Replacing dictionary key values

[John McNulty]

Hi, can someone explain this to me please. 

Say I have a dictionary in a role defaults like this ..

at_service:
  pkg_name: at
  pkg_state: present
  svc_name: atd
  svc_enabled: no
  svc_state: stopped

I can replace that with a higher priority declaration in group_vars, or a role vars file and change some of the values, e.g.

at_service:
  pkg_name: at
  pkg_state: present
  svc_name: atd
  svc_enabled: yes
  svc_state: started

.. and that's fine.   But why can't I just replace the key valuesI want to change, like this ..

at_service.svc_enabled: yes
at_service.svc_state: started

If I do that the change is just ignored.  Seems I have no option but to replace all of it.
 
Thanks,  John

[Pascal Mathis]

You do have an option :) Set the hash_behaviour in your ansible.cfg to merge.

Then you're able to write your second file like this:

at_service:
  svc_enabled: yes
  svc_state: started

Which will result in the following dictionary, after merging it with the first one:

at_service:
  pkg_name: at
  pkg_state: present
  svc_name: atd
  svc_enabled: yes
  svc_state: started

Not all people like that setting though, so keep that in mind.

[John McNulty]

So you're saying it's not possible to replace the value of a dictionary key without changing hash_behaviour to "merge"?  

[Adam Hamsik]

Yes.

Question is if it would be possible to write a module which would be able to do this.

merge_val: orig=at_service.svc_state value=started

P.S.
I have had problems with this, too. Only solution with which I was able to come up was using filters to create new variable which is then later used. But I'm not very happy with that.