Update with pickle

[Matteo Dell'Amico]

Nicolas Fleury wrote:
> I just added a feature request on sf to have an updating load function 
> in pickle.  My preferred solution for now might still be the copy of 
> __dict__ (I could copy slots also), it's simple and not intrusive (using 
> proxies is, even if less a hack).  However, the cleanest solution would 
> be to implement your solution, but I have no idea how to do it for now.

Leif is essentially proposing a non-transparent proxy, while the 
cookbook recipe I pointed out is a transparent proxy (i.e. you are using 
a proxy, but you can access it just as if it was the original object).

If I understood correctly, you want something like this:

 >>> pickle.dump(42, 'file')
 >>> foo = foo2 = 'foo'
 >>> pickle.update(foo, 'file')
 >>> foo
42
 >>> foo2
42

Well, it's not possible: variables are just names attached to objects, 
and while you can assign a name to an object, you *can't* change the 
object's type, or replace an object with another one.

And I think that copying __dict__ is a hack: it wouldn't work, for 
instance, with builtin types, when __slots__ are used, or when you are 
using properties.

With a proxy you could do something like this:

 >>> foo = proxy(foo)
(use foo just as before)
 >>> pickle.dump(foo._obj, 'file')

and then resume the file with
 >>> foo._obj = pickle.load('file')

A transparent proxy is a clean way to do it: through delegation, it 
would work with any kind of object, changes in code are localized in the 
load/save part, and the rest of the application wouldn't even notice.

-- 
Ciao,
Matteo