piranha@gto 1 ~/test> cat j.py 
from jinja2 import Environment, FileSystemLoader, ext, nodes

class MediaExt(ext.Extension):
    tags = ['media']

    def parse(self, parser):
        parser.stream.next()
        args = parser.parse_expression()

        return [nodes.Const('test')]

loader = FileSystemLoader('.')
env = Environment(loader=loader, extensions=[MediaExt])

t = env.get_template('tmpl.html')

if __name__ == '__main__':
    print t.render()
piranha@gto ~/test> cat tmpl.html 
{% media 'test' %}
piranha@gto ~/test> python j.py 
Traceback (most recent call last):
  File "j.py", line 15, in <module>
    t = env.get_template('tmpl.html')
  File "/usr/local/Cellar/EasyInstall/2.6/lib/python2.6/site-packages/jinja2/environment.py", line 504, in get_template
    template = self.loader.load(self, name, self.make_globals(globals))
  File "/usr/local/Cellar/EasyInstall/2.6/lib/python2.6/site-packages/jinja2/loaders.py", line 108, in load
    code = environment.compile(source, name, filename)
  File "/usr/local/Cellar/EasyInstall/2.6/lib/python2.6/site-packages/jinja2/environment.py", line 424, in compile
    return compile(source, filename, 'exec')
  File "./tmpl.html", line 6
    if 0: yield None'test'
                         ^
SyntaxError: invalid syntax