我什么时候应该使用from_json filter https://docs.ansible.com/ansible/playbooks_filters.html#filters-for-formatting-data在安西布尔？

我发现使用它有时有效果，有时没有效果。

请考虑以下示例，它说明了我所得到的不一致之处。

^{Included in reverse order are: the questions - expected result - actual result - the playbook - the data. The data is taken from this question https://stackoverflow.com/q/41261680/2947502 and the playbook is based on this answer https://stackoverflow.com/a/41262712/2947502.}

• 问题：

  为什么要存储左侧部分（之前json_query) 将以下表达式放入变量中，然后使用json_query变量上的值会导致表达式的计算结果不同吗？

  "{{ lookup('file','test.json') | json_query(query) }}"
  

  为什么添加from_json过滤器改变结果（但如果处理变量则不会改变）：

  "{{ lookup('file','test.json') | from_json | json_query(query) }}"
  

• 预期结果：

  最后四项任务应该给出相同的结果。或者，最后两个任务应该给出与前两个任务相同的结果。

• 实际结果（仅最后四个任务）：

  一项任务结果不同。

  TASK [This query is run against lookup value with from_json stored in a variable] ***
  ok: [localhost] => {
      "msg": [
          678
      ]
  }
  
  TASK [This query is run against lookup value without from_json stored in a variable] ***
  ok: [localhost] => {
      "msg": [
          678
      ]
  }
  
  TASK [This query is run directly against lookup value with from_json] **********
  ok: [localhost] => {
      "msg": [
          678
      ]
  }
  
  TASK [This query is run directly against lookup value without from_json - the result is empty - why?] ***
  ok: [localhost] => {
      "msg": ""
  }
  

• 剧本：

  ---
  - hosts: localhost
    gather_facts: no
    connection: local
    tasks:   
      - set_fact:
          from_lookup_with_from_json: "{{ lookup('file','test.json') | from_json }}"
  
      - set_fact:
          from_lookup_without_from_json: "{{ lookup('file','test.json') }}"
  
      - name: Save the lookup value stored in a variable in a file for comparison
        copy: content="{{ from_lookup_with_from_json }}" dest=./from_lookup_with_from_json.txt
  
      - name: Save the lookup value stored in a variable in a file for comparison (they are the same)
        copy: content="{{ from_lookup_without_from_json }}" dest=./from_lookup_without_from_json.txt
  
      - name: This query is run against lookup value with from_json stored in a variable
        debug: msg="{{ from_lookup_with_from_json | json_query(query) }}"
        vars:
          query: "Foods[].{id: Id, for: (Tags[?Key=='For'].Value)[0]} | [?for=='Tigger'].id"
  
      - name: This query is run against lookup value without from_json stored in a variable
        debug: msg="{{ from_lookup_without_from_json | json_query(query) }}"
        vars:
          query: "Foods[].{id: Id, for: (Tags[?Key=='For'].Value)[0]} | [?for=='Tigger'].id"
  
      - name: This query is run directly against lookup value with from_json
        debug: msg="{{ lookup('file','test.json') | from_json | json_query(query) }}"
        vars:
          query: "Foods[].{id: Id, for: (Tags[?Key=='For'].Value)[0]} | [?for=='Tigger'].id"
  
      - name: This query is run directly against lookup value without from_json - the result is empty - why?
        debug: msg="{{ lookup('file','test.json') | json_query(query) }}"
        vars:
          query: "Foods[].{id: Id, for: (Tags[?Key=='For'].Value)[0]} | [?for=='Tigger'].id"
  

• 数据 （test.json):

  { "Foods" :
    [ { "Id": 456
      , "Tags":
        [ {"Key":"For", "Value":"Heffalump"}
        , {"Key":"Purpose", "Value":"Food"}
        ]
      }
    , { "Id": 678
      , "Tags":
        [ {"Key":"For", "Value":"Tigger"}
        , {"Key":"Purpose", "Value":"Food"}
        ]
      }
    , { "Id": 911
      , "Tags":
        [ {"Key":"For", "Value":"Roo"}
        , {"Key":"Purpose", "Value":"Food"}
        ]
      }
    ]
  }
  

json_query需要 Python 对象（dict）作为输入，如果你用字符串提供它，它会给出空字符串作为结果。

由于 Ansible 模板引擎的棘手工作，您会得到不同的结果。
我绝对应该在我的网站上写一篇关于它的文章......

在评估 jijna2 表达式之后，Ansible 尝试将复杂类型转换为 Python 对象（例如 dict 或 list）。看我的other https://stackoverflow.com/a/41147661/2795592 answer.

在你的情况下：

1.

- set_fact:
    from_lookup_with_from_json: "{{ lookup('file','test.json') | from_json }}"

from_lookup_with_from_json是一个字典，因为您手动将 JSON 字符串从文件转换为字典from_json filter.

2.

- set_fact:
    from_lookup_without_from_json: "{{ lookup('file','test.json') }}"

from_lookup_with_from_json变成 dict，因为当 jinja2 表达式以以下结尾时 Ansible 会转换它}}. So from_json实际上是不必要的the last链式过滤器。

3.

  debug: msg="{{ lookup('file','test.json') | from_json | json_query(query) }}"

同样，您可以在此处手动转换 JSON 字符串。所以json_query获取 dict 作为输入。

4.

  debug: msg="{{ lookup('file','test.json') | json_query(query) }}"

在这种情况下，您将 JSON 字符串（而不是字典）作为输入提供给json_query筛选。由于所有事情都发生在一个 jinja2 表达式内，因此 Ansible 不会尝试转换两者之间的任何内容。

您还可以通过以下方式使用变量获取空字符串结果：

- set_fact:
    from_lookup_force_string: "{{ lookup('file','test.json') | string }}"

在这种情况下from_lookup_force_string不会被 Ansible 临时引擎转换，并且json_query会给你空的答复。