Hacking on cloud-init

This document describes how to contribute changes to cloud-init. It assumes you have a GitHub account, and refers to your GitHub user as GH_USER throughout.

Do these things once

  • To contribute, you must sign the Canonical contributor license agreement

    • If you have already signed it as an individual, your Launchpad user will be listed in the contributor-agreement-canonical group. (Unfortunately there is no easy way to check if an organization or company you are doing work for has signed.)
    • When signing it:
      • ensure that you fill in the GitHub username field.
      • when prompted for ‘Project contact’ or ‘Canonical Project Manager’, enter ‘Josh Powers’.
    • If your company has signed the CLA for you, please contact us to help in verifying which Launchpad/GitHub accounts are associated with the company.
    • For any questions or help with the process, please email Josh Powers with the subject, “Cloud-Init CLA”
    • You also may contact user powersj in the #cloud-init channel on the Freenode IRC network.
  • Configure git with your email and name for commit messages.

    Your name will appear in commit messages and will also be used in changelogs or release notes. Give yourself credit!:

    git config user.name "Your Name"
    git config user.email "Your Email"
  • Sign into your GitHub account

  • Fork the upstream repository on Github and clicking on the Fork button

  • Create a new remote pointing to your personal GitHub repository.

    git clone git://github.com/canonical/cloud-init
    cd cloud-init
    git remote add GH_USER [email protected]:GH_USER/cloud-init.git
    git push GH_USER master
  • Read through the cloud-init Code Review Process, so you understand how your changes will end up in cloud-init’s codebase.

Transferring CLA Signatures from Launchpad to Github

For existing contributors who have signed the agreement in Launchpad before the Github username field was included, we need to verify the link between your Launchpad account and your GitHub account. To enable us to do this, we ask that you create a branch with both your Launchpad and GitHub usernames against both the Launchpad and GitHub cloud-init repositories. We’ve added a tool (tools/migrate-lp-user-to-github) to the cloud-init repository to handle this migration as automatically as possible.

The cloud-init team will review the two merge proposals and verify that the CLA has been signed for the Launchpad user and record the associated GitHub account.

Do these things for each feature or bug

  • Create a new topic branch for your work:

    git checkout -b my-topic-branch
  • Make and commit your changes (note, you can make multiple commits, fixes, more commits.):

    git commit
  • Run unit tests and lint/formatting checks with tox:

  • Push your changes to your personal GitHub repository:

    git push -u GH_USER my-topic-branch
  • Use your browser to create a merge request:

    • Open the branch on GitHub

      • You can see a web view of your repository and navigate to the branch at:


    • Click ‘Pull Request`

    • Fill out the pull request title, summarizing the change and a longer message indicating important details about the changes included, like

      Activate the frobnicator.
      The frobnicator was previously inactive and now runs by default.
      This may save the world some day.  Then, list the bugs you fixed
      as footers with syntax as shown here.
      The commit message should be one summary line of less than
      74 characters followed by a blank line, and then one or more
      paragraphs describing the change and why it was needed.
      This is the message that will be used on the commit when it
      is sqaushed and merged into trunk.
      LP: #1

      Note that the project continues to use LP: #NNNNN format for closing launchpad bugs rather than GitHub Issues.

    • Click ‘Create Pull Request`

Then, someone in the Ubuntu Server team will review your changes and follow up in the pull request. Look at the Code Review Process doc to understand the following steps.

Feel free to ping and/or join #cloud-init on freenode irc if you have any questions.


This section captures design decisions that are helpful to know when hacking on cloud-init.

Cloud Config Modules

  • Any new modules should use underscores in any new config options and not hyphens (e.g. new_option and not new-option).

Unit Testing

cloud-init uses pytest to run its tests, and has tests written both as unittest.TestCase sub-classes and as un-subclassed pytest tests. The following guidelines should be followed:

  • For ease of organisation and greater accessibility for developers not familiar with pytest, all cloud-init unit tests must be contained within test classes
    • Put another way, module-level test functions should not be used
  • pytest test classes should use pytest fixtures to share functionality instead of inheritance
  • As all tests are contained within classes, it is acceptable to mix TestCase test classes and pytest test classes within the same test file
  • pytest tests should use bare assert statements, to take advantage of pytest’s assertion introspection
    • For == and other commutative assertions, the expected value should be placed before the value under test: assert expected_value == function_under_test()
  • As we still support Ubuntu 16.04 (Xenial Xerus), we can only use pytest features that are available in v2.8.7. This is an inexhaustive list of ways in which this may catch you out:
    • Support for using yield in pytest.fixture functions was only introduced in pytest 3.0. Such functions must instead use the pytest.yield_fixture decorator.
    • Only the following built-in fixtures are available [1]:
      • cache
      • capsys
      • capfd
      • record_xml_property
      • monkeypatch
      • pytestconfig
      • recwarn
      • tmpdir_factory
      • tmpdir
  • Variables/parameter names for Mock or MagicMock instances should start with m_ to clearly distinguish them from non-mock variables
    • For example, m_readurl (which would be a mock for readurl)

Type Annotations

The cloud-init codebase uses Python’s annotation support for storing type annotations in the style specified by PEP-484. Their use in the codebase is encouraged but with one important caveat: types from the typing module cannot be used.

cloud-init still supports Python 3.4, which doesn’t have the typing module in the stdlib. This means that the use of any types from the typing module in the codebase would require installation of an additional Python module on platforms using Python 3.4. As such platforms are generally in maintenance mode, the introduction of a new dependency may act as a break in compatibility in practical terms.

Similarly, only function annotations are appropriate for use, as the variable annotations specified in PEP-526 were introduced in Python 3.6.


This list of fixtures (with markup) can be reproduced by running:

py.test-3 --fixtures -q | grep "^[^ ]" | grep -v no | sed 's/.*/* ``\0``/'

in a xenial lxd container with python3-pytest installed.