[Python-Dev] Class destructor

Nick Maclaren nmm1 at cus.cam.ac.uk
Wed Feb 28 18:24:57 CET 2007

"Phillip J. Eby" <pje at telecommunity.com> wrote:
> >But I need to clean up workspace when a class (not object) is
> >deallocated.  I can't easily use attributes, as people suggested,
> >because there is no anonymous storage built-in type.  I could subvert
> >one of the existing storage types (buffer, string etc.), but that is
> >unclean.  And I could write one, but that is excessive.
> >       
> >So far, I have been unable to track down how to get something called
> >when a class is destroyed.  The obvious attempts all didn't work, in
> >a variety of ways.  Surely there must be a method?  This could be in
> >either Python or C.
> Have you tried a PyCObject?  This is pretty much what they're for:

Oh, yes, I use them in several places, but they don't really help.

Their first problem is that they take a 'void *' and not a request
for space, so I have to allocate and deallocate the space manually.
Now, I could add a destructor to each of them and do that, but it
isn't really much prettier than subverting one of the semi-generic
storage types for an improper purpose!

It would be a heck of a lot cleaner to deallocate all of my space
in exactly the converse way that I allocate and initialise it.  It
would also all me to collect and log statistics, should I so choose.
This could be VERY useful for tuning!  I haven't done that, yet, but
might well do so.

All in all, what I need is some way to get a callback when a class
object is destroyed.  Well, actually, any time from its last use
for object work and the time that its space is reclaimed - I don't
need any more precise time than that.

I suppose that I could add a C object as an attribute that points to
a block of memory that contains copies of all my workspace pointers,
and use the object deallocator to clean up.  If all else fails, I
will try that, but it seems a hell of a long way round for what I
would have thought was a basic requirement.

Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

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