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[Python-Dev] RFC: PEP 445: Add new APIs to customize Python memory allocators

Kristján Valur Jónsson kristjan at ccpgames.com
Wed Jun 19 17:31:35 CEST 2013 
Right, think of the "ctxt" as a "this" pointer from c++.
If you have an allocator object, that you got from some c++ api, and want to ask Python to use that, you need to be able to thunk the "this" pointer to get at the particular allocator instance.
It used to be a common mistake when writing C callback apis to forget to add an opaque "context" pointer along with the callback function.
This omission makes it difficult (but not impossible) to attach c++ methods to such callbacks.

K

> -----Original Message-----
> From: Python-Dev [mailto:python-dev-
> bounces+kristjan=ccpgames.com at python.org] On Behalf Of Scott Dial
> Sent: 19. júní 2013 04:34
> To: ncoghlan at gmail.com
> Cc: Python-Dev at python.org
> Subject: Re: [Python-Dev] RFC: PEP 445: Add new APIs to customize Python
> memory allocators
> 
> On 6/18/2013 11:32 PM, Nick Coghlan wrote:
> > Agreed more of that rationale needs to be moved from the issue tracker
> > into the PEP, though.
> 
> Thanks for the clarification. I hadn't read the issue tracker at all. On it's face
> value, I didn't see what purpose it served, but having read Kristján's
> comments on the issue tracker, he would like to store state for the allocators
> in that ctx pointer.[1] Having read that (previously, I thought the only utility
> was distinguishing which domain it was -- a small, glorified enumeration), but
> his use-case makes sense and definitely is informative to have in the PEP,
> because the utility of that wasn't obvious to me.
> 
> Thanks,
> -Scott
> 
> [1] http://bugs.python.org/issue3329#msg190529
> """
> One particular trick we have been using, which might be of interest, is to be
> able to tag each allocation with a "context" id.  This is then set according to a
> global sys.memcontext variable, which the program will modify according to
> what it is doing.  This can then be used to track memory usage by different
> parts of the program.
> """
> 
> --
> Scott Dial
> scott at scottdial.com
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