A Jinja extension that adds a {% showonmatch 'some css selector' %} tag. The content is removed if (parsed as HTML) nothing in it matches the CSS selector

gistfile1.py

from jinja2 import nodes
from jinja2.ext import Extension
from lxml.cssselect import CSSSelector
from lxml.html import fromstring


class ShowOnMatch(Extension):
    tags = set(['showonmatch'])

    def parse(self, parser):
        lineno = parser.stream.next().lineno
        selector = parser.parse_expression()
        body = parser.parse_statements(['name:endshowonmatch'],
                                       drop_needle=True)
        return nodes.CallBlock(self.call_method('_callback', (selector,)),
                               [], [], body).set_lineno(lineno)

    def _callback(self, selector, caller):
        """Helper callback."""
        rv = caller()
        cssselector = CSSSelector(selector)
        matches = cssselector(fromstring(rv))
        if matches:
            return rv
        else:
            return ''