[Zope3-dev] Re: Proposed widget/schema work for the Rivah sprint (Thursday and Friday this week)

[Jim Fulton]

Florent Guillaume wrote:
> Jim Fulton wrote:
> 
>>> But a schema is useful for more than "specifying the values of an 
>>> object". All notions of schemas I've encountered (CPS Schemas, XML 
>>> Schemas, Archetypes) use the schema to constrain or validate an 
>>> existing object, yes, but also to create new objects from scratch 
>>> (even in the absence of widgets). Being able to specify initial 
>>> values is quite important there.
>>
>>
>> It's not clear to me that XML schemas specify this.  I'm quite skeptical
>> than they do.  (They have this mysterious "initial value" thing that
>> I can't make any sense of.)  I can't believe that XML schemas deal
>> with the history of a document, which would seem necessary to give
>> inital value, as we mean it, any meaning.
> 
> 
> http://www.w3.org/TR/xmlschema-0/#OccurrenceConstraints (2.2.1) says:
> 
>   Default values of both attributes and elements are declared using the
>   'default' attribute, although this attribute has a slightly different
>   consequence in each case. When an attribute is declared with a default
>   value, the value of the attribute is whatever value appears as the
>   attribute's value in an instance document; if the attribute does not
>   appear in the instance document, the schema processor provides the
>   attribute with a value equal to that of the 'default' attribute. Note
>   that default values for attributes only make sense if the attributes
>   themselves are optional, and so it is an error to specify both a
>   default value and anything other than a value of 'optional' for 'use'.
> 
> An "instance document" is comparable to our object, it is what is being 
> validated/filled by the processor.

This is *not* what default/initial-value is in zope schema.  The XML
"default" concept is closest to missing_value in zope schema.

The default attribute of current zope schema fields are defined
(by the README.txt in zope.schema) as:

   "Default field values are assigned to objects when they are first
    created."

Interestingly, we don't really use them that way.  There may
be cases where __init__ honors these specifications.  If there
are, I doubt they are common.  In practice, we use this information to
initialize forms.  That is, we use these as UI hints.  I think this
is a valid use case, but I think it is better served by information
in form fields.

Jim

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