python destructor

Ferencik Ioan ferencikioan at gmail.com
Mon Nov 5 10:07:40 CET 2012


On Monday, November 5, 2012 8:51:00 AM UTC+2, Ferencik Ioan wrote:
> Hello there folks,
> 
> 
> 
> I have a bit of a special issue.
> 
> I'll start by disclosing myself for what i am doing. I am a postgraduate student and I really have good reasons to do what I am doing. At least i think so.
> 
> 
> 
> And not the issue.
> 
> I am building a python web service. This web service has some generic objects and I use a metaclass to customize the classes. 
> 
> Second I use a non-conventional object oriented database, dybase 
> 
> 
> 
> (http://www.garret.ru/dybase/doc/dybase.html#introduction)
> 
> 
> 
> Now these is a OODBMS claiming to support ACID transactions.
> 
> 
> 
> 
> 
> The instances of my objects are recursively organizing themselves into a hierarchical tree-like structure. When I make an instance of this object persistent dybase actually can recursively save all tree structure.
> 
> Everything works well here. 
> 
> 
> 
> I altered the main class situated at the root of my class hierarchy to actually store inside the__dict__ not the instances of its children but their unique ID's. Then when I set a child attribute I create it and instead of being stored in the instance the child goes to a database index object. Thus it becomes Universally addressable. The a parent retrieves the child it actually fetches it from the database. 
> 
> In this way I ended up with very small objects.However these objects can regenerate the treelike structure as if they were storing there children in the __dict__.
> 
> 
> 
> The issue is how to give the instances access to the database and properly handle the opening and closing of the database.
> 
> It seems futile to me to actually open/close the connection through a context. Because the database is a file it will issue an IO operation on every attribute access and we all know __getattribute__ is used extremely often.
> 
> For this reason I thought the best way would be to wrap the dybase Storage (main class) into a local storage version which would have __del__ method.
> 
> The local Storage is a new style class..it opens the DB file but the __del__ is never called.
> 
> This is because the Storage class has at least 2 cyclic references. 
> 
> So my Storage class never closes the database. I would like this class to close the database when it is garbage collected.
> 
> The class is a Singleton FYI as well but this might not be relevant or even necessary.
> 
> So my question is:
> 
> what s the best way to force __del__ on a singleton that has cyclic references. Should i use weakref and alter the original source? Is there a way i can force a singleton to garbage collect itself?.
> 
> 
> 
> I am by no means a software engineer so i would appreciate any advice from some experts on the matter.
> 
> Thank you in advance.

Just in case somebody is interested:
Because my Storage is a singleton I registered the close() method with atexit from the Storage open().

This actually closes the connection. Not sure if this is feasible but it WORKS!

I am using mod_wsgi in daemon mode so I have multithreading issues. If I configure the mod_wsgi with one process dybase works correctly.
I have to override the Persistent.store() and make it thread safe using multiprocessing. This is for ANYONE who uses or plans to use dybase in a web environment.


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