<a href="valid/link.html" 
   tal:attributes="href python:'/base/' + attrs['href']">blah, blah</a>

<a href="valid/link.html"
   tal:define="origHref attrs/href"
   tal:attributes="href string:/base/$origHref">blah, blah</a>

Fix a test which depended upon Python
Fix a test which depended upon Python
