[Grok-dev] schema.List returns a list, but is it persistent?

Gary Poster gary.poster at gmail.com
Wed Aug 26 16:55:54 EDT 2009

It's actually even more subtle that this.

For the ZODB, PersistentList is OK if you are storing a relatively  
small number of Persistent objects, or a relatively small number of  
relatively small pickle-able objects.  But if you are storing a lot of  
objects, or a medium number of very large pickle-able objects like  
strings, and you are going to mutate the list, the PersistentList has  
bad performance characteristics.  zc.blist attempts to solve the  
problem in that case, for instance.

While I agree that this is a gotcha, I don't see a good answer for  
it.  Under the circumstances, zope.schema.List should probably return  
a list.  Maybe grok itself, with its goals, if it is "opinionated" in  
favor of the ZODB, can make a grok.schema subclass that "does the  
right thing" from grok's perspective.


On Aug 26, 2009, at 12:26 PM, Sebastian Ware wrote:

> I eventually realised that :) but I do think it is a bug in applyData,
> since you would really expect applyData to return automatically
> persistent data types (at least as default). Right now it is kind of
> "it just works... most of the time...".
> Mvh Sebastian
> On 26 aug 2009, at 18.13, Leonardo Rochael Almeida wrote:
>> On Wed, Aug 26, 2009 at 12:25, Leonardo Rochael
>> Almeida<leorochael at gmail.com> wrote:
>>> [...]
>>> On the other hand, it's very easy to create your own class that
>>> extends from schema.List to return your own datatype. IIRC you only
>>> have to override one attribute in the subclass, and it will receive
>>> the same widgets as the superclass unless you register differently  
>>> or
>>> alter the implemented interfaces.
>> The attribute is _type, btw. The reason PersistentList is not the
>> default is because zope.schema doesn't assume you're going to use it
>> with zodb.
>> Cheers, Leo
>>> Cheers, Leo
>>> On Wed, Aug 26, 2009 at 11:56, Sebastian
>>> Ware<sebastian at urbantalk.se> wrote:
>>>> Lists need to be implemented as PersistentList in order to be
>>>> automatically persisted to ZODB, however schema.List returns an
>>>> ordinary list and applyData seemingly stores this list as is. Isn't
>>>> this a big problem?
>>>>    tags = schema.List(title=u"Content Tags",
>>>> value_type=schema.Choice(title=u"Content Tag",
>>>>           vocabulary='Content Tags'), default = [])
>>>> Mvh Sebastian
>>>> _______________________________________________
>>>> Grok-dev mailing list
>>>> Grok-dev at zope.org
>>>> http://mail.zope.org/mailman/listinfo/grok-dev
> _______________________________________________
> Grok-dev mailing list
> Grok-dev at zope.org
> http://mail.zope.org/mailman/listinfo/grok-dev

More information about the Grok-dev mailing list