[Python-Dev] Re: print "%X" % id(object()) not so nice

[Bob Ippolito]

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...
>> [snip]
>> I propose that id() always return a positive value ...
>> Comments?
>
> 1. CPython intentionally searches builtins afters globals and 
> pre-imports
> 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
> ...
>>>> id(a)
> 9158224
> # only semi-tested because I don't have negatives to test and can't 
> force
>
> 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 
> contemporaneous
> objects but, for CPython, definitely not with respect to objects with
> non-overlapping lifetimes.  (Newbies often get tripped by the last 
> fact.).
>> From the viewpoint of Python, ids have no meaning and are only useful 
>> for
> 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.  
> But
> 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 
> addresses
> 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 
output anyway
- (Python 2.4) You get a repr with a strange looking negative hex 
number (0x-FF0102)

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 
formatter:

import sys
HUGEINT = (sys.maxint + 1L) * 2L

def uid(obj):
     """
     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
     return rval

-bob