Let's say you need to append YAML to a YAML file. This can be done using copy and to_yaml or to_nice_yaml. As a simple example, let's read YAML from a file and write the same YAML to a new file. For example, let's say original.yml contain the following.

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

• 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 to_yaml or to_nice_yaml are used to create new files that contain the YAML

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

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

  - copy:
      dest: /path/to/to_yaml.yml
      content: "{{ yaml | to_yaml }}"

  - copy:
      dest: /path/to/to_nice_yaml.yml
      content: "{{ yaml | to_nice_yaml }}"
...

Notice that by default to_nice_yaml is properly formatted. Almost always, it makes sense to go with to_nice_yaml.

~]$ cat to_yaml.yml 
domains:
  bar.example.com: {alias: bar, region: us-east-2}
  foo: {alias: foo, region: us-east-1}

~]$ cat to_nice_yaml.yml 
domains:
    bar.example.com:
        alias: bar
        region: us-east-2
    foo:
        alias: foo
        region: us-east-1

By default, each key in the new file will be indented with 4 spaces. indent=2 can be used to indent each key with 2 spaces.

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

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

  - copy:
      dest: /path/to/to_nice_yaml.yml
      content: "{{ yaml | to_nice_yaml(indent=2) }}"
...

Now the original YAML file and new YAML files have the same data, same format. The only difference is that the new YAML file is sorted alphabetically.

~]$ cat original.yml 
domains:
  foo:
    alias: foo
    region: us-east-1
  bar.example.com:
    alias: bar
    region: us-east-2

~]$ cat to_nice_yaml.yml 
domains:
  bar.example.com:
    alias: bar
    region: us-east-2
  foo:
    alias: foo
    region: us-east-1