[New-bugs-announce] [issue7366] weakref module example relies on behaviour not guaranteed by id()
report at bugs.python.org
Fri Nov 20 07:07:35 CET 2009
New submission from James Henstridge <james at jamesh.id.au>:
The documentation for the weakref module contains an example that uses
WeakValueDictionary to implement a id2obj() lookup function that doesn't
store strong references to those objects.
This example implicitly assumes that the id() of an object will be
unique for the lifetime of the interpreter, when it is only unique for
the lifetime of the object. The problem can be demonstrated like so:
1. create an object "foo"
2. function 1 remembers the id of this object with "oid1 = remember(foo)"
3. "foo" gets garbage collected
4. an object "bar" is created and happens to get the same memory location
5. function 2 remembers the id of this object with "oid2 = remember(bar)"
6. function 1 looks up its stored object ID with "id2obj(oid1)"
In step 6, the object "bar" is returned rather than an exception being
As well as the example being broken, the weakref module contains the
functionality a programmer would need to do this kind of thing safely:
use the weakref.ref type directly.
It'd be good to replace the example with a better one.
nosy: georg.brandl, jamesh
title: weakref module example relies on behaviour not guaranteed by id()
versions: Python 2.7, Python 3.2
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