Ansible module development: getting started

A module is a reusable, standalone script that Ansible runs on your behalf, either locally or remotely. Modules interact with your local machine, an API, or a remote system to perform specific tasks like changing a database password or spinning up a cloud instance. Each module can be used by the Ansible API, or by the ansible or ansible-playbook programs. A module provides a defined interface, accepting arguments and returning information to Ansible by printing a JSON string to stdout before exiting. Ansible ships with thousands of modules, and you can easily write your own. If you’re writing a module for local use, you can choose any programming language and follow your own rules. This tutorial illustrates how to get started developing an Ansible module in Python.

Environment setup

Prerequisites via apt (Ubuntu)

Due to dependencies (for example ansible -> paramiko -> pynacl -> libffi):

sudo apt update
sudo apt install build-essential libssl-dev libffi-dev python-dev

Common environment setup

1. Clone the Ansible repository: $ git clone https://github.com/ansible/ansible.git

2. Change directory into the repository root dir: $ cd ansible

3. Create a virtual environment: $ python3 -m venv venv (or for Python 2 $ virtualenv venv. Note, this requires you to install the virtualenv package: $ pip install virtualenv)

4. Activate the virtual environment: $ . venv/bin/activate

5. Install development requirements: $ pip install -r requirements.txt

6. Run the environment setup script for each new dev shell process: $ . hacking/env-setup

Note

After the initial setup above, every time you are ready to start developing Ansible you should be able to just run the following from the root of the Ansible repo: $ . venv/bin/activate && . hacking/env-setup

Starting a new module

To create a new module:

1. Navigate to the correct directory for your new module: $ cd lib/ansible/modules/cloud/azure/

2. Create your new module file: $ touch my_test.py

3. Paste the content below into your new module file. It includes the required Ansible format and documentation and some example code.

4. Modify and extend the code to do what you want your new module to do. See the programming tips and Python 3 compatibility pages for pointers on writing clean, concise module code.

#!/usr/bin/python

# Copyright: (c) 2018, Terry Jones <[email protected]>
# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)

ANSIBLE_METADATA = {
    'metadata_version': '1.1',
    'status': ['preview'],
    'supported_by': 'community'
}

DOCUMENTATION = '''
---
module: my_test

short_description: This is my test module

version_added: "2.4"

description:
    - "This is my longer description explaining my test module"

options:
    name:
        description:
            - This is the message to send to the test module
        required: true
    new:
        description:
            - Control to demo if the result of this module is changed or not
        required: false

extends_documentation_fragment:
    - azure

author:
    - Your Name (@yourhandle)
'''

EXAMPLES = '''
# Pass in a message
- name: Test with a message
  my_test:
    name: hello world

# pass in a message and have changed true
- name: Test with a message and changed output
  my_test:
    name: hello world
    new: true

# fail the module
- name: Test failure of the module
  my_test:
    name: fail me
'''

RETURN = '''
original_message:
    description: The original name param that was passed in
    type: str
    returned: always
message:
    description: The output message that the test module generates
    type: str
    returned: always
'''

from ansible.module_utils.basic import AnsibleModule

def run_module():
    # define available arguments/parameters a user can pass to the module
    module_args = dict(
        name=dict(type='str', required=True),
        new=dict(type='bool', required=False, default=False)
    )

    # seed the result dict in the object
    # we primarily care about changed and state
    # change is if this module effectively modified the target
    # state will include any data that you want your module to pass back
    # for consumption, for example, in a subsequent task
    result = dict(
        changed=False,
        original_message='',
        message=''
    )

    # the AnsibleModule object will be our abstraction working with Ansible
    # this includes instantiation, a couple of common attr would be the
    # args/params passed to the execution, as well as if the module
    # supports check mode
    module = AnsibleModule(
        argument_spec=module_args,
        supports_check_mode=True
    )

    # if the user is working with this module in only check mode we do not
    # want to make any changes to the environment, just return the current
    # state with no modifications
    if module.check_mode:
        module.exit_json(**result)

