/brz/remove-bazaar

==========

Plugin API

==========

:Date: 2009-01-23

.. contents::

Introduction

============

breezy has a very flexible internal structure allowing plugins for many

operations. Plugins can add commands, new storage formats, diff and merge

features and more. This document provides an overview of the API and

conventions for plugin authors.

If you're writing a plugin and have questions not addressed by this

document, please ask us.

See also

--------

 * `Breezy Developer Documentation Catalog <../index.html>`_.

 * `Breezy Plugins Guide <http://doc.bazaar.canonical.com/plugins/en/plugin-development.html>`_ for

   more suggestions about particular APIs.

Structure of a plugin

=====================

Plugins are Python modules under ``breezy.plugins``. They can be installed

either into the PYTHONPATH in that location, or in ~/.bazaar/plugins.

Plugins should have a setup.py.

As for other Python modules, the name of the directory must match the

expected name of the plugin.

Plugin metadata before installation

===================================

Plugins can export a summary of what they provide, and what versions of breezy

they are compatible with. This allows tools to be written to work with plugins,

such as to generate a directory of plugins, or install them via a

symlink/checkout to ~/.bazaar/plugins.

This interface allows Breezy to interrogate a plugin without actually loading

it. This is useful because loading a plugin may have side effects such

as registering or overriding commands, or the plugin may raise an error,

if for example a prerequisite is not present.

Metadata protocol

-----------------

A plugin that supports the Breezy plugin metadata protocol will do two

things. Firstly, the ``setup.py`` for the plugin will guard the call to

``setup()``::

  if __name__ == 'main':

      setup(...)

Secondly, the setup module will have one or more of the following variables

present at module scope. Any variables that are missing will be given the

defaults from the table. An example of every variable is provided after

the full list.

+------------------------+---------+----------------------------------------+

| Variable               | Default | Definition                             |

+========================+=========+========================================+

| brz_plugin_name        | None    | The name the plugin package should be  |

|                        |         | given on disk. The plugin is then      |

|                        |         | available to python at                 |

|                        |         | breezy.plugins.NAME                    |

+------------------------+---------+----------------------------------------+

| brz_commands           | []      | A list of the commands that the plugin |

|                        |         | provides. Commands that already exist  |

|                        |         | in brz and are decorated by the plugin |

|                        |         | do not need to be listed (but it is not|

|                        |         | harmful if you do list them).          |

+------------------------+---------+----------------------------------------+

| brz_plugin_version     | None    | A version_info 5-tuple with the plugins|

|                        |         | version.                               |

+------------------------+---------+----------------------------------------+

| brz_minimum_version    | None    | A version_info 3-tuple for comparison  |

|                        |         | with the breezy minimum and current    |

|                        |         | version, for determining likely        |

|                        |         | compatibility.                         |

+------------------------+---------+----------------------------------------+

| brz_maximum_version    | None    | A version_info 3-tuple like            |

|                        |         | brz_minimum_version but checking the   |

|                        |         | upper limits supported.                |

+------------------------+---------+----------------------------------------+

| brz_control_formats    | {}      | A dictionary of descriptions of version|

|                        |         | control directories. See               |

|                        |         | `Control Formats` below.               |

+------------------------+---------+----------------------------------------+

| brz_checkout_formats   | {}      | A dictionary of tree_format_string ->  |

|                        |         | human description strings, for tree    |

|                        |         | formats that drop into the             |

|                        |         | ``.bzr/checkout`` metadir system.      |

+------------------------+---------+----------------------------------------+

| brz_branch_formats     | {}      | As brz_checkout_formats but for        |

|                        |         | branches.                              |

+------------------------+---------+----------------------------------------+

| brz_repository_formats | {}      | As brz_checkout_formats but for        |

|                        |         | repositories.                          |

+------------------------+---------+----------------------------------------+

| brz_transports         | []      | URL prefixes for which this plugin     |

|                        |         | will register transports.              |

+------------------------+---------+----------------------------------------+

Control Formats

---------------

Because disk format detection for formats that bzr does not understand at

all can be useful, we allow a declarative description of the shape of a

control directory. Each description has a name for showing to users, and a

dictonary of relative paths, and the content needed at each path. Paths

that end in '/' are required to be directories and the value for that key

is ignored. Other paths are required to be regular files, and the value

for that key is either None, in which case the file is statted but the

content is ignored, or a literal string which is compared against for

the content of the file. Thus::

