How to write dynamic variable in Ansible playbook - jinja2

Question

Based on extra vars parameter I Need to write variable value in ansible playbook
ansible-playbook playbook.yml -e "param1=value1 param2=value2 param3=value3"
If only param1 passed
myvariable: 'param1'
If only param1,param2 passed
myvariable: 'param1,param2'
If param1,param2,param3 are passed then variable value will be
myvariable: 'param1,param2,param3'
When I try to create variable dynamically through template then my playbook always takes previous variable value. But inside dest=roles/myrole/vars/main.yml its writing correct value.
What I make a try here
- hosts: local
user: roop
gather_facts: yes
connection: local
tasks:
- template: src=roles/myrole/templates/myvar.j2 dest=roles/myrole/vars/main.yml
- debug: var=myvariable
roles:
- { role: myrole }
So inside myrole directory I have created template and vars
- roles
- myrole
- vars/main.yml
- templates/myvar.j2
templates/myvar.j2
{% if param1 is defined and param2 is defined and param3 is defined %}
myvariable: 'param1,param2,param3'
{% elif param1 is defined and param2 is defined %}
myvariable: 'param1,param2'
{% elif param1 is defined %}
myvariable: 'param1'
{% else %}
myvariable: 'default-param'
{% endif %}
As I know if only two condition then I can do this using inline expression like below
{{ 'param1,param2' if param1 is defined and param2 is defined else 'default-param' }}
<do something> if <something is true> else <do something else>
Is it possible if - elif - else in inline expression like above. Or any other way to assign value dynamically in ansible playbook?

Answer 1

I am sure there is a smarter way for doing what you want but this should work:
- name : Test var
hosts : all
gather_facts : no
vars:
myvariable : false
tasks:
- name: param1
set_fact:
myvariable: "{{param1}}"
when: param1 is defined
- name: param2
set_fact:
myvariable: "{{ param2 if not myvariable else myvariable + ',' + param2 }}"
when: param2 is defined
- name: param3
set_fact:
myvariable: "{{ param3 if not myvariable else myvariable + ',' + param3 }}"
when: param3 is defined
- name: default
set_fact:
myvariable: "default"
when: not myvariable
- debug:
var=myvariable
Hope that helps. I am not sure if you can construct variables dynamically and do this in an iterator. But you could also write a small python code or any other language and plug it into ansible

Answer 2

my_var: the variable declared
VAR: the variable, whose value is to be checked
param_1, param_2: values of the variable VAR
value_1, value_2, value_3: the values to be assigned to my_var according to the values of my_var
my_var: "{{ 'value_1' if VAR == 'param_1' else 'value_2' if VAR == 'param_2' else 'value_3' }}"

Answer 3

I would first suggest that you step back and look at organizing your plays to not require such complexity, but if you really really do, use the following:
vars:
myvariable: "{{[param1|default(''), param2|default(''), param3|default('')]|join(',')}}"