else#

if allows you to add a condition to template. For example, you can use if to add parts of template depending on the presence of variables in data dictionary.

if statement must also be within inside {% %}. End of condition must be explicitly stated:

{% if ospf %}
router ospf 1
 router-id 10.0.0.{{ id }}
 auto-cost reference-bandwidth 10000
{% endif %}

Template example templates/if.txt:

hostname {{ name }}

interface Loopback0
 ip address 10.0.0.{{ id }} 255.255.255.255

{% for vlan, name in vlans.items() %}
vlan {{ vlan }}
 name {{ name }}
{% endfor %}

{% if ospf %}
router ospf 1
 router-id 10.0.0.{{ id }}
 auto-cost reference-bandwidth 10000
 {% for networks in ospf %}
 network {{ networks.network }} area {{ networks.area }}
 {% endfor %}
{% endif %}

if ospf expression works the same way as in Python: if variable exists and is not empty, the result is True. If there is no variable or it is empty, the result is False. That is, in this template the OSPF configuration is generated only if variable ospf exists and is not empty. Configuration will be generated with two data variants.

First with data_files/if.yml that does not contain ospf variable:

id: 3
name: R3
vlans:
  10: Marketing
  20: Voice
  30: Management

The result will be:

$ python cfg_gen.py templates/if.txt data_files/if.yml

hostname R3

interface Loopback0
 ip address 10.0.0.3 255.255.255.255

vlan 10
 name Marketing
vlan 20
 name Voice
vlan 30
 name Management

Now a similar template but with data_files/if_ospf.yml file:

id: 3
name: R3
vlans:
  10: Marketing
  20: Voice
  30: Management
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

Now the result will be:

hostname R3

interface Loopback0
 ip address 10.0.0.3 255.255.255.255

vlan 10
 name Marketing
vlan 20
 name Voice
vlan 30
 name Management

router ospf 1
 router-id 10.0.0.3
 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

As in Python, Jinja is allowed to make branches in condition.

Template example templates/if_vlans.txt:

{% for intf, params in trunks.items() %}
interface {{ intf }}
 {% 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 %}
{% endfor %}

Data file data_files/if_vlans.yml:

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

In this example, different commands are generated depending on value of action parameter.

In template you could also use this option to refer to nested dictionaries:

{% for intf in trunks %}
interface {{ intf }}
 {% if trunks[intf]['action'] == 'add' %}
 switchport trunk allowed vlan add {{ trunks[intf]['vlans'] }}
 {% elif trunks[intf]['action'] == 'delete' %}
 switchport trunk allowed vlan remove {{ trunks[intf]['vlans'] }}
 {% else %}
 switchport trunk allowed vlan {{ trunks[intf]['vlans'] }}
 {% endif %}
{% endfor %}

This will result in the following configuration:

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

Using if you can also filter which elements of sequence will be iterated in for loop.

Template example templates/if_for.txt with filter in for loop:

{% for vlan, name in vlans.items() if vlan > 15 %}
vlan {{ vlan }}
 name {{ name }}
{% endfor %}

Data file (data_files/if_for.yml):

vlans:
Marketing
Voice
Management

The result will be:

$ python cfg_gen.py templates/if_for.txt data_files/if_for.yml
vlan 20
 name Voice
vlan 30
 name Management