[Python-Dev] Creating dicts from dict subclasses

[Guido van Rossum]

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?

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