Plugin System

Avocado has a plugin system that can be used to extended it in a clean way.

Listing plugins

The avocado command line tool has a builtin plugins command that lets you list available plugins. The usage is pretty simple:

$ avocado plugins
Plugins that add new commands (avocado.plugins.cli.cmd):
exec-path Returns path to avocado bash libraries and exits.
run       Run one or more tests (native test, test alias, binary or script)
sysinfo   Collect system information
Plugins that add new options to commands (avocado.plugins.cli):
remote  Remote machine options for 'run' subcommand
journal Journal options for the 'run' subcommand

Since plugins are (usually small) bundles of Python code, they may fail to load if the Python code is broken for any reason. Example:

$ avocado plugins
Failed to load plugin from module "avocado.plugins.exec_path": ImportError('No module named foo',)
Plugins that add new commands (avocado.plugins.cli.cmd):
run       Run one or more tests (native test, test alias, binary or script)
sysinfo   Collect system information

Writing a plugin

What better way to understand how an Avocado plugin works than creating one? Let’s use another old time favorite for that, the “Print hello world” theme.

Code example

Let’s say you want to write a plugin that adds a new subcommand to the test runner, hello. This is how you’d do it:

from avocado.core.output import LOG_JOB
from avocado.core.plugin_interfaces import CLICmd

class HelloWorld(CLICmd):

    name = 'hello'
    description = 'The classical Hello World! plugin example.'

    def run(self, args):

As you can see, this plugins inherits from avocado.core.plugin_interfaces.CLICmd. This specific base class allows for the creation of new commands for the Avocado CLI tool. The only mandatory method to be implemented is run and it’s the plugin main entry point.

This plugin uses avocado.core.output.LOG_JOB to produce the hello world output in the Job log. One can also use avocado.core.output.LOG_UI to produce output in the human readable output.

Registering Plugins

Avocado makes use of the Stevedore library to load and activate plugins. Stevedore itself uses setuptools and its entry points to register and find Python objects. So, to make your new plugin visible to Avocado, you need to add to your setuptools based file something like:

   'avocado.plugins.cli': [
      'hello = mypluginpack.hello:HelloWorld',

Then, by running either $ python install or $ python develop your plugin should be visible to Avocado.

Fully qualified named for a plugin

The plugin registry mentioned earlier, (setuptools and its entry points) is global to a given Python installation. Avocado uses the namespace prefix avocado.plugins. to avoid name clashes with other software. Now, inside Avocado itself, there’s no need keep using the avocado.plugins. prefix.

Take for instance, the Job Pre/Post plugins are defined on

'avocado.plugins.job.prepost': [
   'jobscripts = avocado.plugins.jobscripts:JobScripts'

The setuptools entry point namespace is composed of the mentioned prefix avocado.plugins., which is is then followed by the Avocado plugin type, in this case, job.prepost.

Inside avocado itself, the fully qualified name for a plugin is the plugin type, such as job.prepost concatenated to the name used in the entry point definition itself, in this case, jobscripts.

To summarize, still using the same example, the fully qualified Avocado plugin name is going to be job.prepost.jobscripts.

Disabling a plugin

Even though a plugin can be installed and registered under setuptools entry points, it can be explicitly disabled in Avocado.

The mechanism available to do so is to add entries to the disable key under the plugins section of the Avocado configuration file. Example:

disable = ['cli.hello', 'job.prepost.jobscripts']

The exact effect on Avocado when a plugin is disabled depends on the plugin type. For instance, by disabling plugins of type cli.cmd, the command implemented by the plugin should no longer be available on the Avocado command line application. Now, by disabling a job.prepost plugin, those won’t be executed before/after the execution of the jobs.

Default plugin execution order

In many situations, such as result generation, not one, but all of the enabled plugin types will be executed. The order in which the plugins are executed follows the lexical order of the entry point name.

For example, for the JSON result plugin, whose fully qualified name is result.json, has an entry point name of json, as can be seen on its registration code in

   'avocado.plugins.result': [
      'json = avocado.plugins.jsonresult:JSONResult',

If it sounds too complicated, it isn’t. It just means that for plugins of the same type, a plugin named automated will be executed before the plugin named uploader.

In the default Avocado set of result plugins, it means that the JSON plugin (json) will be executed before the XUnit plugin (xunit). If the HTML result plugin is installed and enabled (html) it will be executed before both JSON and XUnit.

Configuring the plugin execution order

On some circumstances it may be necessary to change the order in which plugins are executed. To do so, add a order entry a configuration file section named after the plugin type. For job.prepost plugin types, the section name has to be named plugins.job.prepost, and it would look like this:

order = ['myplugin', 'jobscripts']

That configuration sets the job.prepost.myplugin plugin to execute before the standard Avocado job.prepost.jobscripts does.

Wrap Up

We have briefly discussed the making of Avocado plugins. We recommend the Stevedore documentation and also a look at the avocado.core.plugin_interfaces module for the various plugin interface definitions.

Some plugins examples are available in the Avocado source tree, under examples/plugins.

Finally, exploring the real plugins shipped with Avocado in avocado.plugins is the final “documentation” source.