Explanation of list reference

[Steven D'Aprano]

On Fri, 14 Feb 2014 22:20:35 -0700, Ian Kelly wrote:

> On Fri, Feb 14, 2014 at 9:24 PM, Rustom Mody <rustompmody at gmail.com>
> wrote:
>> In the case of physical objects like dice there is a fairly
>> unquestionable framing that makes identity straightforward --
>> 4-dimensional space-time coordiantes. If the space-time coordinates of
>> 2 objects are all equal then the objects are identical, else not.
>>
>> Now we analogize the space-time identity of physical objects to
>> computer identity of computer objects (so-called) and all sorts of
>> problems ensue.
>>
>> To start with we say two objects are identical if they have the same
>> memory address.
> 
> This is false.  It happens to hold for CPython, but that's an
> implementation detail.  The definition of object identity does not
> depend on memory address.  It also doesn't have anything to do with
> space-time coordinates.  The concept of object identity is an
> abstraction, not an analogy from physics.

Correct. 

CPython does not move objects around in memory during their lifespan, so 
CPython can reuse the memory address as the ID. Jython and IronPython do 
move objects around, so they cannot use memory addresses as IDs. Instead 
they number each object sequentially.

PyPy is even more complicated. Objects, like particles in quantum 
mechanics, can appear and disappear from existence between observations 
as the optimizing compiler does its work. For example, a list of Python 
float objects may be transparently converted into an array of machine 
doubles, then turned back into a Python object when you try to access it 
again. The PyPy compiler has to take great care to ensure that the list 
(and the floats!) get the same IDs before and after, since that is 
defined behaviour in the language spec.



-- 
Steven