Let's say you have a YAML file, perhaps something like this.

domains:
  foo:
    alias: foo
    region: us-east-1
  bar.example.com:
    alias: bar
    region: us-east-2

There are similar utilities:

• kwoodson.yedit - to add/change/remove keys
• ansible.utils.remove_keys - to remove keys
• ansible.utils.keep_keys - to retain keys (this article)

In this example, ansible.utils.keep_keys is used to keep the "foo" key and it's child elements and copy and to_nice_yaml are used to create a new YAML file that will contain the "foo" key and it's child elements.

---
- hosts: localhost
  vars_files:
    - /path/to/original.yml
  tasks:
  - name: the data variable should map to the top level key in the YAML file
    set_fact:
      data: "{{ domains }}"

  - copy:
      dest: /path/to/new.yml
      content: "{{ output | to_nice_yaml(indent=2) }}"
    vars:
      output: "{{ {'domains': data | ansible.utils.keep_keys(target=['foo'])} }}"
...

Or like this, using:

• slurp is used to read the original YAML file
• b64decode is used to decode the original YAML file
• set_fact and from_yaml are used to create a variable that contains the YAML from the original file
• copy and ansible.utils.keep_keys and to_nice_yaml are used to create a new file that contain the YAML with the "foo" key

---
- hosts: localhost
  tasks:
  - slurp:
      path: /path/to/original.yml
    register: slurp

  - set_fact:
      yaml: "{{ slurp.content | b64decode | from_yaml }}"

  - copy:
      dest: /path/to/new.yml
      content: "{{ yaml | ansible.utils.keep_keys(target=['foo']) | to_nice_yaml }}"
...

Now the new.yml file should contain the following, where the "foo" key and it's child elements were removed.

domains:
  foo:
    alias: foo
    region: us-east-2