[Python-Dev] Weak Dictionary Iteration Behavior in Python 3

[Virgil Dupras]

I would also like to point out that I submitted a patch related to  
that a couple of months ago in:

http://bugs.python.org/issue839159

But it never got any attention :( I'm not sure if it is still relevant.

Virgil




On 13-Sep-08, at 10:20 PM, Armin Ronacher wrote:

> Hi everybody,
>
> In Python 2.x when iterating over a weak key dictionary for example,  
> the common
> idom for doing that was calling dictionary.keys() to ensure that a  
> list of all
> objects is returned it was safe to iterate over as a weak reference  
> could stop
> existing during dict iteration which of course raises a runtime  
> error by the
> dict iterator.
>
> This was documented behavior and worked pretty well, with the small  
> problem that
> suddenly all references in the dict wouldn't die until iteration is  
> over because
> the list holds references to the object.
>
> This no longer works in Python 3 because .keys() on the weak key  
> dictionary
> returns a generator over the key view of the internal dict which of  
> course has
> the same problem as iterkeys in Python 2.x.
>
> The following code shows the problem::
>
>    from weakref import WeakKeyDictionary
>
>    f1 = Foo()
>    f2 = Foo()
>    d = WeakKeyDictionary()
>    d[f1] = 42
>    d[f2] = 23
>
>    i = iter(d.keys()) # or use d.keyrefs() here which has the same  
> problem
>    print(next(i))
>    del f2
>    print(next(i))
>
> This example essentially dies with "RuntimeError: dictionary changed
> size during iteration" as soon as f2 is deleted.
>
> Iterating over weak key dictionaries might not be the most common  
> task but I
> know some situations where this is necessary.  Unfortunately I can't  
> see a
> way to achieve that in Python 3.
>
> Regards,
> Armin
>
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