Re: prevent AppConfig.ready() from running twice

IMHO, some of (a lot of ?) the "classical" GOF patterns do not really apply to Python (or at least are not necessary, when not making things more confuse). They are often a consequence of constraints and limitations of statically compiled languages such as C++, Java and alike used at the time they have been created, but they loose most of their interest for languages such as Python.

If the singleton instance is used internally by your application code, and apart if you have really good reasons for making things more complicated, juts use a module level variable to store the instance, and initialize it at application start. Of course, forbid yourself to write to this variable from elsewhere than the app initialization code.

If the singleton creation can be called from other places (f.i. if a lazy initialization strategy is used), and if there are possibilities for it to be called several times, a guard can be implemented by providing a factory function which will test if the instance is currently None, instantiate one if needed, store it in the module variable, and return the module variable at the end. It would look like this:

_singleton_instance = None  

def get_singleton():

    if _singleton_instance is None:

        _singleton_instance = .... 

    return _singleton_instance

The '_' prefix of the singleton variable is use to indicate that this is a "private" variable which must not be referred to (even in read access) from outside the module.

Hope this helps.

Regards

Eric

---

From: django-users@googlegroups.com <django-users@googlegroups.com> on behalf of Mike Dewhirst <miked@dewhirst.com.au>
Sent: Saturday, June 23, 2018 2:01:06 AM
To: django-users@googlegroups.com
Subject: Re: prevent AppConfig.ready() from running twice

 

Is there a python singleton pattern which might work?

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Melvyn Sopacua <m.r.sopacua@gmail.com> wrote:

On donderdag 21 juni 2018 16:23:23 CEST clavierplayer@gmail.com wrote:

> If it helps, here is the reason I need to override this multi-instantiation
> behavior: my application launches a multiprocessing.Process at startup to
> monitor and run background tasks. Having more than one background Process
> running at once is going to wreak havoc on the application. I've looked at
> other options to accomplish similar purposes, but those would all be
> instantiated multiple times, also.
>
> Any suggestions?

Use a locked pidfile to prevent multiple daemons starting up. I recall the
python-daemon package being capable of this (and lots of other good stuff).

https://pagure.io/python-daemon/
--
Melvyn Sopacua

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