Why does a single space in Ansible lineinfile module completely change the result

I solved my initial question how to enter a specific line into a file with Ansible. Now I still struggle to see why.

The minimal example is the following:

---
- name: Test line in file
  hosts: localhost
  tasks:
    - name: Set var
      set_fact:
        test_var: test

    - name: write to file
      lineinfile:
        path: test.txt
        create: True
        line: ' {"socket": "{{test_var}}"},'

This, as expected, creates a file with the line {"socket": "test"}, However, using

    - name: write to file
      lineinfile:
        path: test.txt
        create: True
        line: '{"socket": "{{test_var}}"},'

(Note the missing space ) results in the line [{'socket': 'test'}] . Can someone please explain what is happening? Bonus points if someone knows a solution inserting the expected line without a beginning space.

It is a side effect of how templating works.

Ansible does some magic to try and convert strings that look like Python data structures to Python data structures.

A: Writing what you want in a single Ansible expression is impossible.

• In Ansible, both

{"socket": "test"}

and

{socket: "test"}

are valid dictionaries. See YAML Syntax. The first expression is also valid JSON. Note that JSON is a YAML subset.

• They become valid strings if you quote them

'{"socket": "test"},'

'{socket: "test"},'

• When you put a substitution into the expression JINJA is used. The below expression evaluates to a string {socket: "test"},

'{socket: "{{ test_var }}"},'

, but if the 'key' is quoted

'{"socket": "{{ test_var }}"},'

it evaluates to a list [{'socket': 'test'}]

- hosts: localhost

  vars:

    test_var: test
  
  tasks:

    - debug:
        msg: |
          x: {{ item }}
          type: {{ item | type_debug }}
      loop:
        - {"socket": "test"}
        - {socket: "test"}
        - '{"socket": "test"}'
        - '{socket: "test"}'
        - '{"socket": "test"},'
        - '{socket: "test"},'
        - '{socket: "{{ test_var }}"},'
        - '{"socket": "{{ test_var }}"},'

{"socket": "{{ test_var }}"}
{socket: "{{ test_var }}"}

'{"socket": "{{ test_var }}"}'
'{socket: "{{ test_var }}"}'