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AVD Coding Style

This page lists guidelines for developing Python code or Jinja2 templates in the AVD context. These rules apply to creating or updating any Python or Jinja2 file available in a aristanetworks/avd repository.

Python code style

As AVD is an Ansible collection, we're required to follow guidelines from the official Ansible documentation for all Python code.

Furthermore, the CI Pipeline (& pre-commit) for AVD enforces the following:

Configurations for the above tools can be found in:

Jinja2 Syntax guidelines

SYNTAX-1 - Using variables in Jinja

  • Description

    A single space shall be added between Jinja2 curly brackets and a variable's name.

  • Example

    {{ ethernet_interface }}

SYNTAX-2 - Filter syntax

  • Description

    When variables are used in combination with a filter, | shall be enclosed by space.

  • Example

    {{ my_value | to_json }}

SYNTAX-3 - Indentation for statement blocks

  • Description

    The nested jinja code block shall follow the following rules:

    • All J2 statements must be enclosed by one space
    • All J2 statements must be indented by four more spaces within the jinja delimiter
    • To close a control, the end tag must have the same indentation level as the opening tag
    • Indentation must be four spaces and NOT tabulation

  • Example

    {# Initial block indentation #}
{% if my_variable is arista.avd.defined %}

{# Nested block indentation #}
{% for ethernet_interface in ethernet_interfaces %}
{%     if ethernet_interface.name is arista.avd.defined %}
{%         set result = ethernet_interface.name %}
{# ..... output truncated ..... #}
{%     endif %}
{% endfor %}

SYNTAX-4 - Expand list on a single line

  • Description

    Instead of doing a for loop on a single line, the join filter should be leveraged as much as possible.

  • Example

    {{ ciphers | join(", ") }}

SYNTAX-5 - Test if a variable in a list

  • Description

    To test if a variable is part of a list, the in operator should be used as much as possible to avoid long if/elif/else block.

  • Example

    {% if underlay_routing_protocol is arista.avd.defined and underlay_routing_protocol in ['isis', 'ospf'] %}

SYNTAX-6 - Render long CLI

  • Description

    When a long CLI with multiple options must be built, use pure J2 logic and print.

  • Example

    {% for ip_helper in vlan_interfaces[vlan_interface].ip_helpers | arista.avd.natural_sort %}
{%     set ip_helper_cli = "ip helper-address " ~ ip_helper %}
{%     if vlan_interfaces[vlan_interface].ip_helpers[ip_helper].vrf is arista.avd.defined %}
{%         set ip_helper_cli = ip_helper_cli ~ " vrf " ~ vlan_interfaces[vlan_interface].ip_helpers[ip_helper].vrf %}
{%     endif %}
{%     if vlan_interfaces[vlan_interface].ip_helpers[ip_helper].source_interface is arista.avd.defined %}
{%         set ip_helper_cli = ip_helper_cli ~ " source-interface " ~ vlan_interfaces[vlan_interface].ip_helpers[ip_helper].source_interface %}
{%     endif %}
   {{ ip_helper_cli }}
{% endfor %}

YAML Variable definition

VAR-1 - Variable name case

  • Description

    All variables shall use lowercase.

  • Example

    {{ variable }}

VAR-2 - Variable name format

  • Description

    An underscore _ should be used as a separator for a multi-word variable.

  • Example

    {{ my_variable_name }}

VAR-3 - Iterable variables

  • Description

    For an iterable variable, the plural form shall be used.

  • Example

    {{ ethernet_interfaces }}

VAR-4 - Variables in a For Loop

  • Description

    For variables in a for loop, the singular form shall be used.

  • Example

    {{ ethernet_interfaces[ethernet_interface] }}

VAR-5 - Variables concatenation

  • Description

    Tilde ~ should be used for string concatenation as it automatically converts variables to a string.

  • Example

    {% set ip_helper_cli = ip_helper_cli ~ " source-interface " ~ vlan_interfaces[vlan_interface].ip_helpers[ip_helper].source_interface %}

VAR-6 - Variable type comparison

  • Description

    To test the type of a variable, it's recommended to use is/is not keywords.

  • Example

    {# Test if variable is string #}
{% if ethernet_interface is string %}

{# Test if variable is not a string #}
{% if ethernet_interface is not string %}

VAR-7 - Variable content comparison

  • Description

    To test the content of a variable, it's recommended to use ==/!= keywords.

  • Example

    {# Test if variable is equal to 'Ethernet1' #}
{% if ethernet_interface == 'Ethernet1' %}

{# Test if variable is not equal to 'Ethernet1' #}
{% if ethernet_interface != 'Ethernet1' %}

    !!! info PLUGIN-2 can do a test if the variable is defined and has a specific value

VAR-8 - String comparison

  • Description

    All strings should be compared based on lowercase format.

  • Example

    {% if underlay_routing_protocol is arista.avd.defined and underlay_routing_protocol | lower in ['isis', 'ospf'] %}

AVD Plugins usage

See the menu on the left for Ansible Collection Plugins.

PLUGIN-1 - Test if a variable exists

  • Description

    All tests to check if a variable is defined shall be done with arista.avd.defined. This test also does a deep test and doesn't require a test at an upper level.

  • Example

    {# Simple test #}
{% if ethernet_interfaces is arista.avd.defined %}

{# Deep test #}
{% if router_bgp.vrfs[vrf].rd is arista.avd.defined %}

PLUGIN-2 Test if a variable exists with a given value

  • Description

    The test arista.avd.defined shall be used to test if a variable is defined and has a specific value.

  • Example

    {% if vlan.name is arista.avd.defined('test') %}

PLUGIN-3 - Default value

  • Description

    If a default value must be used, the arista.avd.default plugin shall be used instead of an if/else block. The plugin can be used to fallback to different values until one of them is defined and valid.

  • Example

    {# Simple default test with one default value #}
{{ vlan.name | arista.avd.default('test_vlan') }}

{# Default test with a list of default options #}
{{ vlan.name | arista.avd.default(default.vlan.name, 'test_vlan') }}