New submission from James Henstridge ja...@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
raised.
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.
--
assignee: georg.brandl
components: Documentation
messages: 95530
nosy: georg.brandl, jamesh
severity: normal
status: open
title: weakref module example relies on behaviour not guaranteed by id()
versions: Python 2.7, Python 3.2
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http://bugs.python.org/issue7366
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