Nested Config Lists

Jinja

Having a jinja2 layer on our config files helps us centralise information and keep configurations as DRY as possible.

Recall our test YAML:

title: "Lévy the cat"
colors:
  - "black"
  - "white"
hobby:
  eating:
    what: "anything"
friends:
  {% set friends = [ "cartman", "lima" ] %}
  {% for friend in friends %}
  - name: ${ friend }
    type: "cat"
  {% endfor %}

Namedtuples

The colors list has nothing fancy in it, as we have simple types. However, we want to parse nested configurations as Config, while being able to access them by name as attributes.

// Access a list element by name
$ cfg.friends.lima
Config(lima)

// Extract information as usual
$ cfg.friends.lima.type
cat

To fit this spot we have namedtuples. The list attribute becomes a namedtuple where the properties are the names of the nested items. If we check the type...

// Check list instance
$ isinstance(cfg.friends, tuple)
True

Identifiers

name is set as the default identifier, i.e., will be used to name the namedtuple, but we can pass others as parameter. For example, we can set it as id if we had the following YAML:

[...]
friends:
  {% set friends = [ "cartman", "lima" ] %}
  {% for friend in friends %}
  - id: ${ friend }
    type: "cat"
  {% endfor %}

from levy.config import Config

cfg = Config.read_file(file, list_id="id")

Handling Exceptions

If we encounter an error while defining the namedtuples structure, we will get a ListParseException. We should then check how are we defining the lists and our list_id argument.

OBS: Note that the list_id field should be a valid namedtuple key. This means that it cannot contain spaces or other not supported special characters.