Django-创建自定义标签以按模板中的索引访问变量

Question

I have a HTML template in django. It get's two variables: list of categories (queryset, as it it returned by .objects.all() function on model in django) and dictionary of contestants. As a key of the dictionary, I'm using id of category, and value is list of contestats. I want to print name of the category and then all the contestants. Now I have this:

{% for category in categories_list %}
    <h1>category.category_name</h1>
    {% for contestant in contestants_dict[category.id] %}
        {{ contestant }} </br>
    {% endfor %}
{% endfor %}

However, when I run it, I get error:

TemplateSyntaxError at /olympiada/contestants/
Could not parse the remainder: '[category.id]' from 'contestants_dict[category.id]'

What I know so far is that I can't use index in template. I thought that {% something %} contains pure Python, but it shoved up it's just a tag. I know that I have to create my own simple_tag, but I don't know how. I read the docs Writing custom template tags , but there is such a little information and I wasn't able to fiqure out how to create (and mainly use in a for loop) a tag, that will take dict, key and return the value. What I tried is: templatetags/custom_tags.py:

from django import template


register = template.Library()


@register.simple_tag
def list_index(a, b):
    return a[b]

and in template:

{% for contestant in list_index contestants_dict category.id %}

But I get TemplateSyntaxError. Could you please explain/show me how to create the tag, or is there a better way to do this? Thanks.

//EDIT: I managed to do it this way:

{% list_index contestants_list category.id as cont %}
    {% for contestant in  cont %}

it works, but it takes 2 lines and I need to create another variable. Is there any way to do it without it?

Answer 1

If you don't want 2 lines like that you should be able to use a filter i think

@register.filter
def list_index(a, b):
    return a[b]

Then the usage like this

{% for contestant in contestants_dict|list_index:category.id %}
    {{ contestant }} </br>
{% endfor %}