Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/cuu508/cronsim

Cron expression parser and evaluator.
https://github.com/cuu508/cronsim

cron

Last synced: 3 days ago
JSON representation

Cron expression parser and evaluator.

Awesome Lists containing this project

README 

        
# CronSim



[![Tests](https://github.com/cuu508/cronsim/actions/workflows/pytest.yml/badge.svg)](https://github.com/cuu508/cronsim/actions/workflows/pytest.yml)



Cron Sim(ulator), a cron expression parser and evaluator. Works with Python 3.9+.

CronSim is written for and being used in

[Healthchecks](https://github.com/healthchecks/healthchecks/)

(a cron job monitoring service).



Development priorities:



* Correctness. CronSim tries to match Debian's cron as closely as possible,

  including its quirky behaviour during DST transitions.

* Readability. Prefer simple over clever.

* Minimalism. Don't implement features that Healthchecks will not use

  (for example, the seconds field in cron expressions).



## Installation



```

pip install cronsim

```



## Usage



```python

from datetime import datetime

from cronsim import CronSim



it = CronSim("0 0 * 2 MON#5", datetime(2020, 1, 1))

for x in range(0, 5):

    print(next(it))

```



Produces:



```

2044-02-29 00:00:00

2072-02-29 00:00:00

2112-02-29 00:00:00

2140-02-29 00:00:00

2168-02-29 00:00:00

```



To iterate backwards in time, add `reverse=True` in the constructor:



```python

from datetime import datetime

from cronsim import CronSim



it = CronSim("0 0 * 2 MON#5", datetime(2020, 1, 1), reverse=True)

print(next(it))

```



Produces:



```

2016-02-29 00:00:00

```



If CronSim receives an invalid cron expression, it raises `cronsim.CronSimError`:



```python

from datetime import datetime

from cronsim import CronSim



CronSim("123 * * * *", datetime(2020, 1, 1))

```



Produces:



```

cronsim.cronsim.CronSimError: Bad minute

```



If CronSim cannot find a matching datetime in the next 50 years from the start

date or from the previous match, it stops iteration by raising `StopIteration`:



```python

from datetime import datetime

from cronsim import CronSim



# Every minute of 1st and 21st of month,

# if it is also the *last Monday* of the month:

it = CronSim("* * */20 * 1L", datetime(2020, 1, 1))

print(next(it))

```



Produces:



```

StopIteration

```



## CronSim Works With zoneinfo



CronSim starting from version 2.0 is designed to work with timezones provided by

the zoneinfo module.



A previous version, CronSim 1.0, was designed for pytz and relied on its

following non-standard features:



* the non-standard `is_dst` flag in the `localize()` method

* the `pytz.AmbiguousTimeError` and `pytz.NonExistentTimeError` exceptions

* the `normalize()` method



## Supported Cron Expression Features



CronSim aims to match [Debian's cron implementation](https://salsa.debian.org/debian/cron/-/tree/master/)

(which itself is based on Paul Vixie's cron, with modifications). If CronSim evaluates

an expression differently from Debian's cron, that's a bug.



CronSim is open to adding support for non-standard syntax features, as long as

they don't conflict or interfere with the standard syntax.



## DST Transitions



CronSim handles Daylight Saving Time transitions the same as

Debian's cron. Debian has special handling for jobs with a granularity

greater than one hour:



```

Local time changes of less than three hours, such as  those  caused  by

the  start or end of Daylight Saving Time, are handled specially.  This

only applies to jobs that run at a specific time and jobs that are  run

with  a    granularity  greater  than  one hour.  Jobs that run more fre-

quently are scheduled normally.



If time has moved forward, those jobs that would have run in the inter-

val that has been skipped will be run immediately.  Conversely, if time

has moved backward, care is taken to avoid running jobs twice.

```



See test cases in `test_cronsim.py`, `TestDstTransitions` class

for examples of this special handling.



## Cron Expression Feature Matrix



| Feature                              | Debian | Quartz | croniter | cronsim |

| ------------------------------------ | :----: | :----: | :------: | :-----: |

| Seconds in the 6th field             | No     | Yes    | Yes      | No      |

| "L" as the day-of-month              | No     | Yes    | Yes      | Yes     |

| "LW" as the day-of-month             | No     | Yes    | No       | Yes     |

| "L" in the day-of-week field         | No     | Yes    | No       | Yes     |

| Nth weekday of month                 | No     | Yes    | Yes      | Yes     |



**Seconds in the 6th field**: an optional sixth field specifying seconds.

Supports the same syntax features as the minutes field. Quartz Scheduler

expects seconds in the first field, croniter expects seconds in the last field.



Quartz example: `*/15 * * * * *` (every 15 seconds).



**"L" as the day-of-month**: support for the "L" special character in the

day-of-month field. Interpreted as "the last day of the month".



Example: `0 0 L * *` (at the midnight of the last day of every month).



**"LW" as the day-of-month**: support for the "LW" special value in the

day-of-month field. Interpreted as "the last weekday (Mon-Fri) of the month".



Example: `0 0 LW * *` (at the midnight of the last weekday of every month).



**"L" in the day-of-week field**: support for the "{weekday}L" syntax.

For example, "5L" means "the last Friday of the month".



Example: `0 0 * * 6L` (at the midnight of the last Saturday of every month).



**Nth weekday of month**: support for "{weekday}#{nth}" syntax.

For example, "MON#1" means "first Monday of the month", "MON#2" means "second Monday

of the month".



Example: `0 0 * * MON#1` (at midnight of the first monday of every month).



## The `explain()` Method



Starting from version 2.4, the CronSim objects have an `explain()` method

which generates a text description of the supplied cron expression.



```python

from datetime import datetime

from cronsim import CronSim



expr = CronSim("*/5 9-17 * * *", datetime.now())

print(expr.explain())

```



Outputs:



```

Every fifth minute from 09:00 through 17:59

```



The text descriptions are available in English only. The text descriptions

use the 24-hour time format ("23:00" instead of "11:00 PM").



For examples of generated descriptions see `tests/test_explain.py`.