Need help porting Prevayler (Java) to Python

[Michal Wallace]

On Wed, 2 Oct 2002, Patrick K. O'Brien wrote:

> Prevayler is an interesting object persistence system written in Java that
> I'd like to see ported to Python. The system is simple, and the code is only
> a couple of hundred lines, so this should be relatively easy to do. I'm
> looking for people with good Java skills to help create a similar system in
> Python. Anyone interested?

This is pretty smart.

I wrote a couple packages ("arlo" and "storage") that do
something very much like this. It's meant to be a front-end
to various storage systems (MetaKit, MySQL, etc), and for
testing purposes, I implemented a purely in-memory database.

My stuff actually transforms the objects to and from
dictionaries, but you wouldn't have to.

I like the idea of persisting the commands, but the idea of
writing your own for every Add/Insert/Delete seems to be
java nastiness to work around the static typing.

I don't think I would port it directly; it's just not very
pythonic. Instead, wouldn't you want something like:

class PyPrevalance:
   def __init__(self):
       self._data = {}

   def add(self, obj):
       klass = obj.__class__
       try:           
           self._data.setdefault(klass, [])
           self._data[klass].append(obj)
           self.log("add", object)
       except Exception, e:
           pass # really, rollback...

   # etc...
   

It *is* nice to be able to fire off actions on an
add/edit/delete, which their use of the Command pattern
gives you. In that case, add() might look like:

    def add(self, obj):
        self._do(GenericAddCommand(obj))

    def _do(self, cmd):
        cmd.execute(self)
        self.log.append(cmd)

    def undo(self):
        cmd = self.log.pop()
        cmd.rollback(self)


I marked _do as private because it seems like you're meant
to subclass the server itself to add your own commands and
queries.

Anyway, doesn't ZODB do all this?


Cheers,

- Michal   http://www.sabren.net/   sabren at manifestation.com 
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