[Python-Dev] Need discussion for a PR about memory and objects

[Jeff Allen]

On 20/11/2018 00:14, Chris Barker via Python-Dev wrote:
> On Mon, Nov 19, 2018 at 1:41 AM Antoine Pitrou <solipsis at pitrou.net 
> <mailto:solipsis at pitrou.net>> wrote:
>
>     I'd rather keep the reference to memory addressing than start
>     doing car
>     analogies in the reference documentation.
>
>
> I agree -- and any of the car analogies will probably be only valid in 
> some jurisdictions, anyway.
>
> I think being a bit more explicit about what properties an ID has, and 
> how the id() function works, and we may not need an anlogy at all, 
> it's not that difficult a concept. And methions that in c_python the 
> id is (currently) the memory address is a good idea for those that 
> will wonder about it, and if there is enough explanation, folks that 
> don't know about memory addresses will not get confused.
...
> I suggest something like the following:
>
> """
> Every object has an identity, a type and a value. An object’s identity 
> uniquely identifies the object. It will remain the same as long as 
> that object exists. No two different objects will have the same id at 
> the same time, but the same id may be re-used for future objects once 
> one has been deleted. The ‘is’ operator compares the identity of two 
> objects; the id() function returns an integer representing its 
> identity. ``id(object_a) == id(object_b)`` if and only if they are the 
> same object.
>
> **CPython implementation detail:** For CPython, id(x) is the memory 
> address where x is stored.
> """
I agree that readers of a language reference should be able to manage 
without the analogies.

I want to suggest that identity and id() are different things.

The notion of identity in Python is what we access in phrases like "two 
different objects" and "the same object" in the text above. For me it 
defies definition, although one may make statements about it. A new 
object, wherever stored, is identically different from all objects 
preceding it.

Any Python has to implement the concept of identity in order to refer, 
without confusion, to objects in structures and bound by names. In 
practice, Python need only identify an object while the object exists in 
the interpreter, and the object exists as long as something refers to it 
in this way. To this extent, the identifier in the implementation need 
not be unique for all time.

The id() function returns an integer approximating this second idea. 
There is no mechanism to reach the object itself from the result, since 
it does not keep the object in existence, and worse, it may now be the 
id of a different object. In defining id(), I think it is confusing to 
imply that this number *is* the identity of the object that provided it.

Jeff Allen
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