How to inherit undefined variables with Jinja2? - ansible

Question

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?

Answer 1

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

Answer 2

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

Answer 3

try adding
when: ( vars[bar] is defined )