Create, modify, refill dictionary in Ansible based on other variables. Way of Ansible rename dictionary keys.

ansible_template_dict.yml

---
- hosts: all
  gather_facts: false
  vars:
    data_source:
      - {src_host: 172.16.0.10, src_user: www-data, dst_host: 172.16.0.11, dst_user: mysql}
      - {src_host: 172.16.0.10, src_user: ftp, dst_host: 172.16.0.11, dst_user: mysql}
      - {src_host: 172.16.0.10, src_user: ftp, dst_host: 172.16.0.12, dst_user: nat}
      - {src_host: 172.16.0.10, src_user: www-data, dst_host: 172.16.0.14, dst_user: mysql}
    list_of_users:
      "{{ data_source|map(attribute='src_user')|union(data_source|map(attribute='dst_user'))|unique }}"
    list_of_users_pairs_yml: |-
      {% for item in data_source %}
      - src: {{ item.src_user }}
        dst: {{ item.dst_user }}
        opt: {{ item.optional_value|d('') }}
      {% endfor %}
    list_of_users_pairs: "{{ list_of_users_pairs_yml|from_yaml|unique }}"

  tasks:
    - debug: var=list_of_users
    - debug: var=list_of_users_pairs