[Zope3-dev] _validate on schema fields gets called before services exist

[Jim Fulton]

Steve Alexander wrote:
> I'm trying to write a field that validates its contents by asking a 
> service a question.
> 
> However, I can't assume that the service exists and use getService in 
> the _validate method, because when zcml is read in as zope3 is started 
> up, for some reason, the field's _validate gets called. I guess this is 
> because the Schema I'm using the field in gets executed by the python 
> interpreter.
> 
> Does _validate have to be called at this time?
> 
> What is the best thing to do when writing a Field that wants to use a 
> service to help it validate things?

Actually, your problem was caused by something else.  You initialized
your field like this:

permission = PermissionField("The permission needed to use the component.")

but should have initialized it like this:

permission = PermissionField(
         title=u"The permission needed to use the component.")

This is a common mistake. We really do eventually need to:

   - Make it possible to initialize fields with positional doc strings, and

   - catch and prohibit other positional args to catch this common mistake.

Jim

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