Storing empties

[Alex Martelli]

Aahz <aahz at pythoncraft.com> wrote:
   ...
> >For pickling, object() as a unique "nothing here, NOT EVEN a None"
> >marker (AKA sentinel) works fine.
> 
> How does that work?  Maybe I'm missing something obvious.
> 
>     sentinel = object()
>     class C:
>         def __init__(self, foo=sentinel):
>             self.foo = foo
>         def process(self):
>             if self.foo is not sentinel:
>                 ....
> 
> Now, the way I understand this, when your application restarts and an
> instance of C is read from a pickle, your sentinel is going to be a
> different instance of object() and process() will no longer work
> correctly.  Are you suggesting that you need to pickle the sentinel with
> the instance?  Or is there some other trick I'm missing?

Yes, I'd set self.sentinel=sentinel (and test wrt that) -- while in the
abstract it would be better to set sentinel at class level, since
classes are only pickled "by name" that wouldn't work.

If you don't need the absolute ability to pass ANY argument to C(),
there are of course all sorts of workaround to save some small amount of
memory -- any object that's unique and you never need to pass can play
the same role as a sentinel, obviously.


Alex