sorted(self._get_dates_generator(n=n, start=start, end=end))

RRuleSchedule

Bases: PrefectBaseModel

RRule schedule, based on the iCalendar standard (RFC 5545) as implemented in dateutils.rrule.

RRules are appropriate for any kind of calendar-date manipulation, including irregular intervals, repetition, exclusions, week day or day-of-month adjustments, and more.

Note that as a calendar-oriented standard, RRuleSchedules are sensitive to to the initial timezone provided. A 9am daily schedule with a daylight saving time-aware start date will maintain a local 9am time through DST boundaries; a 9am daily schedule with a UTC start date will maintain a 9am UTC time.

Parameters:

Name Type Description Default
rrule str

a valid RRule string

 required
timezone str

a valid timezone string

 required
Source code in prefect/server/schemas/schedules.py 
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class RRuleSchedule(PrefectBaseModel):
    """
    RRule schedule, based on the iCalendar standard
    ([RFC 5545](https://datatracker.ietf.org/doc/html/rfc5545)) as
    implemented in `dateutils.rrule`.

    RRules are appropriate for any kind of calendar-date manipulation, including
    irregular intervals, repetition, exclusions, week day or day-of-month
    adjustments, and more.

    Note that as a calendar-oriented standard, `RRuleSchedules` are sensitive to
    to the initial timezone provided. A 9am daily schedule with a daylight saving
    time-aware start date will maintain a local 9am time through DST boundaries;
    a 9am daily schedule with a UTC start date will maintain a 9am UTC time.

    Args:
        rrule (str): a valid RRule string
        timezone (str, optional): a valid timezone string
    """

    class Config:
        extra = "forbid"

    rrule: str
    timezone: Optional[str] = Field(default=None, example="America/New_York")

    @validator("rrule")
    def validate_rrule_str(cls, v):
        # attempt to parse the rrule string as an rrule object
        # this will error if the string is invalid
        try:
            dateutil.rrule.rrulestr(v, cache=True)
        except ValueError as exc:
            # rrules errors are a mix of cryptic and informative
            # so reraise to be clear that the string was invalid
            raise ValueError(f'Invalid RRule string "{v}": {exc}')
        if len(v) > MAX_RRULE_LENGTH:
            raise ValueError(
                f'Invalid RRule string "{v[:40]}..."\n'
                f"Max length is {MAX_RRULE_LENGTH}, got {len(v)}"
            )
        return v

    @classmethod
    def from_rrule(cls, rrule: dateutil.rrule.rrule):
        if isinstance(rrule, dateutil.rrule.rrule):
            if rrule._dtstart.tzinfo is not None:
                timezone = rrule._dtstart.tzinfo.name
            else:
                timezone = "UTC"
            return RRuleSchedule(rrule=str(rrule), timezone=timezone)
        elif isinstance(rrule, dateutil.rrule.rruleset):
            dtstarts = [rr._dtstart for rr in rrule._rrule if rr._dtstart is not None]
            unique_dstarts = set(pendulum.instance(d).in_tz("UTC") for d in dtstarts)
            unique_timezones = set(d.tzinfo for d in dtstarts if d.tzinfo is not None)

            if len(unique_timezones) > 1:
                raise ValueError(
                    f"rruleset has too many dtstart timezones: {unique_timezones}"
                )

            if len(unique_dstarts) > 1:
                raise ValueError(f"rruleset has too many dtstarts: {unique_dstarts}")

            if unique_dstarts and unique_timezones:
                timezone = dtstarts[0].tzinfo.name
            else:
                timezone = "UTC"

