Jinja2: format + join the items of a list

Question

play_hosts is a list of all machines for a play. I want to take these and use something like format() to rewrite them like rabbitmq@%s and then join them together with something like join(). So:

{{ play_hosts|format(???)|join(', ') }}

All the examples of format use piping where the input is the format string and not a list. Is there a way to use these (or something else) to accomplish what I want? The output should looks something like:

['rabbitmq@server1', 'rabbitmq@server2', rabbitmq@server3', ...]

The jinja2 doc describes format like this:

format(value, *args, **kwargs)

Apply python string formatting on an object:

{{ "%s - %s"|format("Hello?", "Foo!") }}
-> Hello? - Foo!

So it gives three kinds of input but doesn't describe those inputs in the example, which shows one in the pipe and the other two passed in via args. Is there a keyword arg to specify the string that's piped? Please help, python monks!

Answer 1

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 %}
]

Answer 2

You could simply join not only by , but also add the prefix together with it. Now that's not very pythonic or sophisticated but a very simple working solution:

[rabbitmq@{{ play_hosts | join(', rabbitmq@') }}]

Answer 3

In ansible you can use regex_replace filter:

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

Answer 4

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') }}