On Fri, Jan 24, 2020 at 05:45:35AM -0500, Terry Reedy wrote:
> On 1/24/2020 3:36 AM, Victor Stinner wrote:
> >CPython current behavior rely on the fact that it's possible to get
> >the memory address of an object.
>
> No, this behavior relies on the language specification that all objects
> have temporally unique integer ids that can be compared with 'is'.
>
> >I don't think that it should be part of Python language specification,
> >but seen as a CPython implementation detail.
>
> Ids are a language feature; ids being addresses is a CPython detail, but
> this detail is not relevant to equality comparison of items within
> containers.
Thanks Terry for raising this. Of course CPython can optimize identity
tests by checking for the same memory address, and *memory address* is
an implementation-detail of object identity.
Any Python implementation ought to have a sense of object identity,
however it is implemented or emulated. For instance, both Java and .Net
have compacting memory models, which means objects can move around in
memory and there's no well-defined "memory address", but both have to be
able to implement the `is` operator:
* Java has the `==` operator;
* .Net, or at least C#, has Object.ReferenceEquals
If there happens to be a Python interpreter where testing object
identity is expensive, then they should be free to *not* bypass the
equality test.
Victor:
> >In PyPy, you may or may not have an "object". It depends how PyPy
> >decides to store values. For example, if a list only contains small
> >integers: PyPy uses a list specialized for integers and store
> >integers, not objects which stores integers.
I believe that PyPy nevertheless offers the illusion that such lists
contain objects, complete with IDs. They even manage to keep the ID
consistent across copies. For example:
>>>> L = [99]
>>>> id(L[0])
1585
>>>> id(L[:][0])
1585
so even if the list is implemented as an array of native integers rather
than int objects, as far as Python code is concerned there is no
behavioural difference.
--
Steven
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