            rruleset_string = ""
            if rrule._rrule:
                rruleset_string += "\n".join(str(r) for r in rrule._rrule)
            if rrule._exrule:
                rruleset_string += "\n" if rruleset_string else ""
                rruleset_string += "\n".join(str(r) for r in rrule._exrule).replace(
                    "RRULE", "EXRULE"
                )
            if rrule._rdate:
                rruleset_string += "\n" if rruleset_string else ""
                rruleset_string += "RDATE:" + ",".join(
                    rd.strftime("%Y%m%dT%H%M%SZ") for rd in rrule._rdate
                )
            if rrule._exdate:
                rruleset_string += "\n" if rruleset_string else ""
                rruleset_string += "EXDATE:" + ",".join(
                    exd.strftime("%Y%m%dT%H%M%SZ") for exd in rrule._exdate
                )
            return RRuleSchedule(rrule=rruleset_string, timezone=timezone)
        else:
            raise ValueError(f"Invalid RRule object: {rrule}")

    def to_rrule(self) -> dateutil.rrule.rrule:
        """
        Since rrule doesn't properly serialize/deserialize timezones, we localize dates
        here
        """
        rrule = dateutil.rrule.rrulestr(self.rrule, cache=True)
        timezone = dateutil.tz.gettz(self.timezone)
        if isinstance(rrule, dateutil.rrule.rrule):
            kwargs = dict(dtstart=rrule._dtstart.replace(tzinfo=timezone))
            if rrule._until:
                kwargs.update(
                    until=rrule._until.replace(tzinfo=timezone),
                )
            return rrule.replace(**kwargs)
        elif isinstance(rrule, dateutil.rrule.rruleset):
            tz = self.timezone

            # update rrules
            localized_rrules = []
            for rr in rrule._rrule:
                kwargs = dict(dtstart=rr._dtstart.replace(tzinfo=timezone))
                if rr._until:
                    kwargs.update(
                        until=rr._until.replace(tzinfo=timezone),
                    )
                localized_rrules.append(rr.replace(**kwargs))
            rrule._rrule = localized_rrules

            # update exrules
            localized_exrules = []
            for exr in rrule._exrule:
                kwargs = dict(dtstart=exr._dtstart.replace(tzinfo=timezone))
                if exr._until:
                    kwargs.update(
                        until=exr._until.replace(tzinfo=timezone),
                    )
                localized_exrules.append(exr.replace(**kwargs))
            rrule._exrule = localized_exrules

            # update rdates
            localized_rdates = []
            for rd in rrule._rdate:
                localized_rdates.append(rd.replace(tzinfo=timezone))
            rrule._rdate = localized_rdates

            # update exdates
            localized_exdates = []
            for exd in rrule._exdate:
                localized_exdates.append(exd.replace(tzinfo=timezone))
            rrule._exdate = localized_exdates

            return rrule

    @validator("timezone", always=True)
    def valid_timezone(cls, v):
        if v and v not in pytz.all_timezones_set:
            raise ValueError(f'Invalid timezone: "{v}"')
        elif v is None:
            return "UTC"
        return v

    async def get_dates(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> List[pendulum.DateTime]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to None.  If a timezone-naive datetime is
                provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): The maximum scheduled date to return. If
                a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: A list of dates
        """
        return sorted(self._get_dates_generator(n=n, start=start, end=end))

    def _get_dates_generator(
        self,
        n: int = None,
        start: datetime.datetime = None,
        end: datetime.datetime = None,
    ) -> Generator[pendulum.DateTime, None, None]:
        """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
        following the start date.

        Args:
            n (int): The number of dates to generate
            start (datetime.datetime, optional): The first returned date will be on or
                after this date. Defaults to the current date. If a timezone-naive
                datetime is provided, it is assumed to be in the schedule's timezone.
            end (datetime.datetime, optional): No returned date will exceed this date.
                If a timezone-naive datetime is provided, it is assumed to be in the
                schedule's timezone.

