[Short Tip] Flatten nested dict/list structures in Ansible with json_query

A few days ago I was asked how to best deal with structures in Ansible which are mixing dictionaries and lists. json_query can help here!

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A few days ago I was asked how to best deal with structures in Ansible which are mixing dictionaries and lists. Basically, the following example was provided and the questioned remained how to deal with this – for example how to flatten it:

          - name: "city1"
          - size: "large"
          - param: "alpha"
          - name: "city2"
          - size: "small"
          - param: "beta"
          - name: "city1"
          - size: "large"
          - param: "gamma"

I was wondering a lot how to deal with this – after all dict2items only deals with dicts and fails when it reaches the lists in there. I also fooled around with the map filter, but most of my results also required some previous knowledge about the data structure, were only acting by providing “cloud1.region1” or similar.

The solution was the json_query filter: it is based on jmespath and can deal with the above mentioned structure by list and object projections:

  - name: Projections using json_query
      msg: "Item value is: {{ item }}"
    loop: "{{ myhash|json_query(projection_query)|list }}"
      projection_query: "*.*[]"

And indeed, the loop does create a simplified output of all the elements in this nested structure:

TASK [Projections using json_query] **********************************************************
ok: [localhost] => (item=[{'name': 'city1'}, {'size': 'large'}, {'param': 'alpha'}]) => {
    "msg": "Item value is: [{'name': 'city1'}, {'size': 'large'}, {'param': 'alpha'}]"
ok: [localhost] => (item=[{'name': 'city2'}, {'size': 'small'}, {'param': 'beta'}]) => {
    "msg": "Item value is: [{'name': 'city2'}, {'size': 'small'}, {'param': 'beta'}]"
ok: [localhost] => (item=[{'name': 'city1'}, {'size': 'large'}, {'param': 'gamma'}]) => {
    "msg": "Item value is: [{'name': 'city1'}, {'size': 'large'}, {'param': 'gamma'}]"

Of course, some knowledge is still needed to make this work: you need to know if you are projecting on a list or on a dictionary. So if your data structure changes on that level between executions, you might need something else.

Image by Andrew Martin from Pixabay

[Short Tip] verify YAML in Shell via Python one-liner [Update]

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Today the question came up how to verify YAML files easily. Of course, there are many very good online parser. But I was wondering if it is possible to do it simply in Bash/ZSH, using a Python one-liner. Here is the code:

$ python -c 'import yaml,sys;yaml.safe_load(sys.stdin)' < yamltest.txt

It throws an exception if the file is not a proper (aka importable) YAML file. Otherwise it just returns with a 0 exit code.

Please note that I am not sure how tolerant yaml.safe_load is. And note that PyYAML needs to be installed.

Updated to avoid cat abuse – safe the kittens! Thanks to ichor!