test_loop_filter.py

from jinja2 import Environment

tmpl = Environment().from_string("""\
<ul>
{% for item in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] if item % 2 == 0 %}
    <li>{{ loop.index }} / {{ loop.length }}: {{ item }}</li>
{% endfor %}
</ul>
{{ 1 if foo else 0 }}
""")

print tmpl.render(foo=True)