remove-omit-entries

Ansible Role: remove-omit-entries

In a nutshell, this role gives you behavior similar to Ansible's omit, but for dictionaries and lists instead of modules' parameters.

For this, the role defines two filters (remove_omit_entries and remove_omit_entries_get_marker [only used internally]) and one var (remove_omit_entries_marker). The role itself does not run any tasks.

Usage

Add the role to the roles section of your playbook.
Set remove_omit_entries_marker as a default value for entries that should be removed by the filter.
Run dicts or lists through remove_omit_entries to remove entries having remove_omit_entries_marker as their value.

Requirements and Dependencies

None

Example

The need for the role arose from the inadequacy of Ansible's omit when passing dicts as modules' parameters. omit can be used as default value for possibly undefined variables. When passing parameters to modules, Ansible will not pass omitted parameters.

debug:
  # debug_msg will not be passed to the debug module if undefined
  msg: "{{ debug_msg|default(omit) }}"

However, this does not work for entries of dictionaries. Ansible will pass the complete error_info dictionary to the debug module:

vars:
  error_info:
    code: 10
    reason: "{{ error_reason|default(omit) }}"
    
tasks:
  - debug:
      msg: error_info

This will print something like:

TASK [debug] ********************************************************************
ok: [localhost] => {
    "msg": {
        "code": 10,
        "reason": "__omit_place_holder__ab8264664a4f0a83fa01e9ee3f221d79acecefc8"
    }
}

To prevent the debug module from seeing the undefined reason entry of the error_info dictionary, we can use the remove-omit-entries-role.

roles:
  # Include the remove-omit-entries role
  - role: remove-omit-entries
  
vars:
  error_info:
    code: 10
    # Use remove_omit_entries_marker as default value for possibly undefined values
    reason: "{{ error_reason|default(remove_omit_entries_marker) }}"

tasks:
  - debug:
      # Pass the dict through the remove_omit_entries filter
      msg: "{{ error_info|remove_omit_entries }}"

Now, the debug module will receive a dictionary that only contains the defined entry code.

TASK [debug] ********************************************************************
ok: [localhost] => {
    "msg": {
        "code": 10
    }
}

Behavior of `remove_omit_entries` filter

Removes dict entries whose value is OMIT_MARKER:

>>> remove_omit_entries({'key1': 'some_value', 'key2': OMIT_MARKER})
{'key1': 'some_value'}

Removes list items whose value is OMIT_MARKER:

>>> remove_omit_entries([OMIT_MARKER])
[]

Works recursively:

>>> remove_omit_entries({'key1': {'key11': OMIT_MARKER}})
{'key1': {}}

Returns everything else as is:

>>> remove_omit_entries('')
''
>>> remove_omit_entries({OMIT_MARKER: 'some_value'}) == {OMIT_MARKER: 'some_value'}
True
>>> remove_omit_entries(OMIT_MARKER) == OMIT_MARKER
True