AttributeError: 'list' object has no attribute 'splitlines'

Question

Considering the following test playbook:

---
- name: Node Tolerations
  hosts: localhost
  gather_facts: false
  vars:
    tolerations:
      - key: node.cilium.io/agent-not-ready
        operator: Exists
        effect: NoExecute
      - key: node-role.kubernetes.io/control-plane
        operator: Exists
        effect: NoSchedule
  tasks:
    - name: Set node tolerations fact
      ansible.builtin.set_fact:
        node_tolerations: "{{ (node_tolerations | default([]) | union([':'.join((item.key, item.effect))])) }}"
      with_items: '{{ tolerations }}'

    - name: Set node taint fact
      ansible.builtin.set_fact:
        node_taint: "{{ node_tolerations | select('search', 'control-plane') }}"

    - name: Variable output
      ansible.builtin.debug:
        var: node_taint

Which produces the expected result:

ok: [localhost] =>
  node_taint:
  - node-role.kubernetes.io/control-plane:NoSchedule

While using the node_taint fact into a Jinja2 template:

node-taint:
  {{ node_taint | indent(2) }}

The following error is generated:

AttributeError: 'list' object has no attribute 'splitlines'

As temporary workaround, I used:

node-taint:
  - {{ node_taint | join }}

But I prefer the initial Jinja2 format, allowing me to define the proper indentation. I was wondering if you can provide some insight, what would be the proper fix.

Edit: A second usage I will have for the above detailed playbook is:

tolerations:
  {{ tolerations | indent(2) }}

Which produces the same error. I would prefer to avoid the use of for loops inside Jinja template and manipulate the data at fact level, allowing me to use a proper indent inside template.

Answer 1

Given the mre list

  node_taint:
    - node-role.0
    - node-role.1
    - node-role.2

the below debug

   - debug:
       var: node_taint

gives in YAML format

  node_taint:
  - node-role.0
  - node-role.1
  - node-role.2

If you want to indent the lines

    - debug:
        msg: |
          node_taint:
            {{ node_taint | indent(2) }}

you get the below error because the value of node_taint isn't a string

... The error was: AttributeError: 'list' object has no attribute 'splitlines'

Quoting from the function indent:

jinja-filters.indent(s: str, width: int | str = 4, ...

Return a copy of the string with each line indented by 4 spaces. The first line and blank lines are not indented by default.

The solution is to convert the value of node_taint to a string. Use the filter to_nice_yaml

    - debug:
        msg: |
          node_taint:
            {{ node_taint | to_nice_yaml | indent(2) }}

gives

  msg: |-
    node_taint:
      - node-role.0
      - node-role.1
      - node-role.2

---

Example of a complete playbook for testing

- hosts: localhost

  vars:

    node_taint:
      - node-role.0
      - node-role.1
      - node-role.2

  tasks:

    - debug:
        var: node_taint

    - debug:
        msg: |
          node_taint:
            {{ node_taint | to_yaml | indent(2) }}

    - debug:
        msg: |
          node_taint:
            {{ node_taint | to_nice_yaml | indent(2) }}