~bzr-pqm/bzr/bzr.dev

======================
Bazaar Developer Guide
======================

This document describes the Bazaar internals and the development process.
It's meant for people interested in developing Bazaar, and some parts will
also be useful to people developing Bazaar plugins.

If you have any questions or something seems to be incorrect, unclear or
missing, please talk to us in ``irc://irc.freenode.net/#bzr``, or write to
the Bazaar mailing list.  To propose a correction or addition to this
document, send a merge request or new text to the mailing list.

The latest developer documentation can be found online at
http://doc.bazaar-vcs.org/developers/.


Getting Started
###############

Exploring the Bazaar Platform
=============================

Before making changes, it's a good idea to explore the work already
done by others. Perhaps the new feature or improvement you're looking
for is available in another plug-in already? If you find a bug,
perhaps someone else has already fixed it?

To answer these questions and more, take a moment to explore the
overall Bazaar Platform. Here are some links to browse:

* The Plugins page on the Wiki - http://bazaar-vcs.org/BzrPlugins

* The Bazaar product family on Launchpad - https://launchpad.net/bazaar

* Bug Tracker for the core product - https://bugs.launchpad.net/bzr/

* Blueprint Tracker for the core product - https://blueprints.launchpad.net/bzr/

If nothing else, perhaps you'll find inspiration in how other developers
have solved their challenges.

Finding Something To Do
=======================

Ad-hoc performance work can also be done. One useful tool is the 'evil' debug
flag. For instance running ``bzr -Devil commit -m "test"`` will log a backtrace
to the bzr log file for every method call which triggers a slow or non-scalable
part of the bzr library. So checking that a given command with ``-Devil`` has
no backtraces logged to the log file is a good way to find problem function
calls that might be nested deep in the code base.

Planning and Discussing Changes
===============================

There is a very active community around Bazaar. Mostly we meet on IRC
(#bzr on irc.freenode.net) and on the mailing list. To join the Bazaar
community, see http://bazaar-vcs.org/BzrSupport.

If you are planning to make a change, it's a very good idea to mention it
on the IRC channel and/or on the mailing list. There are many advantages
to involving the community before you spend much time on a change.
These include:

* you get to build on the wisdom of others, saving time

* if others can direct you to similar code, it minimises the work to be done

* it assists everyone in coordinating direction, priorities and effort.

In summary, maximising the input from others typically minimises the
total effort required to get your changes merged. The community is
friendly, helpful and always keen to welcome newcomers.


Bazaar Development in a Nutshell
================================

.. was from bazaar-vcs.org/BzrGivingBack

One of the fun things about working on a version control system like Bazaar is
that the users have a high level of proficiency in contributing back into
the tool.  Consider the following very brief introduction to contributing back
to Bazaar.  More detailed instructions are in the following sections.

Making the change
-----------------

First, get a local copy of the development mainline (See `Why make a local
copy of bzr.dev?`_.)
::

 $ bzr init-repo ~/bzr
 $ cd ~/bzr
 $ bzr branch http://bazaar-vcs.org/bzr/bzr.dev/ bzr.dev

Now make your own branch::

 $ bzr branch bzr.dev 123456-my-bugfix

This will give you a branch called "123456-my-bugfix" that you can work on
and commit in. Here, you can study the code, make a fix or a new feature.
Feel free to commit early and often (after all, it's your branch!).

Documentation improvements are an easy place to get started giving back to the
Bazaar project.  The documentation is in the `doc/` subdirectory of the Bazaar
source tree.

When you are done, make sure that you commit your last set of changes as well!
Once you are happy with your changes, ask for them to be merged, as described
below.

Making a Merge Proposal
-----------------------

The Bazaar developers use Launchpad to further enable a truly distributed
style of development.  Anyone can propose a branch for merging into the Bazaar
trunk.  To start this process, you need to push your branch to Launchpad.  To
do this, you will need a Launchpad account and user name, e.g.
`your_lp_username`.  You can push your branch to Launchpad directly from
Bazaar::

  $ bzr push lp:~your_lp_username/bzr/giveback

After you have pushed your branch, you will need to propose it for merging to
the Bazaar trunk.  Go to <https://launchpad.net/your_lp_username/bzr/giveback>
and choose "Propose for merging into another branch".  Select "~bzr/bzr/trunk"
to hand your changes off to the Bazaar developers for review and merging.

Why make a local copy of bzr.dev?
---------------------------------

Making a local mirror of bzr.dev is not strictly necessary, but it means

- You can use that copy of bzr.dev as your main bzr executable, and keep it
  up-to-date using ``bzr pull``.
- Certain operations are faster, and can be done when offline.  For example:

  - ``bzr bundle``
  - ``bzr diff -r ancestor:...``
  - ``bzr merge``

- When it's time to create your next branch, it's more convenient.  When you
  have further contributions to make, you should do them in their own branch::

    $ cd ~/bzr
    $ bzr branch bzr.dev additional_fixes
    $ cd additional_fixes # hack, hack, hack



Understanding the Development Process
=====================================

The development team follows many practices including:

* a public roadmap and planning process in which anyone can participate

* time based milestones everyone can work towards and plan around

* extensive code review and feedback to contributors

* complete and rigorous test coverage on any code contributed

* automated validation that all tests still pass before code is merged
  into the main code branch.

The key tools we use to enable these practices are:

* Launchpad - https://launchpad.net/

* Bazaar - http://bazaar-vcs.org/

* Bundle Buggy - http://bundlebuggy.aaronbentley.com/

* Patch Queue Manager - https://launchpad.net/pqm/

For further information, see http://bazaar-vcs.org/BzrDevelopment.




Preparing a Sandbox for Making Changes to Bazaar
================================================

Bazaar supports many ways of organising your work. See
http://bazaar-vcs.org/SharedRepositoryLayouts for a summary of the
popular alternatives.

Of course, the best choice for you will depend on numerous factors:
the number of changes you may be making, the complexity of the changes, etc.
As a starting suggestion though:

* create a local copy of the main development branch (bzr.dev) by using
  this command::

    bzr branch http://bazaar-vcs.org/bzr/bzr.dev/ bzr.dev

* keep your copy of bzr.dev pristine (by not developing in it) and keep
  it up to date (by using bzr pull)

* create a new branch off your local bzr.dev copy for each issue
  (bug or feature) you are working on.

This approach makes it easy to go back and make any required changes
after a code review. Resubmitting the change is then simple with no
risk of accidentally including edits related to other issues you may
be working on. After the changes for an issue are accepted and merged,
the associated branch can be deleted or archived as you wish.


