Jinja2: Change the value of a variable inside a loop

Try also dictionary-based approach. It seems to be less ugly.

{% set vars = {'foo': False} %}

{% for item in items %}
  {% if vars.update({'foo': True}) %} {% endif %}
  {% if vars.foo %} Ok(1)! {% endif %}
{% endfor %}

{% if vars.foo %} Ok(2)! {% endif %}

This also renders:

Ok(1)!
Ok(2)!

as mentioned in the documentation:

Please note that assignments in loops will be cleared at the end of the iteration and cannot outlive the loop scope.

but as of version 2.10 you can use namespaces:

    {% set ns = namespace(foo=false) %}      
    {% for item in items %}
      {% set ns.foo = True %}
      {% if ns.foo %} Ok(1)! {% endif %}
    {% endfor %}

    {% if ns.foo %} Ok(2)! {% endif %}