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

[Richard Damon]

On 11/18/18 7:32 AM, Nick Coghlan wrote:
> On Sun, 4 Nov 2018 at 23:33, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Sun, Nov 04, 2018 at 11:43:50AM +0100, Stephane Wirtel wrote:
>>> In this PR [https://github.com/python/cpython/pull/3382] "Remove reference
>>> to
>>> address from the docs, as it only causes confusion", opened by Chris
>>> Angelico, there is a discussion about the right term to use for the
>>> address of an object in memory.
>> Why do we need to refer to the address of objects in memory?
> Following up on this discussion from a couple of weeks ago, note that
> Stephane misstated Chris's question/proposal from the PR slightly.
>
> The context is that the data model documentation for objects currently
> describes them as having an identity, a type, and a value, and then
> uses "address in memory" as an analogy for the properties that the
> object identity has (i.e. only one object can have a given identifier
> at any particular point in time, but identifiers can be re-used over
> time as objects are created and destroyed).
>
> That analogy is problematic, since it encourages the "object
> identities are memory addresses" mindset that happens to be true in
> CPython, but isn't true for Python implementations in general, and
> also isn't helpful for folks that have never learned a lower level
> language where you're manipulating pointers directly.
>
> However, simply removing the analogy entirely leaves that paragraph in
> the documentation feeling incomplete, so it would be valuable to
> replace it with a different real world analogy that will make sense to
> a broad audience.
>
> Chris's initial suggestion was to use "license number" or "social
> security number" (i.e. numbers governments assign to people), but I'm
> thinking a better comparison might be to vehicle registration numbers,
> since that analogy can be extended to the type and value
> characteristics in a fairly straightforward way:
>
> - the object identity is like the registration number or license plate
> (unique within the particular system of registering vehicles, but not
> unique across systems, and may sometimes be transferred to a new
> vehicle after the old one is destroyed)
> - the object type is like the make and model (e.g. a 2007 Toyota
> Corolla Ascent Sedan)
> - the object value is a specific car (e.g. "that white Corolla over
> there with 89000 km on the odometer")
>
> On the other hand, we're talking about the language reference here,
> not the tutorial, and understanding memory addressing seems like a
> reasonable assumed pre-requisite in that context.
>
> Cheers,
> Nick.

One issue with things like vehicle registration numbers is that the VIN
of a vehicle is really a UUID, it is globally unique no other vehicle
will every have that same ID number, and people may not think of the
fact that some other ID numbers, like the SSN do get reused. Since the
Python Object Identities can get reused after the object goes away, the
analogy really needs to keep that clear, and not give the other extreme
of a false impression that the ID won't get reused after the object goes
away.

-- 
Richard Damon