Ansible import_playbook fails with variable undefined error - ansible

Question

I am trying to conditionally import a playbook by dynamically generating the name of the playbook to import.
I have the following 2 files:
root#ubuntu:~/test# ls -ltr
total 8
-rw-r--r-- 1 root root 117 sij 2 12:07 child_playbook_1.yaml
-rw-r--r-- 1 root root 210 sij 2 12:11 start.yaml
root#ubuntu:~/test#
start.yaml:
---
- name: main playbook
hosts: localhost
tasks:
- set_fact:
var: "child_playbook"
- debug:
msg: "{{ var }}"
- import_playbook: "{{ var + '_1.yaml' }}"
when: var is defined
child_playbook_1.yaml:
---
- name: child_playbook
hosts: localhost
tasks:
- debug:
msg: "Message from the child playbook"
When I execute start.yaml, I get this as output:
root#ubuntu:~/test# ansible-playbook start.yaml
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
ERROR! 'var' is undefined
root#ubuntu:~/test#
Why is var not seen and why do I get this message?
How to overcome this?
To put things into a bit more perspective, the 'var' variable in start.yaml is being dynamically read which means that the name of the playbook to be imported will also be dynamic.

Answer 1

var is a fact defined for localhost (i.e. the only host in the play) in the first play. So it does not exist "globally", only for the given host.
Moreover, imports are made statically at time of playbook parsing (by opposition to includes which are dynamic but to not exist for playbooks).
One could then try to use the var defined for localhost by referencing hostvars['localhost'].var but:
set_fact has not yet run since it is an import.
Moreover hostvars is not yet defined at time of import.
Lastly, you are miss-interpreting how a when clause is actually working. Basically, the condition is passed to all the tasks contained in your included/imported object. So the imported object must exist. If you use an undefined var to get its name, it will always fire an error.
The only solution I currently see to pass a variable playbook name is to use an extra var on the command line. If you want to be able to 'skip' the import when no variable is defined, you could default to an empty playbook.
Here is a dummy empty.yml playbook
---
- name: Dummy empty playbook
hosts: localhost
gather_facts: false
For my tests, I used the same child playbook as in your question except I disabled facts gathering not needed here:
---
- name: child_playbook
hosts: localhost
gather_facts: false
tasks:
- debug: msg="Message from the child playbook"
The new main playbook start.yml looks like this:
---
- name: Play written in start.yml directly
hosts: localhost
gather_facts: false
tasks:
- debug:
msg: "Message from start playbook"
- import_playbook: "{{ var is defined | ternary( var | default('') + '_1.yml', 'empty.yml') }}"
Note than even though we use var is defined as a condition, var will still be interpreted in the ternary filter in all cases (defined or not). So we have to use a default value to make sure we don't have an error when we don't pass a value.
We can now call the playbook with or without an extra var. Here is the result:
$ ansible-playbook start.yml
PLAY [Play written in start.yml directly] *************************************************************************************************************************************************************************
TASK [debug] ******************************************************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Message from main playbook"
}
PLAY [Dummy empty playbook] ***************************************************************************************************************************************************************************************
PLAY RECAP ********************************************************************************************************************************************************************************************************
localhost : ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
$ ansible-playbook start.yml -e var=child_playbook
PLAY [Play written in start.yml directly] *************************************************************************************************************************************************************************
TASK [debug] ******************************************************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Message from main playbook"
}
PLAY [child_playbook] *********************************************************************************************************************************************************************************************
TASK [debug] ******************************************************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Message from the child playbook"
}
PLAY RECAP ********************************************************************************************************************************************************************************************************
localhost : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0