Navigating the Code Base
========================

.. Was at <http://bazaar-vcs.org/NewDeveloperIntroduction>

Some of the key files in this directory are:

bzr
    The command you run to start Bazaar itself.  This script is pretty
    short and just does some checks then jumps into bzrlib.

README
    This file covers a brief introduction to Bazaar and lists some of its
    key features.

NEWS
    Summary of changes in each Bazaar release that can affect users or
    plugin developers.

setup.py
    Installs Bazaar system-wide or to your home directory.  To perform
    development work on Bazaar it is not required to run this file - you
    can simply run the bzr command from the top level directory of your
    development copy. Note: That if you run setup.py this will create a
    'build' directory in your development branch. There's nothing wrong
    with this but don't be confused by it. The build process puts a copy
    of the main code base into this build directory, along with some other
    files. You don't need to go in here for anything discussed in this
    guide.

bzrlib
    Possibly the most exciting folder of all, bzrlib holds the main code
    base. This is where you will go to edit python files and contribute to
    Bazaar.

doc
    Holds documentation on a whole range of things on Bazaar from the
    origination of ideas within the project to information on Bazaar
    features and use cases.  Within this directory there is a subdirectory
    for each translation into a human language.  All the documentation
    is in the ReStructuredText markup language.

doc/developers
    Documentation specifically targeted at Bazaar and plugin developers.
    (Including this document.)



Automatically-generated API reference information is available at
<http://starship.python.net/crew/mwh/bzrlibapi/>.

See also the `Bazaar Architectural Overview
<http://doc.bazaar-vcs.org/developers/overview.html>`_.


The Code Review Process
#######################

All code changes coming in to Bazaar are reviewed by someone else.
Normally changes by core contributors are reviewed by one other core
developer, and changes from other people are reviewed by two core
developers.  Use intelligent discretion if the patch is trivial.

Good reviews do take time. They also regularly require a solid
understanding of the overall code base. In practice, this means a small
number of people often have a large review burden - with knowledge comes
responsibility. No one likes their merge requests sitting in a queue going
nowhere, so reviewing sooner rather than later is strongly encouraged.





Review cover letters
====================

Please put a "cover letter" on your merge request explaining:

* the reason **why** you're making this change

* **how** this change achieves this purpose

* anything else you may have fixed in passing

* anything significant that you thought of doing, such as a more
  extensive fix or a different approach, but didn't or couldn't do now

A good cover letter makes reviewers' lives easier because they can decide
from the letter whether they agree with the purpose and approach, and then
assess whether the patch actually does what the cover letter says.
Explaining any "drive-by fixes" or roads not taken may also avoid queries
from the reviewer.  All in all this should give faster and better reviews.
Sometimes writing the cover letter helps the submitter realize something
else they need to do.  The size of the cover letter should be proportional
to the size and complexity of the patch.


Reviewing proposed changes
==========================

Anyone is welcome to review code, and reply to the thread with their
opinion or comments.

The simplest way to review a proposed change is to just read the patch on
the list or in Bundle Buggy.  For more complex changes it may be useful
to make a new working tree or branch from trunk, and merge the proposed
change into it, so you can experiment with the code or look at a wider
context.

There are three main requirements for code to get in:

* Doesn't reduce test coverage: if it adds new methods or commands,
  there should be tests for them.  There is a good test framework
  and plenty of examples to crib from, but if you are having trouble
  working out how to test something feel free to post a draft patch
  and ask for help.

* Doesn't reduce design clarity, such as by entangling objects
  we're trying to separate.  This is mostly something the more
  experienced reviewers need to help check.

* Improves bugs, features, speed, or code simplicity.

Code that goes in should not degrade any of these aspects.  Patches are
welcome that only cleanup the code without changing the external
behaviour.  The core developers take care to keep the code quality high
and understandable while recognising that perfect is sometimes the enemy
of good.

It is easy for reviews to make people notice other things which should be
fixed but those things should not hold up the original fix being accepted.
New things can easily be recorded in the Bug Tracker instead.

It's normally much easier to review several smaller patches than one large
one.  You might want to use ``bzr-loom`` to maintain threads of related
work, or submit a preparatory patch that will make your "real" change
easier.


Checklist for reviewers
=======================

* Do you understand what the code's doing and why?

* Will it perform reasonably for large inputs, both in memory size and
  run time?  Are there some scenarios where performance should be
  measured?

* Is it tested, and are the tests at the right level?  Are there both
  blackbox (command-line level) and API-oriented tests?

* If this change will be visible to end users or API users, is it
  appropriately documented in NEWS?

* Does it meet the coding standards below?

* If it changes the user-visible behaviour, does it update the help
  strings and user documentation?

* If it adds a new major concept or standard practice, does it update the
  developer documentation?

* (your ideas here...)


Reviews on Launchpad
====================

From May 2009 on, we prefer people to propose code reviews through
Launchpad.

 * <https://launchpad.net/+tour/code-review>

 * <https://help.launchpad.net/Code/Review>

Anyone can propose or comment on a merge proposal just by creating a
Launchpad account.

There are two ways to create a new merge proposal: through the web
interface or by email.


Proposing a merge through the web
---------------------------------

To create the proposal through the web, first push your branch to Launchpad.
For example, a branch dealing with documentation belonging to the Launchpad
User mbp could be pushed as ::

  bzr push lp:~mbp/bzr/doc

Then go to the branch's web page, which in this case would be
<https://code.launchpad.net/~mbp/bzr/doc>.  You can simplify this step by just
running ::

  bzr lp-open

You can then click "Propose for merging into another branch", and enter your
cover letter (see above) into the web form.  Typically you'll want to merge
into ``~bzr/bzr/trunk`` which will be the default; you might also want to
nominate merging into a release branch for a bug fix.  There is the option to
specify a specific reviewer or type of review, and you shouldn't normally
change those.

Submitting the form takes you to the new page about the merge proposal
containing the diff of the changes, comments by interested people, and
controls to comment or vote on the change.

Proposing a merge by mail
-------------------------

To propose a merge by mail, send a bundle to ``merge@code.launchpad.net``.

You can generate a merge request like this::

  bzr send -o bug-1234.diff

``bzr send`` can also send mail directly if you prefer; see the help.

Reviewing changes
-----------------

From <https://code.launchpad.net/bzr/+activereviews> you can see all
currently active reviews, and choose one to comment on.  This page also
shows proposals that are now approved and should be merged by someone with
PQM access.