    # manipulate or modify the state as needed (this is going to be the
    # part where your module will do what it needs to do)
    result['original_message'] = module.params['name']
    result['message'] = 'goodbye'

    # use whatever logic you need to determine whether or not this module
    # made any modifications to your target
    if module.params['new']:
        result['changed'] = True

    # during the execution of the module, if there is an exception or a
    # conditional state that effectively causes a failure, run
    # AnsibleModule.fail_json() to pass in the message and the result
    if module.params['name'] == 'fail me':
        module.fail_json(msg='You requested this to fail', **result)

    # in the event of a successful module execution, you will want to
    # simple AnsibleModule.exit_json(), passing the key/value results
    module.exit_json(**result)

def main():
    run_module()

if __name__ == '__main__':
    main()

Exercising your module code

Once you’ve modified the sample code above to do what you want, you can try out your module. Our debugging tips will help if you run into bugs as you exercise your module code.

Exercising module code locally

If your module does not need to target a remote host, you can quickly and easily exercise your code locally like this:

• Create an arguments file, a basic JSON config file that passes parameters to your module so you can run it. Name the arguments file /tmp/args.json and add the following content:

{
    "ANSIBLE_MODULE_ARGS": {
        "name": "hello",
        "new": true
    }
}

• If you are using a virtual environment (highly recommended for development) activate it: $ . venv/bin/activate

• Setup the environment for development: $ . hacking/env-setup

• Run your test module locally and directly: $ python -m ansible.modules.cloud.azure.my_test /tmp/args.json

This should return output like this:

{"changed": true, "state": {"original_message": "hello", "new_message": "goodbye"}, "invocation": {"module_args": {"name": "hello", "new": true}}}

Exercising module code in a playbook

The next step in testing your new module is to consume it with an Ansible playbook.

• Create a playbook in any directory: $ touch testmod.yml

• Add the following to the new playbook file:

  - name: test my new module
    hosts: localhost
    tasks:
    - name: run the new module
      my_test:
        name: 'hello'
        new: true
      register: testout
    - name: dump test output
      debug:
        msg: '{{ testout }}'
  

• Run the playbook and analyze the output: $ ansible-playbook ./testmod.yml

Testing basics

These two examples will get you started with testing your module code. Please review our testing section for more detailed information, including instructions for testing module documentation, adding integration tests, and more.

Sanity tests

You can run through Ansible’s sanity checks in a container:

$ ansible-test sanity -v --docker --python 2.7 MODULE_NAME

Note that this example requires Docker to be installed and running. If you’d rather not use a container for this, you can choose to use --tox instead of --docker.

Unit tests

You can add unit tests for your module in ./test/units/modules. You must first setup your testing environment. In this example, we’re using Python 3.5.

• Install the requirements (outside of your virtual environment): $ pip3 install -r ./test/lib/ansible_test/_data/requirements/units.txt

• To run all tests do the following: $ ansible-test units --python 3.5 (you must run . hacking/env-setup prior to this)

Note

Ansible uses pytest for unit testing.

To run pytest against a single test module, you can do the following (provide the path to the test module appropriately):

$ pytest -r a --cov=. --cov-report=html --fulltrace --color yes test/units/modules/.../test/my_test.py

Contributing back to Ansible

If you would like to contribute to the main Ansible repository by adding a new feature or fixing a bug, create a fork of the Ansible repository and develop against a new feature branch using the devel branch as a starting point. When you you have a good working code change, you can submit a pull request to the Ansible repository by selecting your feature branch as a source and the Ansible devel branch as a target.

If you want to contribute your module back to the upstream Ansible repo, review our submission checklist, programming tips, and strategy for maintaining Python 2 and Python 3 compatibility, as well as information about testing before you open a pull request. The Community Guide covers how to open a pull request and what happens next.

Communication and development support

Join the IRC channel #ansible-devel on irc.libera.chat for discussions surrounding Ansible development.

For questions and discussions pertaining to using the Ansible product, use the #ansible channel.

Credit

Thank you to Thomas Stringer (@trstringer) for contributing source material for this topic.