Conditionally add properties to task/module in Ansible Playbook - ansible

Question

I'm developing a role where I want to launch a docker container, among the tasks in my role I have one using the docker_container module to do that:
- name: Launch docker container
docker_container:
name: abc
...
This works fine but now I want to have a variable that will define whether this container needs to be attached to a particular docker network.
If I require it is fine:
- name: Launch docker container
docker_container:
name: abc
networks:
- name: '{{ network_name_var}}'
...
But I want to allow the users to not define it, in which case no networks: ... property should be added.
I have found no easy way of achieving this, is there one?
Semantically I want something like this:
- name: Launch docker container
docker_container:
name: abc
{% if network_name_var is defined %}
networks:
- name: '{{ network_name_var}}'
...
{% endif %}

Answer 1

Here is a possible scenario you can use. The key points:
We keep your single network_name_var that is exposed to your user. I took for granted that this var could be either undefined, or empty.
We define the full network list definition dynamically if the var has a value set. This list stays unset otherwise.
We use the omit place holder to not define any networks in the module if need be.
- name: demo playbook for omit
hosts: localhost
tasks:
- name: set the list of networks for our container
# don't define anywhere else. it should only exist
# if network_name_var is set
set_fact:
my_networks:
- name: '{{ network_name_var }}'
when: network_name_var | default('') | length > 0
- name: make sure container is started
docker_container:
name: abc
networks: "{{ my_networks | default(omit) }}"