Reviews through Bundle Buggy
============================

The Bundle Buggy tool used up to May 2009 is still available as a review
mechanism.

Sending patches for review
--------------------------

If you'd like to propose a change, please post to the
bazaar@lists.canonical.com list with a bundle, patch, or link to a
branch. Put ``[PATCH]`` or ``[MERGE]`` in the subject so Bundle Buggy
can pick it out, and explain the change in the email message text.
Remember to update the NEWS file as part of your change if it makes any
changes visible to users or plugin developers. Please include a diff
against mainline if you're giving a link to a branch.

You can generate a merge request like this::

  bzr send -o bug-1234.patch

A ``.patch`` extension is recommended instead of .bundle as many mail clients
will send the latter as a binary file.

``bzr send`` can also send mail directly if you prefer; see the help.

Please do **NOT** put [PATCH] or [MERGE] in the subject line if you don't
want it to be merged. If you want comments from developers rather than
to be merged, you can put ``[RFC]`` in the subject line.

If this change addresses a bug, please put the bug number in the subject
line too, in the form ``[#1]`` so that Bundle Buggy can recognize it.

If the change is intended for a particular release mark that in the
subject too, e.g. ``[1.6]``.
Anyone can "vote" on the mailing list by expressing an opinion. Core
developers can also vote using Bundle Buggy. Here are the voting codes and
their explanations.

:approve:  Reviewer wants this submission merged.
:tweak:    Reviewer wants this submission merged with small changes. (No
  re-review required.)
:abstain:  Reviewer does not intend to vote on this patch.
:resubmit: Please make changes and resubmit for review.
:reject:   Reviewer doesn't want this kind of change merged.
:comment:  Not really a vote. Reviewer just wants to comment, for now.

If a change gets two approvals from core reviewers, and no rejections,
then it's OK to come in.  Any of the core developers can bring it into the
bzr.dev trunk and backport it to maintenance branches if required.  The
Release Manager will merge the change into the branch for a pending
release, if any. As a guideline, core developers usually merge their own
changes and volunteer to merge other contributions if they were the second
reviewer to agree to a change.

To track the progress of proposed changes, use Bundle Buggy. See
http://bundlebuggy.aaronbentley.com/help for a link to all the
outstanding merge requests together with an explanation of the columns.
Bundle Buggy will also mail you a link to track just your change.

Coding Style Guidelines
#######################

hasattr and getattr
===================

``hasattr`` should not be used because it swallows exceptions including
``KeyboardInterrupt``.  Instead, say something like ::

  if getattr(thing, 'name', None) is None


Code layout
===========

Please write PEP-8__ compliant code.

__ http://www.python.org/peps/pep-0008.html

One often-missed requirement is that the first line of docstrings
should be a self-contained one-sentence summary.

We use 4 space indents for blocks, and never use tab characters.  (In vim,
``set expandtab``.)

Trailing white space should be avoided, but is allowed.
You should however not make lots of unrelated white space changes.

Unix style newlines (LF) are used.

Each file must have a newline at the end of it.

Lines should be no more than 79 characters if at all possible.
Lines that continue a long statement may be indented in either of
two ways:

within the parenthesis or other character that opens the block, e.g.::

    my_long_method(arg1,
                   arg2,
                   arg3)

or indented by four spaces::

    my_long_method(arg1,
        arg2,
        arg3)

The first is considered clearer by some people; however it can be a bit
harder to maintain (e.g. when the method name changes), and it does not
work well if the relevant parenthesis is already far to the right.  Avoid
this::

     self.legbone.kneebone.shinbone.toebone.shake_it(one,
                                                     two,
                                                     three)

but rather ::

     self.legbone.kneebone.shinbone.toebone.shake_it(one,
         two,
         three)

or ::

     self.legbone.kneebone.shinbone.toebone.shake_it(
         one, two, three)

For long lists, we like to add a trailing comma and put the closing
character on the following line.  This makes it easier to add new items in
future::

    from bzrlib.goo import (
        jam,
        jelly,
        marmalade,
        )

There should be spaces between function parameters, but not between the
keyword name and the value::

    call(1, 3, cheese=quark)

In emacs::

    ;(defface my-invalid-face
    ;  '((t (:background "Red" :underline t)))
    ;  "Face used to highlight invalid constructs or other uglyties"
    ;  )

    (defun my-python-mode-hook ()
     ;; setup preferred indentation style.
     (setq fill-column 79)
     (setq indent-tabs-mode nil) ; no tabs, never, I will not repeat
    ;  (font-lock-add-keywords 'python-mode
    ;                         '(("^\\s *\t" . 'my-invalid-face) ; Leading tabs
    ;                            ("[ \t]+$" . 'my-invalid-face)  ; Trailing spaces
    ;                            ("^[ \t]+$" . 'my-invalid-face)); Spaces only
    ;                          )
     )

    (add-hook 'python-mode-hook 'my-python-mode-hook)

The lines beginning with ';' are comments. They can be activated
if one want to have a strong notice of some tab/space usage
violations.


Module Imports
==============

* Imports should be done at the top-level of the file, unless there is
  a strong reason to have them lazily loaded when a particular
  function runs.  Import statements have a cost, so try to make sure
  they don't run inside hot functions.

* Module names should always be given fully-qualified,
  i.e. ``bzrlib.hashcache`` not just ``hashcache``.


Naming
======

Functions, methods or members that are relatively private are given
a leading underscore prefix.  Names without a leading underscore are
public not just across modules but to programmers using bzrlib as an
API.

We prefer class names to be concatenated capital words (``TestCase``)
and variables, methods and functions to be lowercase words joined by
underscores (``revision_id``, ``get_revision``).

For the purposes of naming some names are treated as single compound
words: "filename", "revno".

Consider naming classes as nouns and functions/methods as verbs.

Try to avoid using abbreviations in names, because there can be
inconsistency if other people use the full name.


Standard Names
==============

``revision_id`` not ``rev_id`` or ``revid``

Functions that transform one thing to another should be named ``x_to_y``
(not ``x2y`` as occurs in some old code.)


Destructors
===========

Python destructors (``__del__``) work differently to those of other
languages.  In particular, bear in mind that destructors may be called
immediately when the object apparently becomes unreferenced, or at some
later time, or possibly never at all.  Therefore we have restrictions on
what can be done inside them.

