Tests

Besides filters, Jinja also supports tests. Tests allow variables to be tested for a certain condition.

Jinja supports a large number of built-in tests. We will look at only a few of them. The rest of tests you can find in documentation.

defined

Test defined allows you to check if variable is present in the data dictionary.

Template example templates/test_defined.txt:

router ospf 1
{% if ref_bw is defined %}
 auto-cost reference-bandwidth {{ ref_bw }}
{% else %}
 auto-cost reference-bandwidth 10000
{% endif %}
{% for networks in ospf %}
 network {{ networks.network }} area {{ networks.area }}
{% endfor %}

This example is more cumbersome than default filter option, but this test may be useful if depending on whether a variable is defined or not, different commands need to be executed.

Data file (data_files/test_defined.yml):

ospf:
  - network: 10.0.1.0 0.0.0.255
    area: 0
  - network: 10.0.2.0 0.0.0.255
    area: 2
  - network: 10.1.1.0 0.0.0.255
    area: 0

The result of execution:

$ python cfg_gen.py templates/test_defined.txt data_files/test_defined.yml
router ospf 1
 auto-cost reference-bandwidth 10000
 network 10.0.1.0 0.0.0.255 area 0
 network 10.0.2.0 0.0.0.255 area 2
 network 10.1.1.0 0.0.0.255 area 0

iterable

Test iterable checks whether the object is an iterator. Due to these checks, it is possible to make branches in template which will take into account the type of variable.

Template templates/test_iterable.txt (indents made to make an idea of branches more clear):

{% for intf, params in trunks | dictsort %}
interface {{ intf }}
 {% if params.vlans is iterable %}
   {% if params.action == 'add' %}
 switchport trunk allowed vlan add {{ params.vlans | join(',') }}
   {% elif params.action == 'delete' %}
 switchport trunk allowed vlan remove {{ params.vlans | join(',') }}
   {% else %}
 switchport trunk allowed vlan {{ params.vlans | join(',') }}
   {% endif %}
 {% else %}
   {% if params.action == 'add' %}
 switchport trunk allowed vlan add {{ params.vlans }}
   {% elif params.action == 'delete' %}
 switchport trunk allowed vlan remove {{ params.vlans }}
   {% else %}
 switchport trunk allowed vlan {{ params.vlans }}
   {% endif %}
 {% endif %}
{% endfor %}

Data file (data_files/test_iterable.yml):

trunks:
  Fa0/1:
    action: add
    vlans:
      - 10
      - 20
  Fa0/2:
    action: only
    vlans:
      - 10
      - 30
  Fa0/3:
    action: delete
    vlans: 10

Note the last line: vlans: 10. In this case, 10 is no longer in the list and join filter does not work. But, due to is iterable test (in this case the result will be false), in this case template goes into else branch.

The result of execution:

$ python cfg_gen.py templates/test_iterable.txt data_files/test_iterable.yml
interface Fa0/1
 switchport trunk allowed vlan add 10,20
interface Fa0/2
 switchport trunk allowed vlan 10,30
interface Fa0/3
 switchport trunk allowed vlan remove 10

Such indents appeared because the template uses indents but does not have lstrip_blocks=True installed (it removes spaces and tabs at the beginning of the line).