how to clean up this jinja2 and/or boolean expression

Question

{% block content %}
    {% for blog_dict in blogs|sort(reverse=True,attribute='date') if ((not file_name_filter) or blog_dict.filename==file_name_filter) and ((not category_filter) or blog_dict.category==category_filter) %}
        <div id="post{{blog_dict.post_number}}-{{blog_dict.link_name}}">
            {{blog_dict.date.strftime("%Y-%m-%d")}}
            <a href="/posts/{{blog_dict.filename}}"><h1>{{blog_dict.heading}}</h1></a>
            <!-- Place this tag where you want the +1 button to render -->
            <div class="g-plusone" data-href="/posts/{{blog_dict.filename}}"></div>
            {{blog_dict.post|safe}}
        </div>
        <br />
        <br />
        <br />
        <img src="/images/page_divider.gif"><br />
    {% endfor %}
{% endblock %}

As you can see the expression is getting messy:

{% for blog_dict in blogs|sort(reverse=True,attribute='date') if ((not file_name_filter) or blog_dict.filename==file_name_filter) and ((not category_filter) or blog_dict.category==category_filter) %}

The other option I tried was putting two nested if statements in the loop to take care of the two filters, but then nesting got large.

what is the most elegant?

Answer 1

As John Keyes mentioned in the comments, I added a filter:

def blog_filter(blogs, file_name_filter, category_filter):
    if file_name_filter:
        blogs = (d for d in blogs if d['filename'] == file_name_filter)
    if category_filter:
        blogs = (d for d in blogs if d['category'] == category_filter)
    return blogs

this is added to jinja2 environment.filters dict

myconfig = {}
myconfig['webapp2_extras.jinja2'] =  {'template_path': ['templates','blog_posts'],
                                      'filters': {'blog_filter': blog_filter}}

app = webapp2.WSGIApplication(_routes,
    debug=True,
    config = myconfig)

the expression becomes:

{% for blog_dict in blogs|blog_filter(file_name_filter,category_filter)|sort(reverse=True,attribute='date') %}

I was also able to use this filter elsewhere such as my side menu:

{% for month, blog_dicts in blogs|blog_filter(False, category_filter)|groupby('date.month')|sort(reverse=True,attribute='grouper') %}

Note I use the jinja2 wrapper (from webapp2_extras import jinja2) and it is a cached property in my basehandler

class BaseHandler(webapp2.RequestHandler):
    @webapp2.cached_property
    def jinja2(self):
        return jinja2.get_jinja2(app=self.app)

    def render_template(self, filename, **kwargs):
        #call self.response.write(self.jinja2.render_template(..)