 0. If you think you need to use a ``__del__`` method ask another
    developer for alternatives.  If you do need to use one, explain
    why in a comment.

 1. Never rely on a ``__del__`` method running.  If there is code that
    must run, do it from a ``finally`` block instead.

 2. Never ``import`` from inside a ``__del__`` method, or you may crash the
    interpreter!!

 3. In some places we raise a warning from the destructor if the object
    has not been cleaned up or closed.  This is considered OK: the warning
    may not catch every case but it's still useful sometimes.


Cleanup methods
===============

Often when something has failed later code, including cleanups invoked
from ``finally`` blocks, will fail too.  These secondary failures are
generally uninteresting compared to the original exception.  So use the
``only_raises`` decorator (from ``bzrlib.decorators``) for methods that
are typically called in ``finally`` blocks, such as ``unlock`` methods.
For example, ``@only_raises(LockNotHeld, LockBroken)``.  All errors that
are unlikely to be a knock-on failure from a previous failure should be
allowed.


Factories
=========

In some places we have variables which point to callables that construct
new instances.  That is to say, they can be used a lot like class objects,
but they shouldn't be *named* like classes:

> I think that things named FooBar should create instances of FooBar when
> called. Its plain confusing for them to do otherwise. When we have
> something that is going to be used as a class - that is, checked for via
> isinstance or other such idioms, them I would call it foo_class, so that
> it is clear that a callable is not sufficient. If it is only used as a
> factory, then yes, foo_factory is what I would use.


Registries
==========

Several places in Bazaar use (or will use) a registry, which is a
mapping from names to objects or classes.  The registry allows for
loading in registered code only when it's needed, and keeping
associated information such as a help string or description.


InterObject and multiple dispatch
=================================

The ``InterObject`` provides for two-way `multiple dispatch`__: matching
up for example a source and destination repository to find the right way
to transfer data between them.

.. __: http://en.wikipedia.org/wiki/Multiple_dispatch

There is a subclass ``InterObject`` classes for each type of object that is
dispatched this way, e.g. ``InterRepository``.  Calling ``.get()`` on this
class will return an ``InterObject`` instance providing the best match for
those parameters, and this instance then has methods for operations
between the objects.

  inter = InterRepository.get(source_repo, target_repo)
  inter.fetch(revision_id)

``InterRepository`` also acts as a registry-like object for its
subclasses, and they can be added through ``.register_optimizer``.  The
right one to run is selected by asking each class, in reverse order of
registration, whether it ``.is_compatible`` with the relevant objects.

Lazy Imports
============

To make startup time faster, we use the ``bzrlib.lazy_import`` module to
delay importing modules until they are actually used. ``lazy_import`` uses
the same syntax as regular python imports. So to import a few modules in a
lazy fashion do::

  from bzrlib.lazy_import import lazy_import
  lazy_import(globals(), """
  import os
  import subprocess
  import sys
  import time

  from bzrlib import (
     errors,
     transport,
     revision as _mod_revision,
     )
  import bzrlib.transport
  import bzrlib.xml5
  """)

At this point, all of these exist as a ``ImportReplacer`` object, ready to
be imported once a member is accessed. Also, when importing a module into
the local namespace, which is likely to clash with variable names, it is
recommended to prefix it as ``_mod_<module>``. This makes it clearer that
the variable is a module, and these object should be hidden anyway, since
they shouldn't be imported into other namespaces.

While it is possible for ``lazy_import()`` to import members of a module
when using the ``from module import member`` syntax, it is recommended to
only use that syntax to load sub modules ``from module import submodule``.
This is because variables and classes can frequently be used without
needing a sub-member for example::

  lazy_import(globals(), """
  from module import MyClass
  """)

  def test(x):
      return isinstance(x, MyClass)

This will incorrectly fail, because ``MyClass`` is a ``ImportReplacer``
object, rather than the real class.

It also is incorrect to assign ``ImportReplacer`` objects to other variables.
Because the replacer only knows about the original name, it is unable to
replace other variables. The ``ImportReplacer`` class will raise an
``IllegalUseOfScopeReplacer`` exception if it can figure out that this
happened. But it requires accessing a member more than once from the new
variable, so some bugs are not detected right away.


The Null revision
=================

The null revision is the ancestor of all revisions.  Its revno is 0, its
revision-id is ``null:``, and its tree is the empty tree.  When referring
to the null revision, please use ``bzrlib.revision.NULL_REVISION``.  Old
code sometimes uses ``None`` for the null revision, but this practice is
being phased out.


Object string representations
=============================

Python prints objects using their ``__repr__`` method when they are
written to logs, exception tracebacks, or the debugger.  We want
objects to have useful representations to help in determining what went
wrong.

If you add a new class you should generally add a ``__repr__`` method
unless there is an adequate method in a parent class.  There should be a
test for the repr.

Representations should typically look like Python constructor syntax, but
they don't need to include every value in the object and they don't need
to be able to actually execute.  They're to be read by humans, not
machines.  Don't hardcode the classname in the format, so that we get the
correct value if the method is inherited by a subclass.  If you're
printing attributes of the object, including strings, you should normally
use ``%r`` syntax (to call their repr in turn).

Try to avoid the representation becoming more than one or two lines long.
(But balance this against including useful information, and simplicity of
implementation.)

Because repr methods are often called when something has already gone
wrong, they should be written somewhat more defensively than most code.
The object may be half-initialized or in some other way in an illegal
state.  The repr method shouldn't raise an exception, or it may hide the
(probably more useful) underlying exception.

Example::

    def __repr__(self):
        return '%s(%r)' % (self.__class__.__name__,
                           self._transport)


Exception handling
==================

A bare ``except`` statement will catch all exceptions, including ones that
really should terminate the program such as ``MemoryError`` and
``KeyboardInterrupt``.  They should rarely be used unless the exception is
later re-raised.  Even then, think about whether catching just
``Exception`` (which excludes system errors in Python2.5 and later) would
be better.


Test coverage
=============

All code should be exercised by the test suite.  See the `Bazaar Testing
Guide <http://doc.bazaar-vcs.org/developers/testing.html>`_ for detailed
information about writing tests.


Core Topics
###########

Evolving Interfaces
===================

We don't change APIs in stable branches: any supported symbol in a stable
release of bzr must not be altered in any way that would result in
breaking existing code that uses it. That means that method names,
parameter ordering, parameter names, variable and attribute names etc must
not be changed without leaving a 'deprecated forwarder' behind. This even
applies to modules and classes.

