Class method to process crontab string

[padgy]

Convenience method for creating a crontab object from a string. Many configurations in systems are written as a cron string, so adding a helper function for this would make reading schedules easier.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.79%. Comparing base (54f3824) to head (8113e5a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
celery/schedules.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8250      +/-   ##
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- Coverage   77.80%   77.79%   -0.01%     
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  Files         150      150              
  Lines       18680    18684       +4     
  Branches     3192     3193       +1     
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+ Hits        14534    14536       +2     
- Misses       3854     3856       +2     
  Partials      292      292              

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[auvipy]

I would like to know more context and examples and unit tests for the proposed change

[stumpylog]

I can give at least one example where this would be useful.

In paperless-ngx, the user is able to control the timing of tasks by providing a cron expression to overwrite the default scheduling. We manually process what they give us here to create a celery crontab to configure celery beat.

Having a convenient method built in (and tested by) celery would be useful for us as a celrey beat user.

[auvipy]

we need proper unit tests for considering that, also wait for feedback from more users

[jayeff]

I too think having a convenience method to get a crontab from a cron expression would be helpful in cases where you use configration (e.g. enviroment variable) to define a schedule for some tasks. We use this quite a lot to have different schedules for different runtime enviroments

I would suggest to use the official cron format though as per suggestion

@auvipy In case you give a go ahead I could update the format and add some unit tests to this PR so that it can be merged

[auvipy]

@auvipy In case you give a go ahead I could update the format and add some unit tests to this PR so that it can be merged

sorry for the late response. Please give it a shot if you can!