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 <mi...@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?
Connected by Motorola
Melvyn Sopacua <m.r.sopa...@gmail.com> wrote:
On donderdag 21 juni 2018 16:23:23 CEST clavierpla...@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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