Method similar to 'startswith' in Jinja2/Flask

I'm looking for method/way which is similar to python's startswith. What I would like to do is link some fields in table which start with "i-".

My steps:

1. I have created filter, which return True/False:

   @app.template_filter('startswith')
   def starts_with(field):
       if field.startswith("i-"):
               return True
       return False
   

then linked it to template:

{% for field in row %}
            {% if {{ field | startswith }} %}
               <td><a href="{{ url_for('munin') }}">{{ field | table_field | safe }}</a></td>
            {% else %}
               <td>{{ field | table_field | safe}}</td>
            {% endif %}
     {% endfor %}

Unfortunatetly, it doesn't work.

Second step. I did it without filter, but in template

{% for field in row %}
            {% if field[:2] == 'i-' %}
               <td><a href="{{ url_for('munin') }}">{{ field | table_field | safe }}</a></td>
            {% else %}
               <td>{{ field | table_field | safe}}</td>
            {% endif %}
     {% endfor %}

That works, but to that template are sending different datas, and it works only for this case. I'm thinking that [:2] could be buggy a little bit.

So I try to write filter or maybe there is some method which I skip in documentation.

The expression {% if {{ field | startswith }} %} will not work because you cannot nest blocks inside each other. You can probably get away with {% if (field|startswith) %} but a custom test rather than a filter, would be a better solution.

Something like

def is_link_field(field):
    return field.startswith("i-"):

environment.tests['link_field'] = is_link_field

Then in your template, you can write {% if field is link_field %}