If you wish to change the behaviour of a supported API in an incompatible
way, you need to change its name as well. For instance, if I add an optional keyword
parameter to branch.commit - that's fine. On the other hand, if I add a
keyword parameter to branch.commit which is a *required* transaction
object, I should rename the API - i.e. to 'branch.commit_transaction'.

  (Actually, that may break code that provides a new implementation of
  ``commit`` and doesn't expect to receive the parameter.)

When renaming such supported API's, be sure to leave a deprecated_method (or
_function or ...) behind which forwards to the new API. See the
bzrlib.symbol_versioning module for decorators that take care of the
details for you - such as updating the docstring, and issuing a warning
when the old API is used.

For unsupported API's, it does not hurt to follow this discipline, but it's
not required. Minimally though, please try to rename things so that
callers will at least get an AttributeError rather than weird results.


Deprecation decorators
----------------------

``bzrlib.symbol_versioning`` provides decorators that can be attached to
methods, functions, and other interfaces to indicate that they should no
longer be used.  For example::

   @deprecated_method(deprecated_in((0, 1, 4)))
   def foo(self):
        return self._new_foo()

To deprecate a static method you must call ``deprecated_function``
(**not** method), after the staticmethod call::

    @staticmethod
    @deprecated_function(deprecated_in((0, 1, 4)))
    def create_repository(base, shared=False, format=None):

When you deprecate an API, you should not just delete its tests, because
then we might introduce bugs in them.  If the API is still present at all,
it should still work.  The basic approach is to use
``TestCase.applyDeprecated`` which in one step checks that the API gives
the expected deprecation message, and also returns the real result from
the method, so that tests can keep running.

Deprecation warnings will be suppressed for final releases, but not for
development versions or release candidates, or when running ``bzr
selftest``. This gives developers information about whether their code is
using deprecated functions, but avoids confusing users about things they
can't fix.


Getting Input
=============

Processing Command Lines
------------------------

bzrlib has a standard framework for parsing command lines and calling
processing routines associated with various commands. See builtins.py
for numerous examples.


Standard Parameter Types
------------------------

There are some common requirements in the library: some parameters need to be
unicode safe, some need byte strings, and so on. At the moment we have
only codified one specific pattern: Parameters that need to be unicode
should be checked via ``bzrlib.osutils.safe_unicode``. This will coerce the
input into unicode in a consistent fashion, allowing trivial strings to be
used for programmer convenience, but not performing unpredictably in the
presence of different locales.


Writing Output
==============

(The strategy described here is what we want to get to, but it's not
consistently followed in the code at the moment.)

bzrlib is intended to be a generically reusable library.  It shouldn't
write messages to stdout or stderr, because some programs that use it
might want to display that information through a GUI or some other
mechanism.

We can distinguish two types of output from the library:

 1. Structured data representing the progress or result of an
    operation.  For example, for a commit command this will be a list
    of the modified files and the finally committed revision number
    and id.

    These should be exposed either through the return code or by calls
    to a callback parameter.

    A special case of this is progress indicators for long-lived
    operations, where the caller should pass a ProgressBar object.

 2. Unstructured log/debug messages, mostly for the benefit of the
    developers or users trying to debug problems.  This should always
    be sent through ``bzrlib.trace`` and Python ``logging``, so that
    it can be redirected by the client.

The distinction between the two is a bit subjective, but in general if
there is any chance that a library would want to see something as
structured data, we should make it so.

The policy about how output is presented in the text-mode client
should be only in the command-line tool.


Progress and Activity Indications
---------------------------------

bzrlib has a way for code to display to the user that stuff is happening
during a long operation.  There are two particular types: *activity* which
means that IO is happening on a Transport, and *progress* which means that
higher-level application work is occurring.  Both are drawn together by
the `ui_factory`.

Transport objects are responsible for calling `report_transport_activity`
when they do IO.

Progress uses a model/view pattern: application code acts on a
`ProgressTask` object, which notifies the UI when it needs to be
displayed.  Progress tasks form a stack.  To create a new progress task on
top of the stack, call `bzrlib.ui.ui_factory.nested_progress_bar()`, then
call `update()` on the returned ProgressTask.  It can be updated with just
a text description, with a numeric count, or with a numeric count and
expected total count.  If an expected total count is provided the view
can show the progress moving along towards the expected total.

The user should call `finish` on the `ProgressTask` when the logical
operation has finished, so it can be removed from the stack.

Progress tasks have a complex relationship with generators: it's a very
good place to use them, but because python2.4 does not allow ``finally``
blocks in generators it's hard to clean them up properly.  In this case
it's probably better to have the code calling the generator allocate a
progress task for its use and then call `finalize` when it's done, which
will close it if it was not already closed.  The generator should also
finish the progress task when it exits, because it may otherwise be a long
time until the finally block runs.


Displaying help
===============

Bazaar has online help for various topics through ``bzr help COMMAND`` or
equivalently ``bzr command -h``.  We also have help on command options,
and on other help topics.  (See ``help_topics.py``.)

As for python docstrings, the first paragraph should be a single-sentence
synopsis of the command.

The help for options should be one or more proper sentences, starting with
a capital letter and finishing with a full stop (period).

All help messages and documentation should have two spaces between
sentences.


Handling Errors and Exceptions
==============================

Commands should return non-zero when they encounter circumstances that
the user should really pay attention to - which includes trivial shell
pipelines.

Recommended values are:

    0. OK.
    1. Conflicts in merge-like operations, or changes are present in
       diff-like operations.
    2. Unrepresentable diff changes (i.e. binary files that we cannot show
       a diff of).
    3. An error or exception has occurred.
    4. An internal error occurred (one that shows a traceback.)

Errors are handled through Python exceptions. Exceptions should be defined
inside bzrlib.errors, so that we can see the whole tree at a glance.

We broadly classify errors as either being either internal or not,
depending on whether ``internal_error`` is set or not.  If we think it's our
fault, we show a backtrace, an invitation to report the bug, and possibly
other details.  This is the default for errors that aren't specifically
recognized as being caused by a user error.  Otherwise we show a briefer
message, unless -Derror was given.

Many errors originate as "environmental errors" which are raised by Python
or builtin libraries -- for example IOError.  These are treated as being
our fault, unless they're caught in a particular tight scope where we know
that they indicate a user errors.  For example if the repository format
is not found, the user probably gave the wrong path or URL.  But if one of
the files inside the repository is not found, then it's our fault --
either there's a bug in bzr, or something complicated has gone wrong in
the environment that means one internal file was deleted.

