On 11/17/05, Thomas Lotze <[EMAIL PROTECTED]> wrote:
> I just noticed two things about persistent.PersistentMapping:
> - It inherits from UserDict.UserDict. Is there any reason not to inherit
> from dict directly, given that this has been possible since Python 2.3
It has been possible to inherit from dictionary since Python 2.2, but
it is not possible for a persistent object and it would not do what
you expect even if it were possible. A persistent object has a custom
C layout and so does dict, so it is not possible to have them both as
base classes. (TypeError: multiple bases have instance lay-out
conflict. (I don't know why there is a hypen in lay-out.)) If it
were possible, it wouldn't work anyway. If you inherit from dict and
override its builtin methods, like __setitem__, your overridden method
will be ignored by C code using methods like PyDict_SetItem().
> - Not all methods of the mapping interface are handled. In particular,
> there's no reason not to handle pop() as popitems() is handled.
> Unhandled methods that change the content of the dict lead to especially
> nasty bugs as they seem to work OK during a transaction while their
> effect is not permanent.
On the other hand, I think that this omission is just an oversight.
It would probably be a good idea to change PersistentDict/Mapping to
use the DictMixin instead of UserDict.
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