How to set multiple values for the same attribute in tal:attributes using TALES

Question

I'm trying to set multiple css classes on one element.

Unfortunately this doesn't work, as it returns: LanguageError: Duplicate attribute name in attributes.

<ul>
    <li tal:repeat="item mainnav"
        tal:attributes="class 'first' if repeat.item.start else nothing; 
                        class 'last' if repeat.item.end else nothing;
                        class 'active' if item.active else nothing">
        <a tal:attributes="href item.href" tal:content="item.title">title</a>
    </li>
</ul>

Combining those 3 cases into one expression makes it quite complicated, because there are 6 different css states:

• first + active
• first
• last + active
• last
• active
• (none)

There are 2 possible solutions that I can think of:

-> check each combination inline:

<ul>
    <li tal:repeat="item mainnav" 
        tal:attributes="
            class 'first active' if (repeat.item.start and item.active) else
                  'first'        if repeat.item.start else
                  'last active'  if (repeat.item.end and item.active) else
                  'last'         if repeat.item.end else
                  'active'       if item.active else nothing">
        <a tal:attributes="href item.href" tal:content="item.title">title</a>
    </li>
</ul>

-> create a method that returns the combined css classes

Now, is there a better approach and if not, which of those 2 is better (probably the latter one, as if it gets more complicating the inline script will become unreadable/unmanageable).

BTW, are there any good resources and examples about Chameleon, TALES (other than http://chameleon.repoze.org/docs/latest)

Answer 1

You're not using tal:condition, it has a purpose. I don't like overly nested conditionals, gets you no where. Haven't tested this but you may get the idea.

<ul>
    <tal:myloop tal:repeat="item mainnav">
        <li tal:condition="item.active" tal:attributes="class 
            'active first' if repeat.item.start 
            else 'active last' if repeat.item.end 
            else 'active'">
            <a tal:attribute="href item.href" tal:content="item.title"></a>
        </li>
        <li tal:condition="not item.active" tal:attributes="class 
            'first' if repeat.item.start 
            else 'last' if repeat.item.end else None">
            <a tal:attribute="href item.href" tal:content="item.title"></a>
        </li>
    </tal:myloop>
</ul>

Answer 2

You can use tal:define multiple times to define the various parts of your class string, then construct the actual attribute from those parts:

<tal:loop repeat="item mainnav">
    <li tal:define="class_first  'first'  if repeat.item.start else '';
                    class_last   'last'   if repeat.item.end else '';
                    class_active 'active' if item.active else '';"
        tal:attributes="class string:$class_first $class_last $class_active">
       <a tal:attributes="href item.href" tal:content="item.title">title</a>
    </li>
</tal>

This could result in an empty class attribute, which is harmless.

As for additional documentation; Chameleon is an implementation of TAL, originally developed for Zope Page Templates. As such, you'll find a lot of documentation for the latter also applies to Chameleon, as long as you take into account that Chameleon's default TALES modus is python:, while ZPT defaults to path: instead. The Advanced Page Templates chapter of the Zope Book applies to Chameleon as well, for example.

Answer 3

In Chameleon you can do:

<ul>
    <li tal:repeat="item mainnav"
        class="${'first' if repeat.item.start else ''}
               ${'last' if repeat.item.end else ''}
               ${'active' if item.active else ''">
        <a tal:attributes="href item.href" tal:content="item.title">title</a>
    </li>
</ul>

[Edit] Or better like this:

<ul>
    <li tal:repeat="item mainnav"
        class="${('first ' if repeat.item.start else '') +
                 ('last ' if repeat.item.end else '') +
                 ('active' if item.active else '')}">
        <a tal:attributes="href item.href" tal:content="item.title">title</a>
    </li>
</ul>