snowflake.core.task.Cron

class snowflake.core.task.Cron(expr: str, timezone: str)

Bases: NamedTuple

Specifies a cron expression and time zone for periodically running the task.

Supports a subset of standard cron utility syntax.

Example

>>> cron1 = Cron("0 0 10-20 * TUE,THU", "America/Los_Angeles")
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Attributes

expr: str

The cron expression. The minimum interval is 1 minute.

It consists of the following fields:

# __________ minute (0-59)
# | ________ hour (0-23)
# | | ______ day of month (1-31, or L)
# | | | ____ month (1-12, JAN-DEC)
# | | | | _ day of week (0-6, SUN-SAT, or L)
# | | | | |
# | | | | |
  * * * * *
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The following special characters are supported:

  • * Wildcard. Specifies any occurrence of the field.

  • L Stands for “last”. When used in the day-of-week field, it allows you to specify constructs such as “the last Friday” (“5L”) of a given month. In the day-of-month field, it specifies the last day of the month.

  • /n Indicates the nth instance of a given unit of time. Each quanta of time is computed independently. For example, if 4/3 is specified in the month field, then the task is scheduled for April, July and October (i.e. every 3 months, starting with the 4th month of the year). The same schedule is maintained in subsequent years. That is, the task is not scheduled to run in January (3 months after the October run).

Tasks scheduled during specific times on days when the transition from standard time to daylight saving time (or the reverse) occurs can have unexpected behaviors. For example:

  • During the autumn change from daylight saving time to standard time, a task scheduled to start at 1 AM in the America/Los_Angeles time zone (i.e. 0 1 * * * America/Los_Angeles) would run twice: once at 1 AM and then again when 1:59:59 AM shifts to 1:00:00 AM local time. That is, there are two points in time when the local time is 1 AM.

  • During the spring change from standard time to daylight saving time, a task scheduled to start at 2 AM in the America/Los_Angeles time zone (i.e. 0 2 * * * America/Los_Angeles) would not run at all because the local time shifts from 1:59:59 AM to 3:00:00 AM. That is, there is no point during that day when the local time is 2 AM.

To avoid unexpected task executions due to daylight saving time, use one of the following:

  • Do not schedule tasks to run at a specific time between 1 AM and 3 AM (daily, or on days of the week that include Sundays), or

  • Manually adjust the cron expression for tasks scheduled during those hours twice each year to compensate for the time change due to daylight saving time, or

  • Use a time format that does not apply daylight savings time, such as UTC.

timezone: str

The timezone for the cron expression

For a list of time zones, see the list of tz database time zones in Wikipedia (link removed).

Methods

__init__()
count(value, /)

Return number of occurrences of value.

index(value, start=0, stop=9223372036854775807, /)

Return first index of value.

Raises ValueError if the value is not present.

语言: 中文