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

[Josiah Carlson]

On Sat, Sep 13, 2008 at 1:20 PM, Armin Ronacher
<armin.ronacher at active-4.com> 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.

i = list(d.keys())

 - Josiah