ansible/docs/docsite/rst/dev_guide/developing_plugins.rst

.. _developing_plugins:
.. _plugin_guidelines:

******************
Developing plugins
******************

.. contents:: Topics

Plugins augment Ansible's core functionality with logic and features that are accessible to all modules. Ansible ships with a number of handy plugins, and you can easily write your own. All plugins must:

* be written in Python
* raise errors
* return strings in unicode
* conform to Ansible's configuration and documentation standards

Once you've reviewed these general guidelines, you can skip to the particular type of plugin you want to develop.

Writing plugins in Python
=========================

You must write your plugin in Python so it can be loaded by the ``PluginLoader`` and returned as a Python object that any module can use. Since your plugin will execute on the controller, you must write it in a :ref:`compatible version of Python <control_machine_requirements>`.

Raising errors
==============

You should return errors encountered during plugin execution by raising ``AnsibleError()`` or a similar class with a message describing the error. When wrapping other exceptions into error messages, you should always use the ``to_text`` Ansible function to ensure proper string compatibility across Python versions:

.. code-block:: python

    from ansible.module_utils._text import to_native

    try:
        cause_an_exception()
    except Exception as e:
        raise AnsibleError('Something happened, this was original exception: %s' % to_native(e))

Check the different `AnsibleError objects <https://github.com/ansible/ansible/blob/devel/lib/ansible/errors/__init__.py>`_ and see which one applies best to your situation.

String encoding
===============

You must convert any strings returned by your plugin into Python's unicode type. Converting to unicode ensures that these strings can run through Jinja2. To convert strings:

.. code-block:: python

    from ansible.module_utils._text import to_text
    result_string = to_text(result_string)

Plugin configuration & documentation standards
==============================================

To define configurable options for your plugin, describe them in the ``DOCUMENTATION`` section of the python file. Callback and connection plugins have declared configuration requirements this way since Ansible version 2.4; most plugin types now do the same. This approach ensures that the documentation of your plugin's options will always be correct and up-to-date. To add a configurable option to your plugin, define it in this format:

.. code-block:: yaml

    options:
      option_name:
        description: describe this config option
        default: default value for this config option
        env:
          - name: NAME_OF_ENV_VAR
        ini:
          - section: section_of_ansible.cfg_where_this_config_option_is_defined
            key: key_used_in_ansible.cfg
        required: True/False
        type: boolean/float/integer/list/none/path/pathlist/pathspec/string/tmppath
        version_added: X.x

To access the configuration settings in your plugin, use ``self.get_option(<option_name>)``. For most plugin types, the controller pre-populates the settings. If you need to populate settings explicitly, use a ``self.set_options()`` call.

Plugins that support embedded documentation (see :ref:`ansible-doc` for the list) must include well-formed doc strings to be considered for merge into the Ansible repo. If you inherit from a plugin, you must document the options it takes, either via a documentation fragment or as a copy. See :ref:`module_documenting` for more information on correct documentation. Thorough documentation is a good idea even if you're developing a plugin for local use.

Developing particular plugin types
==================================

.. _developing_callbacks:

Callback plugins
----------------

Callback plugins add new behaviors to Ansible when responding to events. By default, callback plugins control most of the output you see when running the command line programs.

To create a callback plugin, create a new class with the Base(Callbacks) class as the parent:

.. code-block:: python

  from ansible.plugins.callback import CallbackBase

  class CallbackModule(CallbackBase):
      pass

From there, override the specific methods from the CallbackBase that you want to provide a callback for.
For plugins intended for use with Ansible version 2.0 and later, you should only override methods that start with ``v2``.
For a complete list of methods that you can override, please see ``__init__.py`` in the
`lib/ansible/plugins/callback <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/callback>`_ directory.

The following is a modified example of how Ansible's timer plugin is implemented,
but with an extra option so you can see how configuration works in Ansible version 2.4 and later:

.. code-block:: python

  # Make coding more python3-ish, this is required for contributions to Ansible
  from __future__ import (absolute_import, division, print_function)
  __metaclass__ = type

  # not only visible to ansible-doc, it also 'declares' the options the plugin requires and how to configure them.
  DOCUMENTATION = '''
    callback: timer
    callback_type: aggregate
    requirements:
      - whitelist in configuration
    short_description: Adds time to play stats
    version_added: "2.0"
    description:
        - This callback just adds total play duration to the play stats.
    options:
      format_string:
        description: format of the string shown to user at play end
        ini:
          - section: callback_timer
            key: format_string
        env:
          - name: ANSIBLE_CALLBACK_TIMER_FORMAT
        default: "Playbook run took %s days, %s hours, %s minutes, %s seconds"
  '''
  from datetime import datetime

  from ansible.plugins.callback import CallbackBase


  class CallbackModule(CallbackBase):
      """
      This callback module tells you how long your plays ran for.
      """
      CALLBACK_VERSION = 2.0
      CALLBACK_TYPE = 'aggregate'
      CALLBACK_NAME = 'timer'

