Hi Walter, On Wed, Dec 13, 2006 at 05:57:16PM +0100, Walter D?rwald 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).
This is an instance of a general problem in Python: if you subclass a built-in type, then your overridden methods may or may not be used in various situations. In this case you might have subtle problems with built-in functions and statements that expect a dict and manipulate it directly, because they will see the underlying dict structure. It is also quite fragile: e.g. if a future version of CPython adds a new method to dicts, then your existing code will also grow the new method automatically - but as inherited from 'dict', which produces quite surprizing results for the user. > 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) The latter would allow 'arg' to be a sequence instead of a mapping. It may even not crash but produce nonsense instead, e.g. if 'arg' is a list of small integers. Moreover there are multiple places in the code base that assume that mappings are "something with a 'keys' and a '__getitem__'", so I suppose any change in that should be done carefully. A bientot, Armin _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
