[Python-Dev] ctypes: is it intentional that id() is the only way to get the address of an object?

Antoine Pitrou solipsis at pitrou.net
Fri Jan 18 05:57:08 EST 2019

On Fri, 18 Jan 2019 03:00:54 +0000
MRAB <python at mrabarnett.plus.com> wrote:
> On 2019-01-18 00:48, Gregory P. Smith wrote:
> > I've heard that libraries using ctypes, cffi, or cython code of various 
> > sorts in the real world wild today does abuse the unfortunate side 
> > effect of CPython's implementation of id(). I don't have specific 
> > instances of this in mind but trust what I've heard: that it is happening.
> > 
> > id() should never be considered to be the PyObject*.  In as much as code 
> > shouldn't assume it is running on top of a specific CPython implementation.
> > If there is a _need_ to get a pointer to a C struct handle referencing a 
> > CPython C API PyObject, we should make an explicit API for that rather 
> > than the id() hack.  That way code can be explicit about its need, and 
> > code that is just doing a funky form of identity tracking without using 
> > is and is not can continue using id() without triggering regressive 
> > behavior on VMs that don't have a CPython compatible PyObject under the 
> > hood by default.
> > 
> > [who uses id() anyways?]
> >   
> I use it in some of my code.
> If I want to cache some objects, I put them in a dict, using the id as 
> the key. If I wanted to locate an object in a cache and didn't have 
> id(), I'd have to do a linear search for it.

Indeed.  I've used it for the same purpose in the past (identity-dict).



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