Jinja2 附加到循环时不打印

Question

In my flask application I am creating a list and appending. When I append to the list None prints.

{% set keyword_list = [] %}
{% for keyword in keywords_data %}
    {{ keyword_list.append(keyword['keywords_id']) }} //None prints for each iteration
{% endfor %}

{{ keyword_list | join(',') }} {{ keyword_list | join(',') }} prints 1,2,3 {{ keyword_list }} prints [1,2,3]

Answer 1

Change it to {{ keyword_list.append(keyword['keywords_id']) or "" }} , which uses the feature of or s that returns the first truthy answer. Since strings are always truthy in Jinja, and None is falsy, it returns the empty string, outputting nothing to the end-user.