        Returns:
            List[pendulum.DateTime]: a list of dates
        """
        if start is None:
            start = pendulum.now("UTC")

        start, end = _prepare_scheduling_start_and_end(start, end, self.timezone)

        if n is None:
            # if an end was supplied, we do our best to supply all matching dates (up
            # to MAX_ITERATIONS)
            if end is not None:
                n = MAX_ITERATIONS
            else:
                n = 1

        dates = set()
        counter = 0

        # pass count = None to account for discrepancies with duplicates around DST
        # boundaries
        for next_date in self.to_rrule().xafter(start, count=None, inc=True):
            next_date = pendulum.instance(next_date).in_tz(self.timezone)

            # if the end date was exceeded, exit
            if end and next_date > end:
                break

            # ensure no duplicates; weird things can happen with DST
            if next_date not in dates:
                dates.add(next_date)
                yield next_date

            # if enough dates have been collected or enough attempts were made, exit
            if len(dates) >= n or counter > MAX_ITERATIONS:
                break

            counter += 1

get_dates async

Retrieves dates from the schedule. Up to 1,000 candidate dates are checked following the start date.

Parameters:

Name Type Description Default
n int

The number of dates to generate

 None
start datetime.datetime

The first returned date will be on or after this date. Defaults to None. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None
end datetime.datetime

The maximum scheduled date to return. If a timezone-naive datetime is provided, it is assumed to be in the schedule's timezone.

 None

Returns:

Type Description
List[pendulum.DateTime]

List[pendulum.DateTime]: A list of dates
Source code in prefect/server/schemas/schedules.py 
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async def get_dates(
    self,
    n: int = None,
    start: datetime.datetime = None,
    end: datetime.datetime = None,
) -> List[pendulum.DateTime]:
    """Retrieves dates from the schedule. Up to 1,000 candidate dates are checked
    following the start date.

    Args:
        n (int): The number of dates to generate
        start (datetime.datetime, optional): The first returned date will be on or
            after this date. Defaults to None.  If a timezone-naive datetime is
            provided, it is assumed to be in the schedule's timezone.
        end (datetime.datetime, optional): The maximum scheduled date to return. If
            a timezone-naive datetime is provided, it is assumed to be in the
            schedule's timezone.

    Returns:
        List[pendulum.DateTime]: A list of dates
    """
    return sorted(self._get_dates_generator(n=n, start=start, end=end))

to_rrule

Since rrule doesn't properly serialize/deserialize timezones, we localize dates here

Source code in prefect/server/schemas/schedules.py 
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def to_rrule(self) -> dateutil.rrule.rrule:
    """
    Since rrule doesn't properly serialize/deserialize timezones, we localize dates
    here
    """
    rrule = dateutil.rrule.rrulestr(self.rrule, cache=True)
    timezone = dateutil.tz.gettz(self.timezone)
    if isinstance(rrule, dateutil.rrule.rrule):
        kwargs = dict(dtstart=rrule._dtstart.replace(tzinfo=timezone))
        if rrule._until:
            kwargs.update(
                until=rrule._until.replace(tzinfo=timezone),
            )
        return rrule.replace(**kwargs)
    elif isinstance(rrule, dateutil.rrule.rruleset):
        tz = self.timezone

        # update rrules
        localized_rrules = []
        for rr in rrule._rrule:
            kwargs = dict(dtstart=rr._dtstart.replace(tzinfo=timezone))
            if rr._until:
                kwargs.update(
                    until=rr._until.replace(tzinfo=timezone),
                )
            localized_rrules.append(rr.replace(**kwargs))
        rrule._rrule = localized_rrules

        # update exrules
        localized_exrules = []
        for exr in rrule._exrule:
            kwargs = dict(dtstart=exr._dtstart.replace(tzinfo=timezone))
            if exr._until:
                kwargs.update(
                    until=exr._until.replace(tzinfo=timezone),
                )
            localized_exrules.append(exr.replace(**kwargs))
        rrule._exrule = localized_exrules

        # update rdates
        localized_rdates = []
        for rd in rrule._rdate:
            localized_rdates.append(rd.replace(tzinfo=timezone))
        rrule._rdate = localized_rdates

        # update exdates
        localized_exdates = []
        for exd in rrule._exdate:
            localized_exdates.append(exd.replace(tzinfo=timezone))
        rrule._exdate = localized_exdates

        return rrule