On Sun, 16 May 2021 08:54:29 +0200 Vladimir Botka <vbo...@gmail.com> wrote:
> On Sat, 15 May 2021 16:55:24 -0700
> Scott Mcdermott <sc...@smemsh.net> wrote:
>
> > Hello, why does short circuit only works for the first two cases:
> >
> > - hosts: localhost
> > become: false
> > vars:
> > ivar: '{{hostvars[inventory_hostname].dne}}'
> > tasks:
> > - debug:
> > msg: "works: {{'foo' or dne}}"
> > - debug:
> > msg: "works: {{'bar' or hostvars[inventory_hostname].dne}}"
> > - debug:
> > msg: "fails: {{'baz' or ivar}}
> >
> > The third debug will generate an error:
> >
> > The task includes an option with an undefined variable.
> > The error was: {{hostvars[inventory_hostname].dne}}:
> > 'ansible.vars.hostvars.HostVarsVars object' has no attribute 'dne'
> >
> > Why is it even looking up in hostvars at all? The non-empty
> > string should evaluate True, so why is it reading the ORed
> > condition? The same thing happens with "if true else" and
> > ternary(true, ...) filter. It seems that it's properly
> > short-circuiting in the first two cases, because the variable
> > "dne" does not exist, and there is no error. Even more
> > confusing, the expansion of host vars is the exact same in cases
> > 2 and 3, it's only going through an indirect variable in case 3.
> >
> > This seems to happen regardless of the source of variables ie
> > extra vars, vars_files, etc.
> >
> > Is this a bug, or what am I missing?
>
> Why don't you start debugging the variable?
>
> - debug:
> var: dne
>
> See "Minimal working example"
> https://en.wikipedia.org/wiki/Minimal_working_example
If the variable *dne* is not defined the playbook
shell> cat playbook.yml
- hosts: localhost
vars:
ivar: "{{ hostvars[inventory_hostname].dne }}"
tasks:
- debug:
var: ivar
- debug:
msg: "{{ 'foo' or dne }}"
- debug:
msg: "{{ 'foo' or ivar }}"
gives
shell> ansible-playbook playbook.yml
...
TASK [debug]
*************************************************
ok: [localhost] => ivar: VARIABLE IS NOT DEFINED!
TASK [debug]
*************************************************
ok: [localhost] => msg: foo
TASK [debug]
*************************************************
fatal: [localhost]: FAILED! => msg: |-
... The error was: 'dne' is undefined
In the second and third debug tasks, the expansion of the string "{{
... }}" is needed first. Then the *or* expression can be evaluated.
There is no problem in the second task because the variable *dne*
is neither expanded nor evaluated (the first element of the *or*
expression is True).
But, the third debug task fails because the expansion of the string
"{{ ... }}" fails before the *or* expression could be evaluated.
You'll need a default value if you want to use this kind of "lazy
evaluation" constructs, e.g.
shell> cat group_vars/all.yml
dne: False
--
Vladimir Botka
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