ANSIBLE - how to concatenate a string with a list - list

Question

I am trying to concatenate a string which is referenced as variable with a nested list
I looked into the options of using the set_fact and join but to no avail.
#config.yml
- name: concatenate
module_name:Test
state: present
port: {{ env_dc }}{{item.ports}}
with_items:
- "{{ my_list }}"
#group_vars\all.yml
env_dc: uk
my_list:
- {name: switch1, ports: [p1, p2, p3, p4]}
I am expecting the following output:
ukp1
ukp2
ukp3
ukp4
But I am getting;
"item": {
"ports": [
"p1",
"p2",
"p3",
"p4"
]
Actual Playbook:
Error message:

Answer 1

If you write this:
port: {{ env_dc }}{{item.ports}}
You are not producing a new list formated by concatenating the value in env_dc with each item in item.ports; you are simply creating a new string that has the contents of env_dc followed by the string representation of item.ports. That is, in your example, that would evaluate to something like:
uk['p1', 'p2', 'p3', 'p4']
You can solve this using the map filter (which can apply a filter to all items in a list) and the regex_replace filter, like this:
---
- hosts: localhost
gather_facts: false
vars:
env_dc: uk
my_list:
- name: switch1
ports:
- p1
- p2
- p3
- p4
tasks:
- debug:
msg: "ports: {{ item.ports|map('regex_replace', '^', env_dc)|list }}"
with_items: "{{ my_list }}"
Which given your example data would evaluate to:
TASK [debug] **********************************************************************************
ok: [localhost] => (item={u'name': u'switch1', u'ports': [u'p1', u'p2', u'p3', u'p4']}) => {
"msg": "ports: [u'ukp1', u'ukp2', u'ukp3', u'ukp4']"
}