~bzr/bzr-alldocs/trunk

Developing a plugin

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

Naming

------

By convention, most plugins are named bzr-xxx and are installed into a

directory called xxx. Note that the directory name must be a legal

Python package name, so a plugin called bzr-xxx-yyy need to be installed

into a directory called xxx_yyy, i.e. '-' in a plugin name gets mapped to

an underscore in the directory name.

Licensing

---------

We encourage plugin authors to make their plugins publicly available

under the same license as Bazaar itself, namely GPL v2. However, there

is no requirement to do so. You are free to create Bazaar plugins for

private or internal use within your company and not distribute them.

By sharing your work, a larger number of people benefit. In our experience,

plugin developers also benefit heavily by having more users involved in

the design, testing, bug fixing and longer term maintenance. In other words,

sharing leads to a better plugin faster.

Registration

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

If you decide to make your plugin available to others, you are welcome to

have it hosted on Launchpad and added to our plugins registry.

Non-experimental plugins that you or others in the community can support

may also be added to the Plugins Guide, i.e. inclusion in the Guide implies

a certain degree of *this works as advertised*.

To learn about Launchpad project registration, see

https://help.launchpad.net/Projects/Registering.

To add your plugin to the registry:

1. Fetch a copy of the lp:bzr-alldocs project.

2. Edit plugin-registry.ini and commit the change.

3. Submit a merge proposal.

A Bazaar developer will review your change and make some basic checks, e.g.

the one line summary (``purpose``) is clear and the branch URL is correct.

Once merged, your plugin will be registered and should appear shortly

afterwards in the generated web page.

Information on adding your plugin to this guide is provided later in this

document.

Testing

-------

To ensure your plugin under development is available to Bazaar, set

the ``BZR_PLUGIN_PATH`` environment variable to its parent directory.

Alternatively, you may wish to develop your plugin within a directory

under your personal plugins area (``~/.bazaar/plugins`` on GNU/Linux)

or put a symbolic link in that area pointing to your plugin under

test.  Finally you can use ``BZR_PLUGINS_AT`` to point to a specific

directory for a specific plugin (separated by your platform's value of

os.pathsep), e.g.

  export BZR_PLUGINS_AT=qbzr@/home/me/qbzr:explorer@/home/me/explorer

You can disable loading plugins with ``BZR_DISABLE_PLUGINS``.

If you want to stop loading all but installed plugins you can use:

  BZR_PLUGIN_PATH=-site

We also encourage plugin developers to provide tests for their plugin.

When you run ``bzr selftest``, Bazaar will scan all its plugins to see if

they contain a function named ``test_suite()``.  For each plugin that does,

it calls the function and adds any resulting tests to the master test suite.

To run just the tests for plugin xxx, the command is::

  bzr selftest bp.xxx

Providing help

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

Plugins in this guide have their documentation automatically

generated from the online help provided for a plugin. Sections are

ordered as follows:

1. High level introduction

2. Plugin-specific help topics (COMING SOON)

3. Commands defined or extended by the plugin.

High level help is specified in the docstring for the ``__init__.py`` module.

You can register plugin-specific help topics in ``__init__.py`` like this::

  _xxx_tutorial = """XXX Tutorial

  Welcome to xxx, your new best friend. ...

  """

  topic_registry.register('xxx-tutorial',

      _xxx_tutorial,

      'How to use xxx')

Command level help is specified in the docstring for the relevant 

``cmd_xxx`` Command class.

.. note::

   The final documentation needs to be in ReST format. Keep in mind though

   that the documentation should also be readable via ``bzr help xxx`` so

   it's best to keep markup to a reasonable minimum.

Providing custom code via hooks

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

Hooks let you provide custom code at certain processing points.

The available hook point are documented in the `User Reference

<http://doc.bazaar-vcs.org/development/en/user-reference/index.html#hooks>`_.

Adding a new hook is done with, for example::

  import bzrlib.branch

  bzrlib.branch.Branch.hooks.install_named_hook('post_push', post_push_hook,

                                 'My post_push hook')

For more information on how to write hooks,

see http://doc.bazaar-vcs.org/development/en/user-guide/hooks.html.

Defining a new command

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

Bazaar commands are defined as subclasses of ``bzrlib.commands.Command``, the

command name is specified by the name of the subclass, and they must be

registered into ``bzr`` with the ``bzrlib.commands.register_command`` function

at module import time.

To define the ``bzr foo-bar`` command::

  from bzrlib.commands import Command, register_command

  class cmd_foo_bar(Command):

    # see bzrlib/builtins.py for information about what to put here

    pass

  register_command(cmd_foo_bar)

If the class name starts with ``cmd_``, the prefix will get dropped

and ``_`` will be replaced by ``-`` characters.

Extending an existing command

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

TO BE DOCUMENTED.

GUI integration

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

TO BE DOCUMENTED. Once supported, explain how to:

* Register a toolset in the toolbox, providing easy access to new commands.

* Define custom widgets for command options in qrun.

