The Jinja propertyerror test

Objectives

In a Jinja-type calculation, it is possible to test if a variable is accessible (there is no Access control problem).

Prerequisites

We assume that Rougail’s library is installed on your computer.
It is possible to retrieve the current state of the various Rougail files manipulated in this tutorial step by checking out the corresponding tag of the rougail-tutorials git repository. Each tag corresponds to a stage of progress in the tutorial. Of course, you can also decide to copy/paste or download the tutorial files contents while following the tutorial steps.

If you want to follow this tutorial with the help of the corresponding rougail-tutorials git repository, this workshop page corresponds to the tag v1.1_140 in the repository:

git clone https://forge.cloud.silique.fr/stove/rougail-tutorials.git
git switch --detach v1.1_140

For now, the variable proxy_dns_socks5 defines these access controls as follows:

if the proxy_mode variable is not set to "Manual proxy configuration"
if socks version is v4.

The firefox/55-proxy_dns_socks5.yml

%YAML 1.2
---
version: 1.1

proxy_dns_socks5:
  description: Use proxy DNS when using SOCKS v5
  default: false
  disabled:
    jinja: |-
      {{ _.proxy_mode != "Manual proxy configuration" or _.manual.socks_proxy.version == 'v4' }}
    return_type: boolean
    description: |-
      if "_.proxy_mode" is not "Manual proxy configuration"
      or "_.manual.socks_proxy.version" is "v4"
...

In principle it works, but is that really what we want? We mean, do we want to disable it based on the proxy_mode variable?

No, we actually want to make this variable available if the socks version is accessible and equals v4.

Disabled a variable when an other variable is disabled

The most suitable syntax in Jinja for checking the status of a variable is to use the concept of a test.

Rougail offers a custom test that will verify correct access to the variable.

This test is call propertyerror.

Here is how one could write the Jinja using this feature:

The firefox/55-proxy_dns_socks5.yml structure file with some Jinja code in the hidden property

%YAML 1.2
---
version: 1.1

proxy_dns_socks5:
  description: Use proxy DNS when using SOCKS v5
  default: false
  disabled:
    jinja: |-
      {{ _.manual.socks_proxy.version is propertyerror or _.manual.socks_proxy.version == 'v4' }}
    return_type: boolean
    description: |-
      if "_.manual.socks_proxy.version" is accessible and equals "v4"
...

Key points

We have seen that we can use the test propertyerror in Jinja template.