Parse and use crontab schedules in Python

## Project description

Copyright 2011-2021 Josiah Carlson

Released under the LGPL license version 2.1 and version 3 (you can choose which you’d like to be bound under).

## Description

This package intends to offer a method of parsing crontab schedule entries and determining when an item should next be run. More specifically, it calculates a delay in seconds from when the .next() method is called to when the item should next be executed.

Comparing the below chart to http://en.wikipedia.org/wiki/Cron#CRON_expression you will note that W and # symbols are not supported.

Field Name

Mandatory

Allowed Values

Default Value

Allowed Special Characters

Seconds

No

0-59

0

* / , -

Minutes

Yes

0-59

N/A

* / , -

Hours

Yes

0-23

N/A

* / , -

Day of month

Yes

1-31

N/A

* / , - ? L

Month

Yes

1-12 or JAN-DEC

N/A

* / , -

Day of week

Yes

0-6 or SUN-SAT

N/A

* / , - ? L

Year

No

1970-2099

* / , -

If your cron entry has 5 values, minutes-day of week are used, default seconds is and default year is appended. If your cron entry has 6 values, minutes-year are used, and default seconds are prepended.

As such, only 5-7 value crontab entries are accepted (and mangled to 7 values, as necessary).

## Sample individual crontab fields

Examples of supported entries are as follows:

*
*/5
7/8
3-25/7
3,7,9
0-10,30-40/5

For month or day of week entries, 3 letter abbreviations of the month or day can be used to the left of any optional / where a number could be used.

For days of the week:

mon-fri
sun-thu/2

For month:

apr-jul
mar-sep/3

## Installation

pip install crontab

## Example uses

>>> from crontab import CronTab
>>> from datetime import datetime
>>> # define the crontab for 25 minutes past the hour every hour
... entry = CronTab('25 * * * *')
>>> # find the delay from when this was run (around 11:13AM)
... entry.next()
720.81637899999998
>>> # find the delay from when it was last scheduled
... entry.next(datetime(2011, 7, 17, 11, 25))
3600.0

## Notes

At most one of ‘day of week’ or ‘day of month’ can be a value other than ‘?’ or ‘*’. We violate spec here and allow ‘*’ to be an alias for ‘?’, in the case where one of those values is specified (seeing as some platforms don’t support ‘?’).

This module also supports the convenient aliases:

@yearly
@annually
@monthly
@weekly
@daily
@hourly

Example full crontab entries and their meanings:

30 */2 * * * -> 30 minutes past the hour every 2 hours
15,45 23 * * * -> 11:15PM and 11:45PM every day
0 1 ? * SUN -> 1AM every Sunday
0 1 * * SUN -> 1AM every Sunday (same as above)
0 0 1 jan/2 * 2011-2013 ->
midnight on January 1, 2011 and the first of every odd month until
the end of 2013
24 7 L * * -> 7:24 AM on the last day of every month
24 7 * * L5 -> 7:24 AM on the last friday of every month
24 7 * * Lwed-fri ->
7:24 AM on the last wednesday, thursday, and friday of every month

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