tal python expression with condition - plone

Question

<tal:block tal:repeat="image project/images">
<div
tal:define="onlyone python:if repeat.image.length==1: return 'onlyone'"
tal:attributes="class python:'image-{}'.format(repeat.image.number)">
<img
tal:define="img image/getObject"
tal:replace="structure img/##images/image/custom_1280"
/>
</div>
</tal:block>
I already have a class the prints "image-N" based on the loop index, but how do I add another class if the length is "1"?
The documentation is NOT clear https://zope.readthedocs.io/en/latest/zopebook/AppendixC.html#tales-python-expressions, it says any valid python expression can be used but the syntax is always wrong for me.

Answer 1

Fixed it myself.
<tal:block tal:repeat="image project/images">
<div
tal:define="onlyone python:'onlyone' if repeat.image.length == 1 else ''"
tal:attributes="class string:image-${repeat/image/number} ${onlyone}">
<img
tal:define="img image/getObject"
tal:replace="structure img/##images/image/custom_1280"
/>
</div>
</tal:block>

Answer 2

You define onlyone inside repeat, i.e. every iteration - define it outside:
<div class="project-images"
tal:define="onlyone python: len(project.images)==1 and 'onlyone' or '';">
<tal:project-images repeat="image project/images">
<div tal:attributes="class string:image-${repeat/image/number} ${onlyone};">
<img
tal:define="img image/getObject"
tal:replace="structure img/##images/image/custom_1280"
/>
</div>
</tal:project-images>
</div>
But...
your task can (and should) be solved in CSS3 without additional html class:
.project-images div:first-child:nth-last-child(1) {
/**/
}
.project-images div:only-child {
/**/
}