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ansible - example of using filters to change each item in a list
The problem:
I wanted to use the jinja 'map' filter to modify each item in a string, in this simple
example, adding '.conf' to each item.
The 'format' filter in jinja takes arguments (value, *args, **kwargs). Unfortunately,
it uses 'value' as the pattern. When called inside map 'value' is the current item in
the list, or in other words *args as far as format is concerned. So it's the wrong way
The following example creates a simple filter that has the format args in an order we
can use when calling 'map'. Note this is extremely simplestic, and only allows for
adding a prefix or suffix to an item. i.e. prefix%s or %ssuffix. You can't do more
advanced ones like '%s - %s' with this example as that requires passing the current
item as an array and (at least in python 2x), variable length args have to come after
named args.
_Note: A better approach might be to use the ansible regex_replace filter within map,
but when I tried that, it returned \u0001.conf, so I'm guessing that there's some
unicode foo (or lack thereof) happening. i.e. I tried
mylist2: "{{ mylist | map('regex_replace', '^(.*)$', '\\1.conf' ) | list }}"
# plugins/filter/ (check path in ansible.cfg)
# This code is essentially a direct copy of the jinja 'format' filter with
# some minor mods to the args and their use, see do_format in
# for the original
from jinja2.utils import soft_unicode
def map_format(value, pattern):
Apply python string formatting on an object:
.. sourcecode:: jinja
{{ "%s - %s"|format("Hello?", "Foo!") }}
-> Hello? - Foo!
return soft_unicode(pattern) % (value)
class FilterModule(object):
''' jinja2 filters '''
def filters(self):
return {
'map_format': map_format,
- hosts: all
remote_user: vagrant
sudo: true
- Alice
- Bob
- Carol
mylist2: "{{ mylist | map('map_format', '%s.conf' ) | list }}"
- debug: var=mylist2
TASK: [debug var=mylist2] *****************************************************
ok: [vagrant] => {
"var": {
"mylist2": [
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nabheet commented Feb 21, 2020

When there is a need to add both prefix and suffix (and making everything a list), look at:

    extended_etcd_endpoints_list: "{{ groups['etcd'] | map('extract', hostvars, ['ansible_default_ipv4','address']) | map('regex_replace', '^(.*)$','https://\\1:2379') | list  }}"

What is does: takes the list of all machines in the group etcd, extracts the ipv4 address, adds a prefix of 'https://' and a suffix of ':2379'.
At the end, everything is transformed to a list.

I wish I could upvote this somehow!!!! Thank you!!!!!!! :+10000:

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manji-0 commented Dec 7, 2020

from jinja2.utils import soft_unicode

def _format(value, pattern):
    return soft_unicode(pattern) % (value)

def prefix(value, prefix):
    return soft_unicode(prefix + value)

def suffix(value, suffix):
    return soft_unicode(value + suffix)

class FilterModule(object):
    ''' jinja2 filters '''

    def filters(self):
        return {
            'format': _format,
            'prefix': prefix,
            'suffix': suffix
# mylist -> ["a", "b", "c"]
mylist1: "{{ mylist | map('prefix', 'mod_') | list }}" # -> ["mod_a", "mod_b", "mod_c"]
mylist2: "{{ mylist | map('suffix', '.conf' ) | list }}" # -> ["a.conf", "b.conf", "c.conf"]
mylist3: "{{ mylist | map('format', 'mod_%s.conf' ) | list }}" # -> ["mod_a.conf", "mod_b.conf", "mod_c.conf"]

For more complex cases, It would be better to use regex_replace.

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