[Zope] chain menu

[Nico de Boer]

Hello,

I'm working on a chain menu which displays the following:

Previous | Current objectid | Next

Previous and Next are hyperlinks. When I have peer objects 
(brothers and sisters) it is working fine. But when I have 
no peers, I get the following error: 

File /usr/lib/zope/lib/python/Products/PageTemplates/ZRPythonExpr.py, line 49, in __call__
    (Info: chain[0])
  File Python expression "chain[0]", line 2, in f
  File /usr/lib/zope/lib/python/AccessControl/ZopeGuards.py, line 90, in guarded_getitem
TALESError: Sorry, an error occurred

This is my code (from a Zope Page Template): (thanks David)

<span tal:define="chain here/list_chain">
 <a href="#" tal:condition="python:chain[0]" tal:attributes="href python:chain[0].absolute_url()">Previous</a> 
 <b tal:content="python:chain[1].title_or_id()">Sample title</b>
 <a href="#" tal:condition="python:chain[2]" tal:attributes="href python:chain[2].absolute_url()">Next</a>
</span>

This is the list_chain method that I call in the above ZPTemplate:

    def list_chain(self, sortkey='id'):
        """return (previous,current,next) objects """
        NewsTuples = [] # a list of (sortkeyvalue,object) tuples
        for item in self.REQUEST['PARENTS'][1].objectValues('News'):
            if item.inDateRange():
                NewsTuples.append(
                    (getattr(item,sortkey),item)
                    )
        NewsTuples.sort()
        (previous,current,next)=(None,None,None)
        for (sortkey,object) in NewsTuples:
            if current is not None:
                return(previous,current,object)
            elif object.id == self.id:
                current=object
            else:
                previous=object
        if previous is not None:
            return(previous,current,next)

Can someone help me with this problem?

Greetz Nico