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

[Gregory P. Smith]

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?]

-gps


On Thu, Jan 17, 2019 at 2:26 AM Steven D'Aprano <steve at pearwood.info> wrote:

> Disclaimer: I'm not a ctypes expert, so I might have this completely
> wrong. If so, I apologise for the noise.
>
> The id() function is documented as returning an abstract ID number. In
> CPython, that happens to have been implemented as the address of the
> object.
>
> I understand that the only way to pass the address of an object to
> ctypes is to use that id. Is that intentional?
>
> As I see it, there is a conflict between two facts:
>
> - that id() returns a memory address is an implementation detail; as
> such users should not rely on it, as the implementation could (in
> principle) change without notice;
>
> - but users using ctypes have no choice but to rely on id() returning
> the object memory address, as of it were an offical part of the API.
>
> Implementations like PyPy which emulate ctypes, while objects don't have
> fixed memory locations, will surely have a problem here. I don't know
> how PyPy solves this.
>
> Have I misunderstood something here?
>
>
>
> --
> Steve
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