[Zope3-Users] Re: User preferences for languages- possible?

[Philipp von Weitershausen]

Giovannetti, Mark wrote:
>> The key, as you've correctly identified, is write your own 
>> IUserPreferredLanguages adapter (or a variant thereof). I would store 
>> the user's preferences in a cookie or a session. This makes 
>> things much easier. Simply adapt the request to 
>> ISession(request).
> 
> I'll end up having to do something like this, I suppose.
> It really sucks, though, because I *want* to use the
> preferences machinery.  Why was it built if it can't be
> used in certain places - and why aren't the places it can't
> be used documented?  Arrg.

Well, it's pretty simple, I've tried to outline it to you:

No matter whether you're going the route with sessions or the route with 
the preferences machinery: Both need to store their information 
persistently somewhere. That's why they employ local, persistent 
utilities. Those are only available after certain things have happened: 
the database has been opened and the publisher has successfully 
traversed over the sites that contain registered versions of such 
utilities. Then and only then the getUtility calls work.

The ComponentLookupError you were quoting in your original email is, I 
bet my ass on this, due to the fact that the languages adapter is 
invoked right when the request is instantiated, at which point none of 
the steps I outlined above have happened.

>> Note that this might not always work, e.g. when the request is 
>> instantiated and the request's locale is first created (because local 
>> utilities to store the session data in aren't available at 
>> that point). 
> 
> How am I supposed to deal with what you say above?  I don't
> even understand most of it!  ;-)

I'm sorry if I haven't been clear. I also suggest picking up a copy of 
my 2nd edition and reading through the chapter on local sites. It 
explains the concepts of local components.

>> It seems you're hitting that exact problem with the 
>> preference stuff as well. The trick here is to stub this 
>> out when the adapter is first called, 
> 
> 'stub this out ... ' - I don't understand, sorry.
> 
>> and then later during traversal, when you know the local 
>> components are there, you revisit it all again by calling 
>> request.setUpLocale() or whatever it is. 
> 
> I don't know what you mean.  How is a noob supposed to 
> deal with this?
> 
>> The ++lang++ implementation might give some pointers.
> 
> The ++lang++ implementation (and ensuing errors when I tested using
> it) told me that I needed to change BrowserFormLanguages to 
> use IModifiableUserPreferredLanguages.  Since I had to do that
> in order to allow language preferences I decided to get the
> UserPreferences machinery running for the app.  Now I have 
> this problem.

I'll try to be a bit clearer now:

* Your custom language adapter needs to gracefully deal with an absent 
preference machinery. In other words, you should catch that 
ComponentLookupError and fall back to whatever mechanism you want to 
fallback (I suggest you simply fall back to the super class's 
implementation).

* If the preference stuff is correctly set up, normal views invoking the 
translation machinery will get your adapter, there will be no 
ComponentLookupError (because the local site with the persistent 
utilities is active).

* The only problem lies in request.locale which has been initialized too 
early, when the local site wasn't active and when we caught that 
ComponentLookupError and simply ignored it. This should be revisited 
now. I recommend to write an event handler for IBeforeTraverseEvent for 
ISites that re-initializes request.locale. It would look something like 
this:

from zope.component import adapter
from zope.app.component.interfaces import ISite
from zope.app.publication.interfaces import IBeforeTraverseEvent

@adapter(ISite, IBeforeTraverseEvent)
def reinitializeRequestLocale(site, event):
     event.request.setupLocale()

Register that using <subscriber handler=".reinitializeRequestLocale" />.

HTH

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