Calculation with a jinja type

Objectives

In this section we will learn how to create new ways of calculation.

Up to now, our only way of dynamically (that is, during the runtime) calculating a value is to point on another variable’s value. But this is not the only way.

A jinja calculated variable’s hidden property

We can hide or disable some variables or families with other techniques than pointing on a variable’s value.

Let’s reason on the previous HTTPS proxy configuration’s manual mode:

manual:

  use_for_https:
    description: Also use this proxy for HTTPS
    default: true

  https_proxy:
    type: family
    description: HTTPS Proxy
    hidden:
      variable: manual.use_for_https

This is extracted from the proxy’s manual configuration we discussed before.

We see here that there is an https_proxy family that is going to be hidden depending on the value of another variable:

https_proxy:
  type: family
  hidden:
    variable: manual.use_for_https

Now we could write it like that:

manual:

  use_for_https:
    description: Also use this proxy for HTTPS
    default: true

  https_proxy:
    description: HTTPS Proxy
    type: family
    hidden:
      type: jinja
      jinja: |
        {% if rougail.manual.use_for_https %}
        the HTTPS Proxy family is hidden
        {% endif %}

Yes, it’s done in a more complicated (but more powerful) way. Let’s explain this a little:

We have replaced this simple hidden property declaration:

hidden:
  variable: manual.use_for_https

by this (more complicated) hidden property declaration:

hidden:
  type: jinja
  jinja: |
    {% if rougail.manual.use_for_https %}
    the HTTPS Proxy family is hidden
    {% endif %}

The fact is that it has same result, but here we have more possibilities. The hidden process is done by a calculation.

Another jinja calculation type sample

We can now hide or disable some variables or families with other techniques than pointing on a variable’s value.

Let’s reason upon the proxy’s manual configuration we discussed before. We have the dict/02-proxy_manual.yml structure file:

the structfile/02-proxy_manual.yml file

   ---
   version: '1.1'
   manual:
     description: Manual proxy configuration
     type: family
     disabled:
       type: jinja
       jinja: |
         {% if rougail.proxy.proxy_mode != 'Manual proxy configuration' %}
         the proxy mode is not manual
         {% endif %}

Question

question: OK then. What happens when you select the “Manual proxy configuration”?

Here if the user selects the “Manual proxy configuration” proxy mode, the the manual family will be disabled. This is what the jinja code says. Let’s explain it more precisely.

Note

The “the proxy mode is not manual” output is be used in the log outputs for example while

Why Jinja?

What about this Jinja type?

If the Jinja template returns some text, then the family will be disabled. Otherwise it is accessible. Deactivating a family means that we will not be able to access it as well as the variables or families included in this family.

Note

If the Jinja template does not return any text, the variable will be enabled. Here we are using the Jinja condition statement.

Jinja: Jinja is a template engine. we are using Jinja in a classical way, that is, Jinja allows us to handle different cases, for example with the if statement.

Todo

montrer aussi ici des exemples de calculs de valeurs variables, ce qui est un des usages principaux de jinja

Key points

Here we have come to the possibility of making any kind of calculations based on the state of the configuration. This is an important feature to manage the stateful aspect of a configuration.