Overriding Ansible module options defaults - module

Question

I'm struggling to find out how to override Ansible module options defaults without hand rolling it with variables. Even better if there would be a way to override module options defaults only for a subset of hosts.
Say on couple hosts Git is available at /bin/git, as expected. On couple other hosts Git is at /usr/local/bin/git. How can I override the git module executable option default for the latter group of hosts?
At the moment I'm setting a hosts group variable like:
git_executable=/usr/local/bin/git
and using it with default(omit) filter everywhere git is used like so:
- git: "executable={{git_executable|default(omit)}} ..."
So it gets properly overriden on hosts where it's defined, and ignored on others.
executable may not be the best example here, since that may be controlled with PATH environment variable or something. So what about any kind of module option in general that I'd like to override for just some hosts but otherwise fall back to module default?
Since there is couple more of such basic differences in this environment, it's quite tedious to sprinkle this kind of default lookup using variables all over the place just in case it gets run on a host with non-default setup. Is there a way to do this better?

Answer 1

I don't think there is a better option. Modules only know what you pass to them. They do not have access to global vars, server facts or anything else unless you explicitly pass it as a module parameter.
If this really really is important and you want to invest some time, you could create your own action plugin(s). Action plugins are local actions, therefore have access to the Ansible runnerc class and all its properties including facts etc. So you could handle the default parameters or executable detection in there based on server facts and then call the git or whatever module programatically. Huge overhead in my opinion but that depends on view and might be feasible on your end.
Though take care, action plugins are 100% undocumented. Ansible 2.0 is going to be released in the next days. They claim 100% backwards compatibility but I wouldn't be surprised if that only counts for documented features.

Answer 2

In this specific case git.executable as long as it's in PATH or '/sbin', '/usr/sbin', '/usr/local/sbin' git module would find it because it uses the basic.get_bin_path()
On the larger topic, personally I would go with what you already did. But if you are bent on it one other possible hack would be to [mis]use the include statement to create a wrapper for each module that supplies the default value you want from some variable.
Obviously you would have to specify the path somewhere yourself either in group_vars or host/role/... vars. Or a variable defined in the play's vars section.
$ cat my_echo.yml
- shell: "{{echo_exec}} '{{text}}'"
$ cat playbook.yml
- hosts: localhost
tags: so
gather_facts: False
vars:
echo_exec: echo
tasks:
- include: my_echo.yml text='some text'
changed_when: False
- hosts: localhost
tags: so
gather_facts: False
vars:
echo_exec: printf
tasks:
- include: my_echo.yml text='some text'
changed_when: False
$ ansible-playbook playbook.yml -t so -v
PLAY [localhost] **************************************************************
TASK: [shell {{echo_exec}} '{{text}}'] ****************************************
changed: [localhost] => {"changed": true, "cmd": "echo 'some text'", "delta": "0:00:00.003782", "end": "2015-03-20 17:45:58.352069", "rc": 0, "start": "2015-03-20 17:45:58.348287", "stderr": "", "stdout": "some text", "warnings": []}
PLAY [localhost] **************************************************************
TASK: [shell {{echo_exec}} '{{text}}'] ****************************************
changed: [localhost] => {"changed": true, "cmd": "printf 'some text'", "delta": "0:00:00.003705", "end": "2015-03-20 17:45:58.690657", "rc": 0, "start": "2015-03-20 17:45:58.686952", "stderr": "", "stdout": "some text", "warnings": []}
PLAY RECAP ********************************************************************
localhost : ok=2 changed=2 unreachable=0 failed=0
$