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• There is this problem in Jinja2 that variable does not persist outside a for loop.
• The answer is actually to put the variable I want to loop and update into a dictionary.
• Additional problem is that MKDocs does not support do extension. Hence, updating dictionary is impossible without using {{ any_dictionary.update({ ... }) }} ({{}} will prints the Jinja2 variable into DOM).
• The solution is to keep using {{}} but encapsulate it into a DOM with display:none;.
• This is my example solution from my own StackOverflow thread, https://stackoverflow.com/questions/44913535/variable-modified-in-loop-does-not-persist-outside-the-loop/.
• Screenshot to my own discussion in StackOverflow.

./20170706-1352-cet-mkdocs-jinja2-updating-variable-without-do-extension-2.png

• The codes implementation of my case.

<p style="display:none;">
{% set title_data = {'org': nav_item.title.split(" "), 'mod' : ""} %}
{% for i in title_data.org %}
  {% if loop.index > 3 %}
    {{ title_data.update({'mod' : title_data.mod + ' ' + i}) }}
  {% endif %}
{% endfor %}
</p>
<a href="{{ nav_item.url }}" id="{{ id }}">{{ title_data.mod }}</a>