Q:

How can I use the forloop.counter0 as a key in Django Template

I use a custom Widget to modify the context for the widget template (add more information).
Context:
'selected': [[3, 9], [3, 4, 7, 6]], 'depth_levels': {0: [{'name': 'Category A', 'value': 3, 'depth': 0, 'parent_id': None, 'children': [4, 9]}, {'name': 'Category B', 'value': 8, 'depth': 0, 'parent_id': None, 'children': []}], 1: [{'name': 'Category A_1', 'value': 4, 'depth': 1, 'parent_id': 3, 'children': [5, 7]}

{% for path in widget.selected %}
        {% for selected in path %}

            {% for level in widget.depth_levels.forloop.counter0 %}
                {{ level }}

            {% endfor %}
        {% endfor %}
    {% endfor %}

First I cycle thru the selected(path) and the internal arrays(selected).
I want to use the {{forloop.counter0}} as a key for depth_levels.
The issue: {{widget.depth_levels.forloop.counter0}} doesn't return anything.
The forloop.counter is not used as index for a list, but it is used as key to a dictionary. 
Using '.' to access is the default way in which Django templates access a dictionary*  Using  '0', '1' - {{widget.depth_levels.0}}- etc instead of forloop.counter0 it works. 
Using a custom template tag again has an issue because the access is inside a for and can't use {{}} inside:
{% for level in widget.depth_levels.forloop.counter0 %}

The keys that I need to access depth_levels are from 0 to the length of each array inside the selected, the 'path' array in the example. 
The values in selected arrays tell me later where to add attributes are not related to the keys of depth_levels.
My end goal is to access the name, value in the dictionaries that are in the array.

A:

You can add a filter like this (To know more: https://docs.djangoproject.com/en/1.11/howto/custom-template-tags/#writing-custom-template-filters):
@register.filter
def value_by_key(d, key):    
    return d[key]

Then use it like this:
% for path in widget.selected %}
    {% for level in widget.depth_levels|value_by_key:path|length %}
        {{ level }}
    {% endfor %}
{% endfor %}

