问Ansible: 判断嵌套字典是否定义？

---
- hosts: localhost
  user: root
  gather_facts: false
  vars:
  - a:
      b: 1
  tasks:
  - file:
      path: /etc/ansible/facts.d/
      state: directory
  - name: empty local.fact
    file:
      path: /etc/ansible/facts.d/local.fact
      state: absent

  - get_url:  # get a['b'] from  apache server
      url: "{{ apache_url }}/local.ini"
      dest: /etc/ansible/facts.d/local.fact
      force: yes
    ignore_errors: true # 先忽略吧，可能404
  - name: Refresh local facts
    setup:
      filter: ansible_local
      gather_subset: "!all"
  - debug:
      var: a

[a]
b=2

vars:
- a:
    b: "{% if ansible_local.local is defined and ansible_local.local.a is defined and ansible_local.local.a.b is defined %}{{ ansible_local.local.a.b }}{% else %}1{% endif %}"