Many errors are defined in ``bzrlib/errors.py`` but it's OK for new errors
to be added near the place where they are used.

Exceptions are formatted for the user by conversion to a string
(eventually calling their ``__str__`` method.)  As a convenience the
``._fmt`` member can be used as a template which will be mapped to the
error's instance dict.

New exception classes should be defined when callers might want to catch
that exception specifically, or when it needs a substantially different
format string.

#. If it is something that a caller can recover from, a custom exception
   is reasonable.

#. If it is a data consistency issue, using a builtin like
   ``ValueError``/``TypeError`` is reasonable.

#. If it is a programmer error (using an api incorrectly)
   ``AssertionError`` is reasonable.

#. Otherwise, use ``BzrError`` or ``InternalBzrError``.

Exception strings should start with a capital letter and should not have a
final fullstop.  If long, they may contain newlines to break the text.


Assertions
==========

Do not use the Python ``assert`` statement, either in tests or elsewhere.
A source test checks that it is not used.  It is ok to explicitly raise
AssertionError.

Rationale:

 * It makes the behaviour vary depending on whether bzr is run with -O
   or not, therefore giving a chance for bugs that occur in one case or
   the other, several of which have already occurred: assertions with
   side effects, code which can't continue unless the assertion passes,
   cases where we should give the user a proper message rather than an
   assertion failure.
 * It's not that much shorter than an explicit if/raise.
 * It tends to lead to fuzzy thinking about whether the check is
   actually needed or not, and whether it's an internal error or not
 * It tends to cause look-before-you-leap patterns.
 * It's unsafe if the check is needed to protect the integrity of the
   user's data.
 * It tends to give poor messages since the developer can get by with
   no explanatory text at all.
 * We can't rely on people always running with -O in normal use, so we
   can't use it for tests that are actually expensive.
 * Expensive checks that help developers are better turned on from the
   test suite or a -D flag.
 * If used instead of ``self.assert*()`` in tests it makes them falsely pass with -O.


Documenting Changes
===================

When you change bzrlib, please update the relevant documentation for the
change you made: Changes to commands should update their help, and
possibly end user tutorials; changes to the core library should be
reflected in API documentation.

NEWS File
---------

If you make a user-visible change, please add a note to the NEWS file.
The description should be written to make sense to someone who's just
a user of bzr, not a developer: new functions or classes shouldn't be
mentioned, but new commands, changes in behaviour or fixed nontrivial
bugs should be listed.  See the existing entries for an idea of what
should be done.

Within each release, entries in the news file should have the most
user-visible changes first.  So the order should be approximately:

 * changes to existing behaviour - the highest priority because the
   user's existing knowledge is incorrect
 * new features - should be brought to their attention
 * bug fixes - may be of interest if the bug was affecting them, and
   should include the bug number if any
 * major documentation changes, including fixed documentation bugs
 * changes to internal interfaces

People who made significant contributions to each change are listed in
parenthesis.  This can include reporting bugs (particularly with good
details or reproduction recipes), submitting patches, etc.

To help with merging, NEWS entries should be sorted lexicographically
within each section.

Commands
--------

The docstring of a command is used by ``bzr help`` to generate help output
for the command. The list 'takes_options' attribute on a command is used by
``bzr help`` to document the options for the command - the command
docstring does not need to document them. Finally, the '_see_also'
attribute on a command can be used to reference other related help topics.

API Documentation
-----------------

Functions, methods, classes and modules should have docstrings
describing how they are used.

The first line of the docstring should be a self-contained sentence.

For the special case of Command classes, this acts as the user-visible
documentation shown by the help command.

The docstrings should be formatted as reStructuredText_ (like this
document), suitable for processing using the epydoc_ tool into HTML
documentation.

.. _reStructuredText: http://docutils.sourceforge.net/rst.html
.. _epydoc: http://epydoc.sourceforge.net/


General Guidelines
==================

Copyright
---------

The copyright policy for bzr was recently made clear in this email (edited
for grammatical correctness)::

    The attached patch cleans up the copyright and license statements in
    the bzr source. It also adds tests to help us remember to add them
    with the correct text.

    We had the problem that lots of our files were "Copyright Canonical
    Development Ltd" which is not a real company, and some other variations
    on this theme. Also, some files were missing the GPL statements.

    I want to be clear about the intent of this patch, since copyright can
    be a little controversial.

    1) The big motivation for this is not to shut out the community, but
    just to clean up all of the invalid copyright statements.

    2) It has been the general policy for bzr that we want a single
    copyright holder for all of the core code. This is following the model
    set by the FSF, which makes it easier to update the code to a new
    license in case problems are encountered. (For example, if we want to
    upgrade the project universally to GPL v3 it is much simpler if there is
    a single copyright holder). It also makes it clearer if copyright is
    ever debated, there is a single holder, which makes it easier to defend
    in court, etc. (I think the FSF position is that if you assign them
    copyright, they can defend it in court rather than you needing to, and
    I'm sure Canonical would do the same).
    As such, Canonical has requested copyright assignments from all of the
    major contributers.

    3) If someone wants to add code and not attribute it to Canonical, there
    is a specific list of files that are excluded from this check. And the
    test failure indicates where that is, and how to update it.

    4) If anyone feels that I changed a copyright statement incorrectly, just
    let me know, and I'll be happy to correct it. Whenever you have large
    mechanical changes like this, it is possible to make some mistakes.

    Just to reiterate, this is a community project, and it is meant to stay
    that way. Core bzr code is copyright Canonical for legal reasons, and
    the tests are just there to help us maintain that.


Miscellaneous Topics
####################

Debugging
=========

Bazaar has a few facilities to help debug problems by going into pdb_, the
Python debugger.

.. _pdb: http://docs.python.org/lib/debugger-commands.html

If the ``BZR_PDB`` environment variable is set
then bzr will go into pdb post-mortem mode when an unhandled exception
occurs.

If you send a SIGQUIT or SIGBREAK signal to bzr then it will drop into the
debugger immediately. SIGQUIT can be generated by pressing Ctrl-\\ on
Unix.  SIGBREAK is generated with Ctrl-Pause on Windows (some laptops have
this as Fn-Pause).  You can continue execution by typing ``c``.  This can
be disabled if necessary by setting the environment variable
``BZR_SIGQUIT_PDB=0``.


