[Python-Dev] Creating dicts from dict subclasses

[Guido van Rossum]

On 12/13/06, Walter Dörwald <walter at livinglogic.de> wrote:
> Guido van Rossum wrote:
> > On 12/13/06, Walter Dörwald <walter at livinglogic.de> wrote:
> >> I tried to reimplement weakref.WeakValueDictionary as a subclass of
> >> dict. The test passes except for one problem: To compare results
> >> test_weakref.py converts a weakdict to a real dict via dict(weakdict).
> >> This no longer works because PyDict_Merge() does a PyDict_Check() on the
> >> argument and then ignores all overwritten methods. (The old version
> >> worked because UserDict.UserDict was used).
> >>
> >> The simplest solution is to replace the PyDict_Check() call with
> >> PyDict_CheckExact(), but this might slow things down too much, because
> >> the fallback code basically does:
> >>
> >>    for key in iter(arg.keys()):
> >>       self[key] = arg.__getitem__(key)
> >>
> >> Why can't we use:
> >>
> >>    for key in iter(arg):
> >>       self[key] = arg.__getitem__(key)
> >>
> >> instead?
> >
> > The only reason I can think of is backwards compatibility: not all
> > "mappings" created pre-2.2 would support iteration. Maybe you could
> > check for a tp_iter slot and if non-NULL use the latter otherwise use
> > the original fallback?
>
> This doesn't seem to work. It breaks test_update() in test_dict.py which
> does this:
>
>     d = {}
>     class SimpleUserDict:
>         def __init__(self):
>             self.d = {1:1, 2:2, 3:3}
>         def keys(self):
>             return self.d.keys()
>         def __getitem__(self, i):
>             return self.d[i]
>     d.update(SimpleUserDict())
>     self.assertEqual(d, {1:1, 2:2, 3:3})
>
> This fails with
>
>     KeyError: 0
>
> because SimpleUserDict doesn't implement __iter__, so it gets an
> iterator implementation via __getitem__.
>
> So maybe this only makes sense for Python 3.0 where we can demand that
> dict-like classes implement __iter__?

Ah, right. But I think you should still use PyDict_CheckExact, and
slow fallbacks be damned. (I guess you could look for iterkeys first.)

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)