[Grok-dev] Problems rendering zope.schema.List

Sebastian Ware sebastian at urbantalk.se
Thu Jul 5 10:11:26 EDT 2007

Wonderful! It works. It would have helped if it was mentioned in the  
docstring of the interface, or any of the accompanying help files... :)

Mvh Sebastian

5 jul 2007 kl. 15.41 skrev Leonardo Rochael Almeida:

> On 7/5/07, Luciano Ramalho <luciano at ramalho.org> wrote:
>> This is a gotcha that got me before... Philikon helped me: add a
>> default argument to the list constructor, like this:
>>         creators = schema.List(title=u"Authors", required=False,
>>                                value_type=schema.TextLine(),  
>> default=[])
>> The problem is that the list must never be None. When empty, it  
>> must be [].
>> IMHO, it's a usability bug in the Zope3 API. If the list fails
>> miserably when it's None, then the default=[] should be automatic. Or
>> is there some ratinonale for the way things are?
> I suppose the problem is that you're specifying a mutable object as
> the default for a field. This could cause you problems if someone
> mutates this value afterward.
> In a simple web app where all interactions are caused by form
> handling, it could well be the case that no one will mutate the
> default object (I think the default widget always creates a new list
> when returning the value). But suppose some piece of code takes an
> object that was set with the default value and manipulates the list
> directly, this would change the default value for the schema itself.
> All schema fields like List contain a reference to the type of the
> object they're supposed to create (defaults to the standard "list"). I
> think the proper default behaviour, in this case, would be to set the
> default value with a fresh instance of the underlying type. This could
> be done by making the "default" attribute of the schema field instance
> be a property.
> Cheers, Leo

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