Data manipulation

In many cases, you need to do some complex operation with your variables, while Ansible is not recommended as a data processing/manipulation tool, you can use the existing Jinja2 templating in conjunction with the many added Ansible filters, lookups and tests to do some very complex transformations.

Let’s start with a quick definition of each type of plugin:
  • lookups: Mainly used to query ‘external data’, in Ansible these were the primary part of loops using the with_<lookup> construct, but they can be used independently to return data for processing. They normally return a list due to their primary function in loops as mentioned previously. Used with the lookup or query Jinja2 operators.

  • filters: used to change/transform data, used with the | Jinja2 operator.

  • tests: used to validate data, used with the is Jinja2 operator.

Loops and list comprehensions

Most programming languages have loops (for, while, and so on) and list comprehensions to do transformations on lists including lists of objects. Jinja2 has a few filters that provide this functionality: map, select, reject, selectattr, rejectattr.

  • map: this is a basic for loop that just allows you to change every item in a list, using the ‘attribute’ keyword you can do the transformation based on attributes of the list elements.

  • select/reject: this is a for loop with a condition, that allows you to create a subset of a list that matches (or not) based on the result of the condition.

  • selectattr/rejectattr: very similar to the above but it uses a specific attribute of the list elements for the conditional statement.

Extract keys from a dictionary matching elements from a list

The Python equivalent code would be:

chains = [1, 2]
for chain in chains:
    for config in chains_config[chain]['configs']:
        print(config['type'])

There are several ways to do it in Ansible, this is just one example:

Way to extract matching keys from a list of dictionaries
 tasks:
   - name: Show extracted list of keys from a list of dictionaries
     debug: msg="{{ chains | map('extract', chains_config) | map(attribute='configs') | flatten | map(attribute='type') | flatten }}"
     vars:
       chains: [1, 2]
       chains_config:
           1:
               foo: bar
               configs:
                   - type: routed
                     version: 0.1
                   - type: bridged
                     version: 0.2
           2:
               foo: baz
               configs:
                   - type: routed
                     version: 1.0
                   - type: bridged
                     version: 1.1
Results of debug task, a list with the extracted keys
   ok: [localhost] => {
       "msg": [
           "routed",
           "bridged",
           "routed",
           "bridged"
       ]
   }

Find mount point

In this case, we want to find the mount point for a given path across our machines, since we already collect mount facts, we can use the following:

Use selectattr to filter mounts into list I can then sort and select the last from
  - hosts: all
    gather_facts: True
    vars:
       path: /var/lib/cache
    tasks:
    - name: The mount point for {{path}}, found using the Ansible mount facts, [-1] is the same as the 'last' filter
      debug: msg="{{(ansible_facts.mounts | selectattr('mount', 'in', path) | list | sort(attribute='mount'))[-1]['mount']}}"

Omit elements from a list

The special omit variable ONLY works with module options, but we can still use it in other ways as an identifier to tailor a list of elements:

Inline list filtering when feeding a module option
   - name: enable a list of Windows features, by name
     set_fact:
       win_feature_list: "{{ namestuff | reject('equalto', omit) | list }}"
     vars:
       namestuff:
         - "{{ (fs_installed_smb_v1 | default(False)) | ternary(omit, 'FS-SMB1') }}"
         - "foo"
         - "bar"

Another way is to avoid adding elements to the list in the first place, so you can just use it directly:

Using set_fact in a loop to increment a list conditionally
   - name: build unique list with some items conditionally omittted
     set_fact:
        namestuff: ' {{ (namestuff | default([])) | union([item]) }}'
     when: item != omit
     loop:
         - "{{ (fs_installed_smb_v1 | default(False)) | ternary(omit, 'FS-SMB1') }}"
         - "foo"
         - "bar"

Complex Type transformations

Jinja provides filters for simple data type transformations (int, bool, and so on), but when you want to transform data structures things are not as easy. You can use loops and list comprehensions as shown above to help, also other filters and lookups can be chained and leveraged to achieve more complex transformations.

Create dictionary from list

In most languages it is easy to create a dictionary (a.k.a. map/associative array/hash and so on) from a list of pairs, in Ansible there are a couple of ways to do it and the best one for you might depend on the source of your data.

These example produces {"a": "b", "c": "d"}

Simple list to dict by assuming the list is [key, value , key, value, …]
 vars:
     single_list: [ 'a', 'b', 'c', 'd' ]
     mydict: "{{ dict(single_list) | slice(2) | list }}"
It is simpler when we have a list of pairs:
 vars:
     list_of_pairs: [ ['a', 'b'], ['c', 'd'] ]
     mydict: "{{ dict(list_of_pairs) }}"

Both end up being the same thing, with the slice(2) | list transforming single_list to the same structure as list_of_pairs.

A bit more complex, using set_fact and a loop to create/update a dictionary with key value pairs from 2 lists:

Using set_fact to create a dictionary from a set of lists
    - name: Uses 'combine' to update the dictionary and 'zip' to make pairs of both lists
      set_fact:
        mydict: "{{ mydict | default({}) | combine({item[0]: item[1]}) }}"
      loop: "{{ (keys | zip(values)) | list }}"
      vars:
        keys:
          - foo
          - var
          - bar
        values:
          - a
          - b
          - c

This results in {"foo": "a", "var": "b", "bar": "c"}.

You can even combine these simple examples with other filters and lookups to create a dictionary dynamically by matching patterns to variable names:

Using ‘vars’ to define dictionary from a set of lists without needing a task
   vars:
       myvarnames: "{{ q('varnames', '^my') }}"
       mydict: "{{ dict(myvarnames | zip(q('vars', *myvarnames))) }}"

A quick explanation, since there is a lot to unpack from these two lines:

  • The varnames lookup returns a list of variables that match “begin with my”.

  • Then feeding the list from the previous step into the vars lookup to get the list of values. The * is used to ‘dereference the list’ (a pythonism that works in Jinja), otherwise it would take the list as a single argument.

  • Both lists get passed to the zip filter to pair them off into a unified list (key, value, key2, value2, …).

  • The dict function then takes this ‘list of pairs’ to create the dictionary.

An example on how to use facts to find a host’s data that meets condition X:

vars:
  uptime_of_host_most_recently_rebooted: "{{ansible_play_hosts_all | map('extract', hostvars, 'ansible_uptime_seconds') | sort | first}}"

Using an example from @zoradache on reddit, to show the ‘uptime in days/hours/minutes’ (assumes facts where gathered). https://www.reddit.com/r/ansible/comments/gj5a93/trying_to_get_uptime_from_seconds/fqj2qr3/

- debug:
   msg: Timedelta {{ now() - now().fromtimestamp(now(fmt='%s') | int - ansible_uptime_seconds) }}

See also

Using filters to manipulate data

Jinja2 filters included with Ansible

Tests

Jinja2 tests included with Ansible

Jinja2 Docs

Jinja2 documentation, includes lists for core filters and tests