My Kubernetes Ansible setup - which is, to this date, still the easiest way to bootstrap an on-premises Kubernetes cluster - has a task that installs the packages tied to a specific Kubernetes version. When using it with an Ansible version newer than the one used when it was written, I was bothered by the following deprecation warning:

[DEPRECATION WARNING]: Invoking "apt" only once while using a loop via
squash_actions is deprecated. Instead of using a loop to supply multiple items
and specifying `name: "{{ item }}={{ kubernetes_version }}-00"`, please use
`name: '{{ kubernetes_packages }}'` and remove the loop. This feature will be
removed in version 2.11. Deprecation warnings can be disabled by setting
deprecation_warnings=False in ansible.cfg.

This warning a bit misleading. It’s clear that item, which comes from the kubernetes_packages variable used in a with_items option, is just one part of the equation. The package name is being interpolated with its version, glued together with other characters (= and -00) that will produce something like kubectl=1.17.2-00. Changing it to kubernetes_packages isn’t enough. The process of replacing this in-place interpolation by a proper list, as Ansible wants, can be achieved in some ways like:

  • Write down a list that interpolates hard-coded package names with version, like: kubectl={{ kubernetes_version }}-00. The problem is that this pattern has to be repeated for every package.
  • Find a way do generate this list dynamically, by applying the interpolation to every item of the kubernetes_packages list.

Repetition isn’t always bad, but I prefer to avoid it here. The latter option can be easily achieved in any programming language with functional constructs, like JavaScript, which offers a map() array method that accepts a function (here, an arrow function) as the argument and returns another array:

let pkgs = ['kubelet', 'kubectl', 'kubeadm'];

let version = '1.17.2'; => `${p}=${version}-00`);
(3) ["kubelet=1.17.2-00", "kubectl=1.17.2-00", "kubeadm=1.17.2-00"]

Python, the language in which Ansible is written, offers a map() function which accepts a function (here, a lambda expression) and a list as arguments. The object it returns can then be converted to a list:

In [1]: pkgs = ['kubelet', 'kubectl', 'kubeadm']

In [2]: version = '1.17.2'

In [3]: list(map(lambda p: '{}={}-00'.format(p, version), pkgs))
Out[3]: ['kubelet=1.17.2-00', 'kubectl=1.17.2-00', 'kubeadm=1.17.2-00']

That’s be supposed to be similarly easy in Ansible, given that Jinja, its template language, offers a format() filter. The problem is that it does not - and will not - support combining format() and map() filters. Another way to do the same would be to use the format() filter in a list comprehension, but that’s also not supported. But not all hope is lost, as Ansible supports additional regular expression filters, like regex_replace(). It can be used in many different ways, but here we will use it for doing a single thing: concatenate the package name with a suffix made of another string concatenation operation. This way, the following task:

- name: install packages
    name: "{{ item }}={{ kubernetes_version }}-00"
  with_items: "{{ kubernetes_packages }}"

Is equivalent to:

- name: install packages
    name: "{{ kubernetes_packages |
      map('regex_replace', '$', '=' + kubernetes_version + '-00') |

The key is that the '$' character matches the end of the string, so replacing it is akin to concatenating two strings. The list filter in the end is needed because, just like the equivalent Python built-in function, the Jinja map() filter also returns a generator. This object then needs to be converted to a list, otherwise, it would result in errors like No package matching '<generator object do_map at 0x10bbedba0>' is available, given that its string representation will be used as the package name.