      # only needed if you ship it and don't want to enable by default
      CALLBACK_NEEDS_WHITELIST = True

      def __init__(self):

          # make sure the expected objects are present, calling the base's __init__
          super(CallbackModule, self).__init__()

          # start the timer when the plugin is loaded, the first play should start a few milliseconds after.
          self.start_time = datetime.now()

      def _days_hours_minutes_seconds(self, runtime):
          ''' internal helper method for this callback '''
          minutes = (runtime.seconds // 60) % 60
          r_seconds = runtime.seconds - (minutes * 60)
          return runtime.days, runtime.seconds // 3600, minutes, r_seconds

      # this is only event we care about for display, when the play shows its summary stats; the rest are ignored by the base class
      def v2_playbook_on_stats(self, stats):
          end_time = datetime.now()
          runtime = end_time - self.start_time

          # Shows the usage of a config option declared in the DOCUMENTATION variable. Ansible will have set it when it loads the plugin.
          # Also note the use of the display object to print to screen. This is available to all callbacks, and you should use this over printing yourself
          self._display.display(self._plugin_options['format_string'] % (self._days_hours_minutes_seconds(runtime)))

Note that the ``CALLBACK_VERSION`` and ``CALLBACK_NAME`` definitions are required for properly functioning plugins for Ansible version 2.0 and later. ``CALLBACK_TYPE`` is mostly needed to distinguish 'stdout' plugins from the rest, since you can only load one plugin that writes to stdout.

For example callback plugins, see the source code for the `callback plugins included with Ansible Core <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/callback>`_

.. _developing_connection_plugins:

Connection plugins
------------------

Connection plugins allow Ansible to connect to the target hosts so it can execute tasks on them. Ansible ships with many connection plugins, but only one can be used per host at a time. The most commonly used connection plugins are the ``paramiko`` SSH, native ssh (just called ``ssh``), and ``local`` connection types.  All of these can be used in playbooks and with ``/usr/bin/ansible`` to connect to remote machines.

Ansible version 2.1 introduced the ``smart`` connection plugin. The ``smart`` connection type allows Ansible to automatically select either the ``paramiko`` or ``openssh`` connection plugin based on system capabilities, or the ``ssh`` connection plugin if OpenSSH supports ControlPersist.

To create a new connection plugin (for example, to support SNMP, Message bus, or other transports), copy the format of one of the existing connection plugins and drop it into the ``connection_plugins`` directory.

For example connection plugins, see the source code for the `connection plugins included with Ansible Core <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/connection>`_.

.. _developing_filter_plugins:

Filter plugins
--------------

Filter plugins manipulate data. They are a feature of Jinja2 and are also available in Jinja2 templates used by the ``template`` module. As with all plugins, they can be easily extended, but instead of having a file for each one you can have several per file. Most of the filter plugins shipped with Ansible reside in a ``core.py``.

Filter plugins do not use the standard configuration and documentation system described above.

For example filter plugins, see the source code for the `filter plugins included with Ansible Core <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/filter>`_.

.. _developing_inventory_plugins:

Inventory plugins
-----------------

Inventory plugins parse inventory sources and form an in-memory representation of the inventory. Inventory plugins were added in Ansible version 2.4.

You can see the details for inventory plugins in the :ref:`developing_inventory` page.

.. _developing_lookup_plugins:

Lookup plugins
--------------

Lookup plugins pull in data from external data stores. Lookup plugins can be used within playbooks both for looping --- playbook language constructs like ``with_fileglob`` and ``with_items`` are implemented via lookup plugins --- and to return values into a variable or parameter.

Lookup plugins are very flexible, allowing you to retrieve and return any type of data. When writing lookup plugins, always return data of a consistent type that can be easily consumed in a playbook. Avoid parameters that change the returned data type. If there is a need to return a single value sometimes and a complex dictionary other times, write two different lookup plugins.

Ansible includes many :ref:`filters <playbooks_filters>` which can be used to manipulate the data returned by a lookup plugin. Sometimes it makes sense to do the filtering inside the lookup plugin, other times it is better to return results that can be filtered in the playbook. Keep in mind how the data will be referenced when determing the appropriate level of filtering to be done inside the lookup plugin.

