什么是计算给定当前时间和一个cron规范？事件的下次运行时间的有效方式。

What's an efficient way to calculate the next run time of an event given the current time and a cron spec?

我在找的东西比其他，通过每分钟检查循环，如果它符合规范。

I'm looking for something other than "loop through every minute checking if it matches spec".

规范的例子可能是：

Python的code是可爱的，但伪code或高层次的描述也将是AP preciated。

Python code would be lovely but psuedo code or high level description would also be appreciated.

[更新]假设该规范已经被解析，并在一些合理的格式。

[Update] Assume the spec is already parsed and is in some reasonable format.

推荐答案

只是看着它，我想你需要：

Just looking at it, I think you need to:

我不知道如何处理日常-的同时每周日-的一个月;我相信，有一种方法，但在另一方面，我不认为我见过，实际上指定了一个规范。我认为这将是足以写一个处理程序，或者如果您收到这两个抛出一个错误。的

编辑：显然天的星期天个月，如果都被指定，它应该是开枪的两个 - 也就是说，如果规则是 15日，周三将火上每隔14个和每星期三。

apparently if day-of-week and day-of-month are both specified, it is supposed to fire on both - ie if the rule is '15th, Wednesday' it will fire on every 15th and every Wednesday.

该croniter包你想要做什么：

The croniter package does what you want:

import croniter
import datetime

now = datetime.datetime.now()
sched = '1 15 1,15 * *'    # at 3:01pm on the 1st and 15th of every month
cron = croniter.croniter(sched, now)

for i in range(4):
    nextdate = cron.get_next(datetime.datetime)
    print nextdate

打印

2011-01-15 15:01:00
2011-02-01 15:01:00
2011-02-15 15:01:00
2011-03-01 15:01:00

虽然这将是很好，如果它被写成一个实际的迭代器。也许我有我的下一个项目; - ）

although it would be nice if it were written as an actual iterator. Maybe I've got my next project ;-)