[ZODB-Dev] PersistentMapping

[Jeremy Hylton]

On 11/17/05, Thomas Lotze <tl at gocept.com> wrote:
> Hi,
>
> 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
>   IIRC?

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.

Jeremy