Ansible: Backing up variable value doesn't work as expected

Question

Attempting to save a variable's value for backup before modifying it doesn't work.

I have the following inventory file (note the custom ansible_port and user):

all:
    hosts:
            test:
                    ansible_port: 11111
                    ansible_user: user_foo
                    ansible_host: 192.168.56.101 

And I have the following playbook:

- name: Test
  hosts: test
  vars:
          backup_ansible_port: "{{ ansible_port }}"
          backup_ansible_user: "{{ ansible_user }}"
          ansible_port: "{{ tmp_port }}"
          ansible_user: root
  tasks:
          - name: foo
            debug:
                    msg: 
                    - "{{ backup_ansible_port }}"
                    - "{{ backup_ansible_user }}"
                    - "{{ ansible_port }}"
                    - "{{ ansible_user }}"

Running the following command:

ansible-playbook bootstrap_ssh.yml --ask-pass -e 'target=test tmp_port=22'

The output is the following:

...
TASK [foo] *********************************
  ok: [test] => {
      "msg": [
          "22", 
          "root", 
          "22", 
          "root"
      ]
  }
...

Expected output:

...
TASK [foo] *********************************
  ok: [test] => {
      "msg": [
          "11111", 
          "user_foo", 
          "22", 
          "root"
      ]
  }
...

Why is this not the case?

Answer 1

It is a totally expected result. You define the same variables in different places and ansible applies its variable precedence

First of all, remember that variables passed as extra vars always win: you can never change them in any other place or during the run (i.e. set_fact).

Now let's decompose what happens. The schema is basically the same for all vars where you don't get the expected result. I'll only give the example for backup_ansible_port. You can follow the spaghetti yourself for the others once you got it.

When you hit the debug task to display "{{ backup_ansible_port }}"

1. The first jinja2 template pass finds it should be an expansion of ansible_port as defined in your play vars.
2. ansible_port is defined with value "11111" directly in your inventory as a host var (see #8 in above precedence doc) But it is also defined as an expansion of tmp_port in your play vars (see #12 in above precedence doc). Higher precedence wins, so the second jinja2 template pass finds it should expand tmp_port
3. tmp_port has value "22" in your extra vars which will be the final value returned.