  # (look for a .hg directory)

  brz_control_formats = {"Mercurial":{'.hg/': None}}

  # (look for a file called .svn/format with contents 4\n).

  brz_control_formats = {"Subversion":{'.svn/format': '4\n'}}

Example

-------

An example setup.py follows::

  #!/usr/bin/env python2.4

  from distutils.core import setup

  brz_plugin_name = 'demo'

  brz_commands = [

      'new-command',

      ]

  brz_branch_formats = {

      "Branch label on disk\n":"demo branch",

      }

  brz_control_formats = {"Subversion":{'.svn/format': '4\n'}}

  brz_transports = ["hg+ssh://"]

  brz_plugin_version = (1, 3, 0, 'dev', 0)

  brz_minimum_version = (1, 0, 0)

  if __name__ == 'main':

      setup(name="Demo",

            version="1.3.0dev0",

            description="Demo plugin for plugin metadata.",

            author="Canonical Ltd",

            author_email="bazaar@lists.canonical.com",

            license = "GNU GPL v2",

            url="https://launchpad.net/bzr-demo",

            packages=['breezy.plugins.demo',

                      'breezy.plugins.demo.tests',

                      ],

            package_dir={'breezy.plugins.demo': '.'})

Plugin metadata after installation

==================================

After a plugin has been installed, metadata can be more easily obtained by

looking inside the module object -- in other words, for variables defined

in the plugin's ``__init__.py``.

Help and documentation

----------------------

The module docstring is used as the plugin description shown by ``bzr

plugins``.  As with all Python docstrings, the first line should be a

short complete sentence summarizing the plugin.  The full docstring is

shown by ``bzr help PLUGIN_NAME``.

This is a user-visible docstring so should be prefixed with ``__doc__ =``

to ensure help works under ``python -OO`` with docstrings stripped.

API version

-----------

Plugins can and should declare that they depend on a particular version of

breezy like so::

    from breezy.api import require_api

    require_api(breezy, (1, 11, 0))

Please see `API versioning <api-versioning.html>`_ for more details on the API

metadata protocol used by breezy.

Plugin version

--------------

The plugin should expose a version tuple to describe its own version.

Some plugins use a version number that corresponds to the version of bzr

they're released against, but you can use whatever you want.  For example::

    version_info = (1, 10, 0)

Detecting whether code's being loaded as a plugin

-------------------------------------------------

You may have a Python module that can be used as a bzr plugin and also in

other places.  To detect whether the module is being loaded by bzr, use

something like this::

    if __name__ == 'breezy.plugins.loggerhead':

        # register with breezy...

Plugin performance

==================

Plugins should avoid doing work or loading code from the plugin or

external libraries, if they're just installed but not actually active,

because this slows down every invocation of bzr.  The breezy APIs

generally allow the plugin to 'lazily' register methods to invoke if a

particular disk format or seen or a particular command is run.

Plugin registrations

====================

The plugin ``__init__.py`` runs when the plugin is loaded during bzr

startup.  Generally the plugin won't want to actually do anything at this

time other than register or override functions to be called later.

The plugin can import breezy and call any function.

Some interesting APIs are described in `Breezy Plugins Guide <http://doc.bazaar.canonical.com/plugins/en/plugin-development.html>`_.

Publishing your plugin

======================

When your plugin is basically working you might like to share it with

other people.  Here are some steps to consider:

 * make a project on Launchpad.net like

   <https://launchpad.net/bzr-fastimport>

   and publish the branches or tarballs there

 * include the plugin in <http://wiki.bazaar.canonical.com/BzrPlugins>

 * post about it to the ``bazaar-announce`` list at ``lists.canonical.com``

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