何时在Ansible中使用from_json过滤器？

Question

我什么时候应该在Ansible中使用from_json 过滤器？

我发现有时使用它有时会产生影响.

请考虑以下示例,说明我得到的不一致.

^{反向顺序包括:问题 - 预期结果 - 实际结果 - 剧本 - 数据.数据来自这个问题,剧本基于这个答案.}

• 问题:

  为什么将json_query以下表达式的左侧部分(之前)存储在变量中然后json_query对变量使用会导致表达式的计算方式不同？

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

  为什么添加from_json过滤器会改变结果(但如果处理变量则不会):

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

• 预期结果:

  最后四个任务应该给出相同的结果.或者,最后两个任务应该提供与前两个任务相同的结果.

• 实际结果(仅限最后四个任务):

  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"}
        ]
      }
    ]
  }
  

Answer 1

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

由于Ansible模板引擎棘手的工作,你会得到不同的结果.
我绝对应该在我的网站上写一篇关于它的帖子......

在评估jijna2表达式后,Ansible尝试将复杂类型转换为Python对象(如dict或list).看到我的其他答案.

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带有from_json过滤器的dict .

2.

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

from_lookup_with_from_json成为dict,因为当jinja2表达式结束时,Ansible会转换它}}.所以from_json实际上不需要作为链中的最后一个过滤器.

3.

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

再次,您在此处手动转换JSON字符串.所以json_query得到dict作为输入.

4.

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

在这种情况下,您将JSON-string(不是dict)作为输入json_query过滤.由于一切都发生在一个jinja2表达式中,Ansible不会尝试转换其间的任何内容.

您也可以通过这种方式获得带有变量的空字符串结果:

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

在这种情况下from_lookup_force_string,Ansible tempating引擎不会转换,并json_query会给你空响应.