On 18 Jan 2019, at 11:57, Antoine Pitrou wrote:
On Fri, 18 Jan 2019 03:00:54 +0000
MRAB <pyt...@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).
Its useful in all situations where you do topology preserving
transformations, for example pickling (i.e. object serialization) or a
deep copy of some object structures.
In these cases you need a way to record and quickly detect whether
you've handled a specific object before. In Python we can do that with a
dictionary that has object ids as keys. Java provides IdentityHashMap
for that. Javascript provides neither, so deep-copying objects in
Javascript seems to be impossible.
Regards
Antoine.
Servus,
Walter
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