Conditionally define variable in Ansible

Question

I want to conditionally define a variable in an Ansible playbook like this:

my_var: "{{ 'foo' if my_condition}}" 

I would like the variable to remain undefined if the condition does not resolve to true.

Ansible gives the following error if I try to execute the code:

fatal: [foo.local] => {'msg': 'AnsibleUndefinedVariable: One or more undefined                        variables: the inline if-expression on line 1 evaluated                        to false and no else section was defined.', 'failed': True} 

Why is this an error anyway?

The complete case looks like this:

{role: foo, my_var: "foo"} 

If my_var is defined, the role does something special. In some cases, I don't want the role to do this. I could use when: condition, but then I would have to copy the whole role block. I could also use an extra bool variable, but I would like a solution without having to change the "interface" to the role.

Any ideas?

Answer 1

You could use something like this:

my_var: "{{ 'foo' if my_condition else '' }}" 

The 'else' will happen if condition not match, and in this case will set a empty value for the variable. I think this is a short, readable and elegant solution.