A "space" caused Ansible error!

 Example playbook.

---
- hosts: localhost
  tasks:
    - name: test loop
      vars:
        users:
          - user01
          - user02
      debug:
        var: item
      loop: " {{ users }}"

When running it, below errors will be printed out.

ASK [test loop] ***************************************************************
fatal: [localhost]: FAILED! => {"msg": "Invalid data passed to 'loop', it requires a list, got this instead:  ['user01', 'user02']. Hint: If you passed a list/dict of just one element, try adding wantlist=True to your lookup invocation or use q/query instead of lookup."}  
 

The root cause is the space (marked as red background) before the jinja2 var.

With this space, " {{ users }}" is not a list any more.

So, be careful of extra spaces in your playbook!