Custom jinja2 filter for iterator

How do I most e开发者_如何学JAVAfficiently write a custom filter for Jinja2 that applies to an iterable like the built-in 'sort' filter, for use in a for loop in the template?

For example:

{% for item in iterable|customsort(somearg) %}
...
{% endfor %}

See http://jinja.pocoo.org/docs/api/#writing-filters for general documentation

The same way you'd write any other filter. Here's an example that should get you started:

from jinja2 import Environment, Undefined

def custom_sort(iterable, somearg):
    if iterable is None or isinstance(iterable, Undefined):
        return iterable

    # Do custom sorting of iterable here

    return iterable

# ...

env = Environment()
env.filters['customsort'] = custom_sort

Don't worry about efficiency until it becomes a problem. The template engine is unlikely to be the bottle-neck in any case.