[Zope3-Users] What attributes are made persistent

[Peter Bengtsson]

---------- Forwarded message ----------
From: Peter Bengtsson <peter at fry-it.com>
To: Jeff Shell <eucci.group at gmail.com>
Date: Wed, 15 Feb 2006 13:14:58 +0000
Subject: Re: [Zope3-Users] What attributes are made persistent
On 2/15/06, Jeff Shell <eucci.group at gmail.com> wrote:
> On 2/14/06, Peter Bengtsson <mail at peterbe.com> wrote:
> > D'oh! That's confusing. Isn't there a class that gathers all of these in one.
> >
> > It seems confusing, you derive from Persistent but only some are accepted.
> > Does that mean that there's PersistentFloat and PersistentTuple too?
> > If not, why *only* dicts?
>
> As mentioned above, it applies to *mutable* objects.
>

>
> Instances of Persistent based classes know when they change. Like if you do::
>
>     clive.age = 28
>

>
> On the other hand, if you do::
>
>     clive.favoriteNumbers.append(13)
>
> 'clive' does not change. 'favoriteNumbers' changes. If favoriteNumbers
> is a regular Python list, the persistence machinery has no idea that
> it's changed. It's not being assigned to 'clive', it's already an
> attribute there and is not being replaced. So if it's not a
> PersistentList, it gets forgotten.
>

>
> It's not only dicts, it's dicts and lists (PersistentDict and PersistentList).
>
> I don't know if there's a PersistentSet. Python offers two sets since
> 2.3 - a mutable (list-like) one and an immutable (tuple-like) one. I
> imagine that if you use the mutable set (``sets.Set`` in 2.3, ``set``
> in 2.4), you'd run into the same problems. But if you used the
> immutable set (``sets.ImmutableSet``, ``frozenset`` in 2.4) you
> wouldn't.
>

>
> So - just use PersistentList and PersistentDict (or look into BTrees
> for better storage options).
>
> For more details, visit the ZODB documentation on ZODB programming,
> and visit the section on modifying mutable objects:
>
> http://www.zope.org/Wikis/ZODB/FrontPage/guide/node3.html
>

I understand the mutation stuff and I always do it like this in zope2
(I'm a complete beginner in the zope3 world eager to learn):

def updatesometing(self):
   #self.numbers['Peter'] = "0779 123 456"
   numbers = self.numbers
   numbers['Peter'] = "0779 123 456"
   self.numbers = numbers

But in zope2 land, if I derive from Persistent it magically saves ALL
attributes defined in __init__ assuming that I post-write to them
correct as shown above.

I like this simplicity and was hoping to find it in zope3 land too. I
guess I'm just an old dog and if I want to drag with me the zope2 way
I can do this::

import persistent.mapping.PersistentMapping
import persistent.list.PersistentList
import persistent.dict.PersistentDict

class PersistentAnything(PersistentMapping, PersistentList, PersistentDict):

I'm aware of BTrees and am/will always use it when size of objects
becomes "uncontrollable".

Am I just trying to make it too simple? Have I read one too many books
by Steve Krug?

--
Peter Bengtsson,
work www.fry-it.com
home www.peterbe.com
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