Debug Flags
===========

Bazaar accepts some global options starting with ``-D`` such as
``-Dhpss``.  These set a value in `bzrlib.debug.debug_flags`, and
typically cause more information to be written to the trace file.  Most
`mutter` calls should be guarded by a check of those flags so that we
don't write out too much information if it's not needed.

Debug flags may have effects other than just emitting trace messages.

Run ``bzr help global-options`` to see them all.

These flags may also be set as a comma-separated list in the
``debug_flags`` option in e.g.  ``~/.bazaar/bazaar.conf``.  (Note that it
must be in this global file, not in the branch or location configuration,
because it's currently only loaded at startup time.)  For instance you may
want to always record hpss traces and to see full error tracebacks::

    debug_flags = hpss, error


Jargon
======

revno
    Integer identifier for a revision on the main line of a branch.
    Revision 0 is always the null revision; others are 1-based
    indexes into the branch's revision history.


Unicode and Encoding Support
============================

This section discusses various techniques that Bazaar uses to handle
characters that are outside the ASCII set.

``Command.outf``
----------------

When a ``Command`` object is created, it is given a member variable
accessible by ``self.outf``.  This is a file-like object, which is bound to
``sys.stdout``, and should be used to write information to the screen,
rather than directly writing to ``sys.stdout`` or calling ``print``.
This file has the ability to translate Unicode objects into the correct
representation, based on the console encoding.  Also, the class attribute
``encoding_type`` will effect how unprintable characters will be
handled.  This parameter can take one of 3 values:

  replace
    Unprintable characters will be represented with a suitable replacement
    marker (typically '?'), and no exception will be raised. This is for
    any command which generates text for the user to review, rather than
    for automated processing.
    For example: ``bzr log`` should not fail if one of the entries has text
    that cannot be displayed.

  strict
    Attempting to print an unprintable character will cause a UnicodeError.
    This is for commands that are intended more as scripting support, rather
    than plain user review.
    For example: ``bzr ls`` is designed to be used with shell scripting. One
    use would be ``bzr ls --null --unknowns | xargs -0 rm``.  If ``bzr``
    printed a filename with a '?', the wrong file could be deleted. (At the
    very least, the correct file would not be deleted). An error is used to
    indicate that the requested action could not be performed.

  exact
    Do not attempt to automatically convert Unicode strings. This is used
    for commands that must handle conversion themselves.
    For example: ``bzr diff`` needs to translate Unicode paths, but should
    not change the exact text of the contents of the files.


``bzrlib.urlutils.unescape_for_display``
----------------------------------------

Because Transports work in URLs (as defined earlier), printing the raw URL
to the user is usually less than optimal. Characters outside the standard
set are printed as escapes, rather than the real character, and local
paths would be printed as ``file://`` urls. The function
``unescape_for_display`` attempts to unescape a URL, such that anything
that cannot be printed in the current encoding stays an escaped URL, but
valid characters are generated where possible.


Portability Tips
================

The ``bzrlib.osutils`` module has many useful helper functions, including
some more portable variants of functions in the standard library.

In particular, don't use ``shutil.rmtree`` unless it's acceptable for it
to fail on Windows if some files are readonly or still open elsewhere.
Use ``bzrlib.osutils.rmtree`` instead.


C Extension Modules
===================

We write some extensions in C using pyrex. We design these to work in
three scenarios:

 * User with no C compiler
 * User with C compiler
 * Developers

The recommended way to install bzr is to have a C compiler so that the
extensions can be built, but if no C compiler is present, the pure python
versions we supply will work, though more slowly.

For developers we recommend that pyrex be installed, so that the C
extensions can be changed if needed.

For the C extensions, the extension module should always match the
original python one in all respects (modulo speed). This should be
maintained over time.

To create an extension, add rules to setup.py for building it with pyrex,
and with distutils. Now start with an empty .pyx file. At the top add
"include 'yourmodule.py'". This will import the contents of foo.py into this
file at build time - remember that only one module will be loaded at
runtime. Now you can subclass classes, or replace functions, and only your
changes need to be present in the .pyx file.

Note that pyrex does not support all 2.4 programming idioms, so some
syntax changes may be required. I.e.

 - 'from foo import (bar, gam)' needs to change to not use the brackets.
 - 'import foo.bar as bar' needs to be 'import foo.bar; bar = foo.bar'

If the changes are too dramatic, consider
maintaining the python code twice - once in the .pyx, and once in the .py,
and no longer including the .py file.


Making Installers for OS Windows
================================
To build a win32 installer, see the instructions on the wiki page:
http://bazaar-vcs.org/BzrWin32Installer


Core Developer Tasks
####################

Overview
========

What is a Core Developer?
-------------------------

While everyone in the Bazaar community is welcome and encouraged to
propose and submit changes, a smaller team is reponsible for pulling those
changes together into a cohesive whole. In addition to the general developer
stuff covered above, "core" developers have responsibility for:

* reviewing changes
* reviewing blueprints
* planning releases
* managing releases (see `Releasing Bazaar <http://doc.bazaar-vcs.org/developers/releasing.html>`_)

.. note::
  Removing barriers to community participation is a key reason for adopting
  distributed VCS technology. While DVCS removes many technical barriers,
  a small number of social barriers are often necessary instead.
  By documenting how the above things are done, we hope to
  encourage more people to participate in these activities, keeping the
  differences between core and non-core contributors to a minimum.


Communicating and Coordinating
------------------------------

While it has many advantages, one of the challenges of distributed
development is keeping everyone else aware of what you're working on.
There are numerous ways to do this:

#. Assign bugs to yourself in Launchpad
#. Mention it on the mailing list
#. Mention it on IRC

As well as the email notifcations that occur when merge requests are sent
and reviewed, you can keep others informed of where you're spending your
energy by emailing the **bazaar-commits** list implicitly. To do this,
install and configure the Email plugin. One way to do this is add these
configuration settings to your central configuration file (e.g.
``~/.bazaar/bazaar.conf`` on Linux)::

  [DEFAULT]
  email = Joe Smith <joe.smith@internode.on.net>
  smtp_server = mail.internode.on.net:25

Then add these lines for the relevant branches in ``locations.conf``::

  post_commit_to = bazaar-commits@lists.canonical.com
  post_commit_mailer = smtplib

While attending a sprint, RobertCollins' Dbus plugin is useful for the
same reason. See the documentation within the plugin for information on
how to set it up and configure it.


