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

Tim Peters tim.peters at gmail.com
Fri Jan 18 18:25:48 EST 2019

>>  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.

[Greg Ewing <greg.ewing at canterbury.ac.nz>]
> That sounds dangerous. An id() is only valid as long as the object
> it came from still exists, after which it can get re-used for a different
> object.

The objects are the values in such a dict.

    thedict[id(obj)] is obj

Therefore the objects can't become garbage before id(obj) is deleted
from the dict.

> So when an object is flushed from your cache, you would have
> to chase down all the places its id is being stored and eliminate them.

The dict itself keeps the objects alive.

> Are you sure you couldn't achieve the same thing more safely using
> weak references?

I can't say exactly what MRAB is doing.  I've done things "like that"
for decades, though, and have happily almost never used weakrefs.  I
wouldn't call my uses "caches", though - more like using dicts to
associate info with arbitrary objects (via using the object id as the
dict key), where the object implementations are out of my control and
don't properly support being used as dict keys.

This sometimes includes builtin mutable objects, like lists, or even
other dicts.

No such uses care about object addresses, though - just that id(obj)
returns a value usable as a dict key, unique among all reachable
objects at the time `id()` is called.

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