Here's a simple lookup plugin implementation --- this lookup returns the contents of a text file as a variable:

.. code-block:: python

  # python 3 headers, required if submitting to Ansible
  from __future__ import (absolute_import, division, print_function)
  __metaclass__ = type

  DOCUMENTATION = """
        lookup: file
          author: Daniel Hokka Zakrisson <daniel@hozac.com>
          version_added: "0.9"
          short_description: read file contents
          description:
              - This lookup returns the contents from a file on the Ansible controller's file system.
          options:
            _terms:
              description: path(s) of files to read
              required: True
          notes:
            - if read in variable context, the file can be interpreted as YAML if the content is valid to the parser.
            - this lookup does not understand globing --- use the fileglob lookup instead.
  """
  from ansible.errors import AnsibleError, AnsibleParserError
  from ansible.plugins.lookup import LookupBase

  try:
      from __main__ import display
  except ImportError:
      from ansible.utils.display import Display
      display = Display()


  class LookupModule(LookupBase):

      def run(self, terms, variables=None, **kwargs):


          # lookups in general are expected to both take a list as input and output a list
          # this is done so they work with the looping construct 'with_'.
          ret = []
          for term in terms:
              display.debug("File lookup term: %s" % term)

              # Find the file in the expected search path, using a class method
              # that implements the 'expected' search path for Ansible plugins.
              lookupfile = self.find_file_in_search_path(variables, 'files', term)

              # Don't use print or your own logging, the display class
              # takes care of it in a unified way.
              display.vvvv(u"File lookup using %s as file" % lookupfile)
              try:
                  if lookupfile:
                      contents, show_data = self._loader._get_file_contents(lookupfile)
                      ret.append(contents.rstrip())
                  else:
                      # Always use ansible error classes to throw 'final' exceptions,
                      # so the Ansible engine will know how to deal with them.
                      # The Parser error indicates invalid options passed
                      raise AnsibleParserError()
              except AnsibleParserError:
                  raise AnsibleError("could not locate file in lookup: %s" % term)

          return ret

The following is an example of how this lookup is called::

  ---
  - hosts: all
    vars:
       contents: "{{ lookup('file', '/etc/foo.txt') }}"

    tasks:

       - debug:
           msg: the value of foo.txt is {{ contents }} as seen today {{ lookup('pipe', 'date +"%Y-%m-%d"') }}

For example lookup plugins, see the source code for the `lookup plugins included with Ansible Core <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/lookup>`_.

For more usage examples of lookup plugins, see :ref:`Using Lookups<playbooks_lookups>`.

.. _developing_test_plugins:

Test plugins
------------

Test plugins verify data. They are a feature of Jinja2 and are also available in Jinja2 templates used by the ``template`` module. As with all plugins, they can be easily extended, but instead of having a file for each one you can have several per file. Most of the test plugins shipped with Ansible reside in a ``core.py``. These are specially useful in conjunction with some filter plugins like ``map`` and ``select``; they are also available for conditional directives like ``when:``.

Test plugins do not use the standard configuration and documentation system described above.

For example test plugins, see the source code for the `test plugins included with Ansible Core <https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/test>`_.

.. _developing_vars_plugins:

Vars plugins
------------

Vars plugins inject additional variable data into Ansible runs that did not come from an inventory source, playbook, or command line. Playbook constructs like 'host_vars' and 'group_vars' work using vars plugins.

Vars plugins were partially implemented in Ansible 2.0 and rewritten to be fully implemented starting with Ansible 2.4.

Older plugins used a ``run`` method as their main body/work:

.. code-block:: python

    def run(self, name, vault_password=None):
        pass # your code goes here


Ansible 2.0 did not pass passwords to older plugins, so vaults were unavailable.
Most of the work now  happens in the ``get_vars`` method which is called from the VariableManager when needed.

.. code-block:: python

    def get_vars(self, loader, path, entities):
        pass # your code goes here

The parameters are:

 * loader: Ansible's DataLoader. The DataLoader can read files, auto-load JSON/YAML and decrypt vaulted data, and cache read files.
 * path: this is 'directory data' for every inventory source and the current play's playbook directory, so they can search for data in reference to them. ``get_vars`` will be called at least once per available path.
 * entities: these are host or group names that are pertinent to the variables needed. The plugin will get called once for hosts and again for groups.

This ``get vars`` method just needs to return a dictionary structure with the variables.

Since Ansible version 2.4, vars plugins only execute as needed when preparing to execute a task. This avoids the costly 'always execute' behavior that occurred during inventory construction in older versions of Ansible.

For example vars plugins, see the source code for the `vars plugins included with Ansible Core
<https://github.com/ansible/ansible/tree/devel/lib/ansible/plugins/vars>`_.

.. seealso::

   :ref:`all_modules`
       List of all modules
   :doc:`developing_api`
       Learn about the Python API for task execution
   :doc:`developing_inventory`
       Learn about how to develop dynamic inventory sources
   :doc:`developing_modules`
       Learn about how to write Ansible modules
   `Mailing List <https://groups.google.com/group/ansible-devel>`_
       The development mailing list
   `irc.freenode.net <http://irc.freenode.net>`_
       #ansible IRC chat channel