Managing data

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

Plugin data falls into several categories:

* Configuration settings.

* Data the user can see and version control.

* Data behind the scenes.

Configuration settings are often stored in ``branch.conf``,

``locations.conf`` or ``bazaar.conf``.

User-visible data for a plugin called xxx should be stored in

``.bzrmeta/xxx``. If mutiple files are desirable, make ``.bzrmeta/xxx``

a directory or give them a common prefix within ``.bzrmeta``, e.g.

``xxx-foo``, ``xxx-bar``.

Data managed behind the scenes should be stored in ``.bzr``.

Depending on the nature of the data, it may belong in a subdirectory

within there, e.g. ``checkout``, ``branch`` or ``repository``.

It's your responsibility to ensure behind-the-scenes data is

propagated and merged appropriately via custom code. You may want

to use existing hooks for this or ask for new hooks to help. The

`Branch Baggage <http://wiki.bazaar.canonical.com/DraftSpecs/BranchBaggage>`_

feature may assist as well once implemented.

Useful metadata

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

It is highly recommended that plugins define a version number. This

is displayed by ``bzr plugins`` and by the ``qplugins`` GUI dialog.

To do this, define ``version_info`` in ``__init__.py`` like this::

  version_info = (1, 2, 0, 'beta', 1)

Plugins can also declare other useful metadata such as a mimimum

bzrlib version, new transports and storage formats. See

`Plugin API <http://doc.bazaar-vcs.org/bzr.dev/developers/plugin-api.html>`_

for details.

Performance tips

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

When bzr starts up, it imports every plugin, so plugins can degrade

performance when they're not being used. However, sub-modules are not

loaded, only the main name.

One way you can avoid this slowdown is by putting most of your code

in sub-modules, so that the plugin, itself, is small. All you really

need in the ``__init__.py`` is the plugin's Command classes, the

commands to register them, and the optional ``test_suite()``.

Another way to reduce your plugin's overhead is to use the bzrlib

lazy_import functionality. That looks something like this::

  from bzrlib.lazy_import import lazy_import

  lazy_import(globals(), """

  from bzrlib import (

      branch as _mod_branch,

      option,

      workingtree,

      )

  """)

Lazy importing only works for packages and modules, not classes or

functions. It defers the import until you actually need it.

Adding your plugin to the Plugins Guide

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

To add your (registered) plugin to the Plugins Guide, submit a merge

proposal to lp:bzr-alldocs. Here are the steps to follow:

1. Use ``scripts/document-plugin.py`` to generate ``plugins/en/xxx-plugin.txt``.

   To do this, run the above script in either the top level directory

   (where ``plugins-registry.ini`` is) or in the ``plugins/en`` directory,

   passing the plugin name as the argument.

2. Add *xxx-plugin* to the relevant category in ``plugins/en/index.txt``.

3. Test your changes::

     cd plugins/en

     make html

     Open _build/html/index.html in your web browser.

     Check your plugin appears as expected and it's help formatted correctly.

4. Add the new file and commit the changes.

5. Submit a merge proposal with your changes.

Learning more

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

`Integrating with Bazaar <http://wiki.bazaar.canonical.com/Integrating_with_Bazaar>`_

explains how to do such operations as ``add``, ``commit``, ``log`` and more.

Reference documentation on some key APIs is provided below. For a more

detailed reference, see the `complete bzrlib API documentation

<http://starship.python.net/crew/mwh/bzrlibapi/>`_.

Mini API Reference

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

Command Class

~~~~~~~~~~~~~

Base class for commands. Commands are the heart of the command-line bzr

interface.

The command object mostly handles the mapping of command-line parameters into

one or more bzrlib operations, and of the results into textual output.

Commands normally don't have any state. All their arguments are passed in to

the run method. (Subclasses may take a different policy if the behaviour of the

instance needs to depend on e.g. a shell plugin and not just its Python class.)

The docstring for an actual command should give a single-line summary, then a

complete description of the command. A grammar description will be inserted.

aliases

    Other accepted names for this command.

takes_args

    List of argument forms, marked with whether they are optional,

    repeated, etc.

    For example: ``['to_location', 'from_branch?', 'file*']`` means:

    * 'to_location' is required

    * 'from_branch' is optional

    * 'file' can be specified 0 or more times

takes_options

    List of options that may be given for this command. These can be either

    strings, referring to globally-defined options, or option objects.

    Retrieve through options().

hidden

    If true, this command isn't advertised. This is typically for commands

    intended for expert users.

run()

    Actually run the command. This is invoked with the options and arguments

    bound to keyword parameters.

    Return 0 or None if the command was successful, or a non-zero shell error

    code if not. It's OK for this method to allow an exception to raise up.

register_command Function

~~~~~~~~~~~~~~~~~~~~~~~~~

Utility function to help register a command.

param *cmd*

  Command subclass to register

param *decorate*

  If true, allow overriding an existing command of the same name; the old

  command is returned by this function. Otherwise it is an error to try to

  override an existing command.