Jinja2: format + join the items of a list

I believe another way would be using the joiner global function, as you can read in http://jinja.pocoo.org/docs/2.9/templates/#list-of-global-functions:

A joiner is passed a string and will return that string every time it’s called, except the first time (in which case it returns an empty string). You can use this to join things

So your code would be something like:

[
{% set comma = joiner(",") %}    
{% for host in play_hosts %}
    {{ comma() }}
    {{ "rabbitmq@%s"|format(host) }}
{% endfor %}
]

In ansible you can use regex_replace filter:

{{ play_hosts | map('regex_replace', '^(.*)$', 'rabbitmq@\\1') | list }}

You can create custom filter

# /usr/share/ansible/plugins/filter/format_list.py (check filter_plugins path in ansible.cfg)

def format_list(list_, pattern):
    return [pattern % s for s in list_]


class FilterModule(object):
    def filters(self):
        return {
            'format_list': format_list,
        }

and use it

{{ play_hosts | format_list('rabbitmq@%s') }}