gistfile1.txt

Python:
def attr(_, v, d):
    return d if v == '' else v

def render(templ):
    global xslt
    ex = {('local-ns', 'attr') : attr}
    ns = {'ext' : 'local-ns'}
    return etree.tostring(xslt(templ, namespaces=ns, extensions=ex))


XSLT:
<xsl:template match="somenode">
    <a href="{ext:attr(string('111'),string('222'))}"/>
</xsl:template>


XML:
<somenode/>


Output:

Exception TypeError: TypeError(u'Argument must be string or unicode.',) in 'lxml.etree.XSLT._run_transform' ignored

Traceback (most recent call last):
  File "/home/eugeny/Work/ajenti/ajenti/app/application.py", line 88, in dispatcher
    self.start_response)
...
bla-bla-bla
...
  File "/home/eugeny/Work/ajenti/ajenti/ui/template.py", line 41, in render
    return xslt.render(self._dom.getroot())
  File "/home/eugeny/Work/ajenti/ajenti/ui/xslt.py", line 28, in render
    return etree.tostring(xslt(templ, namespaces=ns, extensions=ex))
  File "xslt.pxi", line 556, in lxml.etree.XSLT.__call__ (src/lxml/lxml.etree.c:110044)
XSLTApplyError: Error applying stylesheet