[Python-Dev] Re: print "%X" % id(object()) not so nice
bob at redivi.com
Sat Nov 20 01:55:08 CET 2004
On Nov 20, 2004, at 2:38 AM, Terry Reedy wrote:
> "James Y Knight" <foom at fuhm.net> wrote in message
> news:582E4A36-3A6C-11D9-AA57-000A95A50FB2 at fuhm.net...
>> I propose that id() always return a positive value ...
> 1. CPython intentionally searches builtins afters globals and
> the former as __builtins__ just so one can wrap a builtin to modify its
> apparent behavior for personal needs.
>>>> def id(o, max = 2**32):
> ... i = __builtins__.id(o)
> ... return (i < 0) and (max - i) or i
> # only semi-tested because I don't have negatives to test and can't
> 2. Given that, why bother changing the language for what must be an
> esoteric need (to formattedly print and view ids)?
> The id of an object is constant and unique with respect to
> objects but, for CPython, definitely not with respect to objects with
> non-overlapping lifetimes. (Newbies often get tripped by the last
>> From the viewpoint of Python, ids have no meaning and are only useful
> identity comparision. For this purpose, arbitrary strings would have
> worked as well as integers.
> For convenience, CPython uses the apparent address stored as an int.
> this is strictly an implementation detail. On modern systems, that
> 'address' is, I believe, a process-specific virtual address which the
> hardware memory management system maps the hidden real address --
> which is
> the only reason why systems with less than 2**31 memory can have
> at or above 2**31 to become negative ints.
The problem, more than anything else, is the following behavior that
can happen during a random __repr__ or repr-like-function if the object
happens to have a certain address range:
- (Python 2.3) You get an unexpected and unwanted warning but expected
- (Python 2.4) You get a repr with a strange looking negative hex
Neither of these are fatal, of course, it's just annoying.. I find the
Python 2.3 behavior more obnoxious than Python 2.4's, personally.
FYI, I have also encountered this "problem" this week on a Powerbook G4
w/ only 1GB physical memory on both Python 2.3 and 2.4. I'm at the
PyPy sprint, and a lot of the tools we are using make use of repr.
Fortunately we have control over all of this code, so I checked in a
workaround that makes sure a sane value was passed to the hex
HUGEINT = (sys.maxint + 1L) * 2L
Return the id of an object as an unsigned number so that its hex
representation makes sense
rval = id(obj)
if rval < 0:
rval += HUGEINT
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