Submitting Changes
==================

An Overview of PQM
------------------

Of the many workflows supported by Bazaar, the one adopted for Bazaar
development itself is known as "Decentralized with automatic gatekeeper".
To repeat the explanation of this given on
http://bazaar-vcs.org/Workflows:

.. pull-quote::
  In this workflow, each developer has their own branch or
  branches, plus read-only access to the mainline. A software gatekeeper
  (e.g. PQM) has commit rights to the main branch. When a developer wants
  their work merged, they request the gatekeeper to merge it. The gatekeeper
  does a merge, a compile, and runs the test suite. If the code passes, it
  is merged into the mainline.

In a nutshell, here's the overall submission process:

#. get your work ready (including review except for trivial changes)
#. push to a public location
#. ask PQM to merge from that location

.. note::
  At present, PQM always takes the changes to merge from a branch
  at a URL that can be read by it. For Bazaar, that means a public,
  typically http, URL.

As a result, the following things are needed to use PQM for submissions:

#. A publicly available web server
#. Your OpenPGP key registered with PQM (contact RobertCollins for this)
#. The PQM plugin installed and configured (not strictly required but
   highly recommended).


Selecting a Public Branch Location
----------------------------------

If you don't have your own web server running, branches can always be
pushed to Launchpad. Here's the process for doing that:

Depending on your location throughout the world and the size of your
repository though, it is often quicker to use an alternative public
location to Launchpad, particularly if you can set up your own repo and
push into that. By using an existing repo, push only needs to send the
changes, instead of the complete repository every time. Note that it is
easy to register branches in other locations with Launchpad so no benefits
are lost by going this way.

.. note::
  For Canonical staff, http://people.ubuntu.com/~<user>/ is one
  suggestion for public http branches. Contact your manager for information
  on accessing this system if required.

It should also be noted that best practice in this area is subject to
change as things evolve. For example, once the Bazaar smart server on
Launchpad supports server-side branching, the performance situation will
be very different to what it is now (Jun 2007).


Configuring the PQM Plug-In
---------------------------

While not strictly required, the PQM plugin automates a few things and
reduces the chance of error. Before looking at the plugin, it helps to
understand  a little more how PQM operates. Basically, PQM requires an
email indicating what you want it to do. The email typically looks like
this::

  star-merge source-branch target-branch

For example::

  star-merge http://bzr.arbash-meinel.com/branches/bzr/jam-integration http://bazaar-vcs.org/bzr/bzr.dev

Note that the command needs to be on one line. The subject of the email
will be used for the commit message. The email also needs to be ``gpg``
signed with a key that PQM accepts.

The advantages of using the PQM plugin are:

#. You can use the config policies to make it easy to set up public
   branches, so you don't have to ever type the full paths you want to merge
   from or into.

#. It checks to make sure the public branch last revision matches the
   local last revision so you are submitting what you think you are.

#. It uses the same public_branch and smtp sending settings as bzr-email,
   so if you have one set up, you have the other mostly set up.

#. Thunderbird refuses to not wrap lines, and request lines are usually
   pretty long (you have 2 long URLs in there).

Here are sample configuration settings for the PQM plugin. Here are the
lines in bazaar.conf::

  [DEFAULT]
  email = Joe Smith <joe.smith@internode.on.net>
  smtp_server=mail.internode.on.net:25

And here are the lines in ``locations.conf`` (or ``branch.conf`` for
dirstate-tags branches)::

  [/home/joe/bzr/my-integration]
  push_location = sftp://joe-smith@bazaar.launchpad.net/%7Ejoe-smith/bzr/my-integration/
  push_location:policy = norecurse
  public_branch = http://bazaar.launchpad.net/~joe-smith/bzr/my-integration/
  public_branch:policy = appendpath
  pqm_email = Bazaar PQM <pqm@bazaar-vcs.org>
  pqm_branch = http://bazaar-vcs.org/bzr/bzr.dev

Note that the push settings will be added by the first ``push`` on
a branch. Indeed the preferred way to generate the lines above is to use
``push`` with an argument, then copy-and-paste the other lines into
the relevant file.


Submitting a Change
-------------------

Here is one possible recipe once the above environment is set up:

#. pull bzr.dev => my-integration
#. merge patch => my-integration
#. fix up any final merge conflicts (NEWS being the big killer here).
#. commit
#. push
#. pqm-submit

.. note::
  The ``push`` step is not required if ``my-integration`` is a checkout of
  a public branch.

  Because of defaults, you can type a single message into commit and
  pqm-commit will reuse that.


Tracking Change Acceptance
--------------------------

The web interface to PQM is https://pqm.bazaar-vcs.org/. After submitting
a change, you can visit this URL to confirm it was received and placed in
PQM's queue.

When PQM completes processing a change, an email is sent to you with the
results.


Reviewing Blueprints
====================

Blueprint Tracking Using Launchpad
----------------------------------

New features typically require a fair amount of discussion, design and
debate. For Bazaar, that information is often captured in a so-called
"blueprint" on our Wiki. Overall tracking of blueprints and their status
is done using Launchpad's relevant tracker,
https://blueprints.launchpad.net/bzr/. Once a blueprint for ready for
review, please announce it on the mailing list.

Alternatively, send an email beginning with [RFC] with the proposal to the
list. In some cases, you may wish to attach proposed code  or a proposed
developer document if that best communicates the idea. Debate can then
proceed using the normal merge review processes.


Recording Blueprint Review Feedback
-----------------------------------

Unlike its Bug Tracker, Launchpad's Blueprint Tracker doesn't currently
(Jun 2007) support a chronological list of comment responses. Review
feedback can either be recorded on the Wiki hosting the blueprints or by
using Launchpad's whiteboard feature.


Planning Releases
=================


Using Releases and Milestones in Launchpad
------------------------------------------

TODO ... (Exact policies still under discussion)


Bug Triage
----------

Keeping on top of bugs reported is an important part of ongoing release
planning. Everyone in the community is welcome and encouraged to raise
bugs, confirm bugs raised by others, and nominate a priority. Practically
though, a good percentage of bug triage is often done by the core
developers, partially because of their depth of product knowledge.

With respect to bug triage, core developers are encouraged to play an
active role with particular attention to the following tasks:

* keeping the number of unconfirmed bugs low
* ensuring the priorities are generally right (everything as critical - or
  medium - is meaningless)
* looking out for regressions and turning those around sooner rather than later.

.. note::
  As well as prioritizing bugs and nominating them against a
  target milestone, Launchpad lets core developers offer to mentor others in
  fixing them.


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