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

[Nick Coghlan]

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.

--
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia