Battenberg

Battenberg is a tool built atop of Cookiecutter to keep Cookiecut projects in sync with their parent templates. Under the hood, Battenberg relies on Git to manage the merging, diffing, and conflict resolution story. The first goal of Battenberg is to provide an upgrade feature to Cookiecutter.

Installation

We publish battenberg to PyPI for easy consumption.

pip install battenberg

If you're on Mac OS X or Windows please follow the installation guides in the pygit2 documentation as well as battenberg relies on libgit2 which needs to be installed first. Please install libgit2 >= 1.0.

If you use SSH to connect to git, please also install libssh2 prior to installing libgit2!! Most like you can do this via brew install libssh2 if you are on Mac OS X.

Prerequistes

It is assumed that your cookiecutter template contains a .cookiecutter.json file at the root directory, or you can override its location by passing in --context-file. Please use the jsonify Jinja2 extension to dump the cookiecutter template context to .cookiecutter.json.

Tip: One problem battenberg has that as divergence between the cookiecutter template and the project itself increase as will the volume of conflicts needed to be manually resolved for each upgrade merge. To minimize these it is often advisable to fit templates with a generate_example boolean flag which will disable including any example code, instead replacing implementation with a pass for example.

Usage

Install a Cookiecutter template:

battenberg [-O <root path>] [--verbose] install [--checkout v1.0.0] [--initial-branch main] <cookiecutter template path/URL>

• --checkout - Specifies a target reference (branch, tag or commit) from the cookiecutter template repo, if not specified is it inferred from the default branch for the template repo.
• -O - Specifies an output folder path, defaults to the current directory.
• --initial-branch - The default branch for the newly created git repo, if not specified is it inferred from the default branch for the template repo.
• --verbose - Enables extra debug logging.

Upgrade your repository with last version of a template:

battenberg upgrade [--checkout v1.0.0] [--no-input] [--merge-target <branch, tag or commit>] [--context-file <context filename>]

• --checkout - Specifies a target reference (branch, tag or commit) from the cookiecutter template repo, if not specified is it inferred from the default branch for the template repo.

• --no-input - Read in the template context from --context-file instead of asking the cookiecutter template questions again.

• --merge-target - Specify where to merge the eventual template updates.

• --context-file - Specifies where to read in the template context from, defaults to .cookiecutter.json.

  Note: --merge-target is useful to set if you are a template owner but each cookiecut repo is owned independently. The value you pass to --merge-target should be the source branch for a PR that'd target main in the cookiecut repo so they can approve any changes.

Onboarding existing cookiecutter projects

A great feature of battenberg is that it's relatively easy to onboard existing projects you've already cookiecut from an existing template. To do this you need to follow the battenberg install instructions above but use the -O output to specify the directory of the existing project and it'll create you a new template branch and attempt to merge just like an upgrade operation.

Once you've completed your first merge from template -> main you can then follow the battenberg upgrade instructions as though it was generated using battenberg initially.

High-level design

At a high level battenberg attempts to provide a continuous history between the upstream template project and the cookiecut project. It does this by maintaining a disjoint template branch which battenberg attempts to keep in sync with the upstream template, it therefore will contain no project-specific changes beyond replacing the template values. Then changes to the template are incorporated into the main and other branches via a git merge --allow-unrelated-histories command for each template update pulled in. This merge commit should be used to resolve any conflicts between the upstream template and the specialized project.

This shows the repo structure immediately after running a battenberg install <template> command

This shows the repo structure immediately after running a battenberg upgrade command on the previously installed project

Development

To get set up run:

python3 -m venv env
source env/bin/activate

# Install in editable mode so you get the updates propagated.
pip install -e .

# If you want to be able to run tests & linting install via:
pip install -e ".[dev]"

Then to actually perform any operations just use the battenberg command which should now be on your $PATH.

To run tests:

pytest

To run linting:

flake8 --config flake8.cfg battenberg

Releasing a new version to PyPI

Reminder to update HISTORY.md with a summary of any updates, especially breaking changes.

We utilize GitHub Actions to deploy to PyPI. We've limited it to just publish on git tags. To release run:

# Change to the appropriate commit you want to base the release on.
vi battenberg/__init__.py  # Update the version string.
git tag <version>
git push origin <version>

Then watch the build for any errors, eventually it should appear on the battenberg PyPI project.

FAQ

• I got an error like _pygit2.GitError: unsupported URL protocol, how do I fix this?

  Likely you're using a git URL with ssh and have installed pygit2 without access to the underlying libssh2 library. To test this run:

  $ python -c "import pygit2; print(bool(pygit2.features & pygit2.GIT_FEATURE_SSH))"
  False

  To remedy this run:

  $ pip uninstall pygit2
  ...
  # Hopefully you have this, but this will install the compiler toolchain for OS X.
  $ xcode-select --install
  ...
  $ brew install libssh2
  ...
  $ brew install libgit2
  ...
  # The python wheels for Mac OS X for pygit2 are not built with SSH support by default so tell pip
  # to install pygit2 from source.
  $ pip install pygit2 --no-binary pygit2
  ...
  # Finally test out to ensure pygit2 picks up the SSH features.
  $ python -c "import pygit2; print(bool(pygit2.features & pygit2.GIT_FEATURE_SSH))"
  True

• Why are you using a new .cookiecutter.json pattern instead of using the replay pattern?

  Frankly the implementation was quite convoluted to get the intentions of these features to align. With the .cookiecutter.json approach we're intended for template state to live at the project level instead of at the user level which the replay functionality defaults to. Overriding that behaviour, whilst possible was convoluted in the current cookiecutter API and would require upstream changes so instead we decided against trying to align these features.

• Why battenberg?

  A tribute to the shoulders this project stands on, cookiecutter & milhoja, and a tasty cake in its own right.

Credits

Original code written by Raphael Medaer from an original idea of Abdó Roig-Maranges.

License

Free software: Apache Software License 2.0