How to inherit undefined variables with Jinja2?

Question 1

In my Ansible roles, some roles derive specific configuration settings from global variables Global variables might be undefined. The following code illustrates the schema:

- hosts: localhost
  vars:
    bar: '{{ foo }}'
  tasks:
    # Assume foo comes from an Ansible environment
    - debug: var=foo
    # Assume bar comes from a role default
    - debug: var=bar
    # Catched by the "is defined" condition
    - debug: msg="foo is defined"
      when: 'foo is defined'
    # Cannot catch undefined exception?!
    - debug: msg="bar is defined"
      when: 'bar is defined'

Everything works as expected, but the last statement: Ansible raises an exception because foo is undefined (yes, it is undefined).

PLAY [localhost] *********************************************************************************************************************************************************

TASK [Gathering Facts] ***************************************************************************************************************************************************
ok: [localhost]

TASK [debug] *************************************************************************************************************************************************************
ok: [localhost] => {
    "foo": "VARIABLE IS NOT DEFINED!"
}

TASK [debug] *************************************************************************************************************************************************************
ok: [localhost] => {
    "bar": "VARIABLE IS NOT DEFINED!"
}

TASK [debug] *************************************************************************************************************************************************************
skipping: [localhost]

TASK [debug] *************************************************************************************************************************************************************
fatal: [localhost]: FAILED! => {"msg": "The conditional check 'bar is defined' failed. The error was: error while evaluating conditional (bar is defined): {{ foo }}: 'foo' is undefined\n\nThe error appears to be in '.../test-undef.yml': line 9, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n      when: 'foo is defined'\n    - debug: msg=\"bar is defined\"\n      ^ here\n"}

So why does bar not “evaluate” to undefined like foo? And how can I trap this “multi-level” undefinedness?

Question 2

Try this:

- hosts: localhost
  vars:
    bar: '{{ foo }}'
  tasks:
    # Assume foo comes from an Ansible environment
    - debug: var=vars.foo
    # Assume bar comes from a role default
    - debug: var=vars.bar
    # Catched by the "is defined" condition
    - debug: msg="foo is defined"
      when: vars.foo is defined
    # Cannot catch undefined exception?!
    - debug: msg="bar is defined"
      when: vars.bar is defined

Question 3

The problem is that bar is technically defined, and your definition of bar uses a possibly undefined variable. When you attempt to do anything with bar it has to be evaluated as an independent Jinja expression, which happens before the is defined check.

One way to address this is to make it so bar can be evaluated without resulting in an undefined value, e.g.

- hosts: localhost
  vars:
    bar: "{{ foo | default(false) }}"
  tasks:
    - debug:
        msg: bar is truthy
      when: bar is truthy

You can also check foo before bar since evaluation is short-circuitable, but baking knowledge of the variable relationship into your tasks can be unwieldy.

- hosts: localhost
  vars:
    bar: "{{ foo }}"
  tasks:
    - debug:
        msg: bar is truthy
      when: 
        - foo is defined
        - bar is defined

Question 4

try adding

when: ( vars[bar] is defined )

Swifty · Answer 1 · 2021-11-23T09:57:21

Try this:

- hosts: localhost
  vars:
    bar: '{{ foo }}'
  tasks:
    # Assume foo comes from an Ansible environment
    - debug: var=vars.foo
    # Assume bar comes from a role default
    - debug: var=vars.bar
    # Catched by the "is defined" condition
    - debug: msg="foo is defined"
      when: vars.foo is defined
    # Cannot catch undefined exception?!
    - debug: msg="bar is defined"
      when: vars.bar is defined

@Frenchy: Yes, indeed... And if you run my script then you will see that it runs fine...
Thanks, @Swifty. However, Ansible shows vars.bar as defined with your code: text TASK [debug] ************************************************************************************************************************************************************* ok: [localhost] => { "msg": "bar is defined" } Probably because it holds the template string?
foo not defined implies bar not defined..you just have to check if foo is defined to check if bar is defined
@Stephan Please understand the difference between '{{ foo }}' and "{{ foo }}"... "{{ foo }}" means a variable... '{{ foo }}' means literally that string...

flowerysong · Answer 2 · 2021-11-23T17:15:56

The problem is that bar is technically defined, and your definition of bar uses a possibly undefined variable. When you attempt to do anything with bar it has to be evaluated as an independent Jinja expression, which happens before the is defined check.

One way to address this is to make it so bar can be evaluated without resulting in an undefined value, e.g.

- hosts: localhost
  vars:
    bar: "{{ foo | default(false) }}"
  tasks:
    - debug:
        msg: bar is truthy
      when: bar is truthy

You can also check foo before bar since evaluation is short-circuitable, but baking knowledge of the variable relationship into your tasks can be unwieldy.

- hosts: localhost
  vars:
    bar: "{{ foo }}"
  tasks:
    - debug:
        msg: bar is truthy
      when: 
        - foo is defined
        - bar is defined

Please note that @Stephan defined the bar variable as a string! bar: '{{ foo }}' literally means a sting!!!

The HCD · Answer 3 · 2021-11-23T15:28:45

-1

try adding

when: ( vars[bar] is defined )

The HCD

2021-11-23 15:28:45

vars is an undocumented internal implementation that may be removed in the future and should not be used.

flowerysong

How to inherit undefined variables with Jinja2?

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