On Fri, Jan 01, 2021 at 02:53:33PM -0500, Eric V. Smith wrote:
> On 1/1/2021 2:00 PM, Jonathan Fine wrote:
> >
> >By the way, this surprised me. Would anyone like to explain this?
> > >>> id(f()), id(f())
> > (139927013695536, 139927013695536)
> >
> id's are only unique while an object exists. Here the object returned by
> the first call is discarded after its id is taken. Then the second
> object is created and reuses the memory address that was used by the
> first object. Since in CPython id() returns object's memory address, the
> id's are the same.
What Eric said, except I would put more emphasis on two things:
1. I would emphasize more strongly that IDs are abstract identity
numbers, not memory addresses except as an accident of implementation.
For example in Jython and IronPython, the IDs are sequential numbers
and won't be reused.
In general, if the Python implementation is using a compacting garbage
collector where objects can move in memory, IDs cannot be memory
addresses.
2. And that it's a matter of happenstance, not design, that the same
memory address happens to be used.
I can't produce a demonstration right now, but I've seen senarios where
every *second* temporary object reuses the previous IDs, so printing a
sequence of IDs goes something like this:
1012, 1042, 1012, 1042, 1012, 1042 ...
Presumably that could happen if the garbage collector takes longer to
reclaim each temporary object than it takes the interpreter to generate
and process the next one.
--
Steve
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