Ansible conditional check failed

I wanted to add a check to one of my Ansible roles so that an application source would be copied and the source recompiled only if no current version existed or if the existing version did not match the expected version:

- name: Check to see if app is installed and the expected version
  command: /usr/local/app/bin/app --version
  register: version_check
  ignore_errors: True
  changed_when: "version_check.rc != 0 or {{ target_version }} not in version_check.stdout"

- name: include app install
  include: tasks/install.yml
  when: "version_check.rc != 0 or {{ target_version }} not in version_check.stdout"

I defined the target version in my role’s defaults/main.yml:

target_version: "2.5.2"

The first time I ran it, I encountered an error:

fatal: []: FAILED! => {"failed": true, "msg": "The conditional check 'version_check.rc != 0 or {{ target_version }} not in version_check.stdout' failed. The error was: error while evaluating conditional (version_check.rc != 0 or {{ target_version }} not in version_check.stdout): Unable to look up a name or access an attribute in template string ({% if version_check.rc != 0 or 2.5.2 not in version_check.stdout %} True {% else %} False {% endif %}).\nMake sure your variable name does not contain invalid characters like '-': coercing to Unicode: need string or buffer, StrictUndefined found"}

It’s a little unclear what is wrong, so I figured it was likely an issue with quotes or a lack of parentheses.

First I tried parentheses:

changed_when: "version_check.rc != 0 or ({{ target_version }} not in version_check.stdout)"

No luck.

changed_when: "version_check.rc != 0 or !('{{ target_version }}' in version_check.stdout)"

You know, trying to google and or not or or or is is tricky. Even if you add terms like Boolean logic or propositional calculus.

I tried to break it down into smaller parts:

changed_when: "version_check.rc != 0"

That worked.

changed_when: "!('{{ target_version }}' in version_check.stdout)"

A different error appeared:

template error while templating string: unexpected char u'!'

OK, that’s getting somewhere! Try a variation:

changed_when: "'{{ target_version }}' not in version_check.stdout"

It worked! But with a warning:

[WARNING]: when statements should not include jinja2 templating delimiters
such as {{ }} or {% %}. Found: ('{{ target_version }}' not in version_check.stdout)

Next try:

changed_when: "target_version not in version_check.stdout"

That worked, and without any warnings. I put the or back in:

changed_when: "version_check.rc != 0 or target_version not in version_check.stdout"

That worked! It was the jinja2 delimiters the whole time. The value of the changed_when key is already interpreted as jinja2 apparently, so the delimiters were redundant. Even though it succeeded (with a warning) in a single propositional statement, it failed when the logical disjunction was added. It was an important reminder: error messages aren’t perfect.