How to output a comma delimited list in jinja python template?

You want your if check to be:

{% if not loop.last %}
    ,
{% endif %}

Note that you can also shorten the code by using If Expression:

{{ ", " if not loop.last else "" }}

You could also use the builtin join filter like this:

{{ users|join(', ') }}

And using the joiner from https://jinja.palletsprojects.com/templates/#joiner

{% set comma = joiner(",") %}
{% for user in userlist %}
    {{ comma() }}<a href="/profile/{{ user }}/">{{ user }}</a>
{% endfor %}  

It's made for this exact purpose. Normally a join or a check of forloop.last would suffice for a single list, but for multiple groups of things it's useful.

A more complex example on why you would use it.

{% set pipe = joiner("|") %}
{% if categories %} {{ pipe() }}
    Categories: {{ categories|join(", ") }}
{% endif %}
{% if author %} {{ pipe() }}
    Author: {{ author() }}
{% endif %}
{% if can_edit %} {{ pipe() }}
    <a href="?action=edit">Edit</a>
{% endif %}