~bzr-pqm/bzr/bzr.dev

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

.. contents::

(The current version of this document is available in the file ``HACKING``
in the source tree, or at http://doc.bazaar-vcs.org/bzr.dev/hacking.html)


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.


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 on 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
================================

Looking for a 10 minute introduction to submitting a change?
See http://bazaar-vcs.org/BzrGivingBack.

TODO: Merge that Wiki page into this document.


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

The development team follows many best-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.


A Closer Look at the Merge & Review Process
===========================================

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 bundle like this::

  bzr bundle > mybundle.patch
  
A .patch extension is recommended instead of .bundle as many mail clients
will send the latter as a binary file. If a bundle would be too long or your
mailer mangles whitespace (e.g. implicitly converts Unix newlines to DOS
newlines), use the merge-directive command instead like this::

  bzr merge-directive http://bazaar-vcs.org http://example.org/my_branch > my_directive.patch

See the help for details on the arguments to merge-directive.

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.

Anyone is welcome to review code.  There are broadly three gates 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 pass all three. 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.)

Anyone can "vote" on the mailing list. Core developers can also vote using
Bundle Buggy. Here are the voting codes and their explanations.

  -1	really don't want it in current form
  -0	somewhat uncomfortable 
  +0	comfortable but resubmission after changes requested
  +1 conditional	good to go after some minor changes
  +1	good to go

+1 conditional is used as a way to avoid another submit/review cycle for
patches that need small changes.

If a change gets two +1 votes from core reviewers, and no
vetos, 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.


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 prestine (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 accidentially 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
========================

TODO: List and describe in one line the purpose of each directory
inside an installation of bzr.

TODO: Refer to a central location holding an up to date copy of the API
documentation generated by epydoc, e.g. something like
http://starship.python.net/crew/mwh/bzrlibapi/bzrlib.html.


Testing Bazaar
##############

The Importance of Testing
=========================

Reliability is a critical success factor for any Version Control System.
We want Bazaar to be highly reliable across multiple platforms while
evolving over time to meet the needs of its community. 

In a nutshell, this is want we expect and encourage:

* New functionality should have test cases.  Preferably write the
  test before writing the code.

  In general, you can test at either the command-line level or the
  internal API level.  See Writing tests below for more detail.

* Try to practice Test-Driven Development: before fixing a bug, write a
  test case so that it does not regress.  Similarly for adding a new
  feature: write a test case for a small version of the new feature before
  starting on the code itself.  Check the test fails on the old code, then
  add the feature or fix and check it passes.

By doing these things, the Bazaar team gets increased confidence that
changes do what they claim to do, whether provided by the core team or
by community members. Equally importantly, we can be surer that changes
down the track do not break new features or bug fixes that you are
contributing today.

As of May 2007, Bazaar ships with a test suite containing over 6000 tests
and growing. We are proud of it and want to remain so. As community
members, we all benefit from it. Would you trust version control on
your project to a product *without* a test suite like Bazaar has?


Running the Test Suite
======================

Currently, bzr selftest is used to invoke tests.
You can provide a pattern argument to run a subset. For example, 
to run just the blackbox tests, run::

  ./bzr selftest -v blackbox

To skip a particular test (or set of tests), use the --exclude option
(shorthand -x) like so::

  ./bzr selftest -v -x blackbox  

To list tests without running them, use the --list-only option like so::

  ./bzr selftest --list-only

This option can be combined with other selftest options (like -x) and
filter patterns to understand their effect.


Writing Tests
=============

In general tests should be placed in a file named test_FOO.py where 
FOO is the logical thing under test. That file should be placed in the
tests subdirectory under the package being tested.

For example, tests for merge3 in bzrlib belong in bzrlib/tests/test_merge3.py.
See bzrlib/tests/test_sampler.py for a template test script.

Tests can be written for the UI or for individual areas of the library.
Choose whichever is appropriate: if adding a new command, or a new command 
option, then you should be writing a UI test.  If you are both adding UI
functionality and library functionality, you will want to write tests for 
both the UI and the core behaviours.  We call UI tests 'blackbox' tests
and they are found in ``bzrlib/tests/blackbox/*.py``. 

When writing blackbox tests please honour the following conventions:

 1. Place the tests for the command 'name' in
    bzrlib/tests/blackbox/test_name.py. This makes it easy for developers
    to locate the test script for a faulty command.

 2. Use the 'self.run_bzr("name")' utility function to invoke the command
    rather than running bzr in a subprocess or invoking the
    cmd_object.run() method directly. This is a lot faster than
    subprocesses and generates the same logging output as running it in a
    subprocess (which invoking the method directly does not).
 
 3. Only test the one command in a single test script. Use the bzrlib 
    library when setting up tests and when evaluating the side-effects of
    the command. We do this so that the library api has continual pressure
    on it to be as functional as the command line in a simple manner, and
    to isolate knock-on effects throughout the blackbox test suite when a
    command changes its name or signature. Ideally only the tests for a
    given command are affected when a given command is changed.

 4. If you have a test which does actually require running bzr in a
    subprocess you can use ``run_bzr_subprocess``. By default the spawned
    process will not load plugins unless ``--allow-plugins`` is supplied.


Doctests
--------

We make selective use of doctests__.  In general they should provide 
*examples* within the API documentation which can incidentally be tested.  We 
don't try to test every important case using doctests -- regular Python
tests are generally a better solution.

Most of these are in ``bzrlib/doc/api``.  More additions are welcome.

  __ http://docs.python.org/lib/module-doctest.html


Skipping tests and test requirements
------------------------------------

In our enhancements to unittest we allow for some addition results beyond
just success or failure.

If a test can't be run, it can say that it's skipped.  This is typically
used in parameterized tests - for example if a transport doesn't support
setting permissions, we'll skip the tests that relating to that.  ::

    try:
        return self.branch_format.initialize(repo.bzrdir)
    except errors.UninitializableFormat:
        raise tests.TestSkipped('Uninitializable branch format')

Raising TestSkipped is a good idea when you want to make it clear that the
test was not run, rather than just returning which makes it look as if it
was run and passed.

A subtly different case is a test that should run, but can't run in the
current environment.  This covers tests that can only run in particular
operating systems or locales, or that depend on external libraries.  Here
we want to inform the user that they didn't get full test coverage, but
they possibly could if they installed more libraries.  These are expressed
as a dependency on a feature so we can summarise them, and so that the
test for the feature is done only once.  (For historical reasons, as of
May 2007 many cases that should depend on features currently raise
TestSkipped.)  The typical use is::

    class TestStrace(TestCaseWithTransport):

        _test_needs_features = [StraceFeature]

which means all tests in this class need the feature.  The feature itself
should provide a ``_probe`` method which is called once to determine if
it's available.


Known failures
--------------

Known failures are when a test exists but we know it currently doesn't
work, allowing the test suite to still pass.  These should be used with
care, we don't want a proliferation of quietly broken tests.  It might be
appropriate to use them if you've committed a test for a bug but not the
fix for it, or if something works on Unix but not on Windows.



Essential Domain Classes
########################

Introducing the Object Model
============================

The core domain objects within the bazaar model are:

* Transport

* Branch

* Repository

* WorkingTree

Transports are explained below. See http://bazaar-vcs.org/Classes/
for an introduction to the other key classes.

Using Transports
================

The ``Transport`` layer handles access to local or remote directories.
Each Transport object acts like a logical connection to a particular
directory, and it allows various operations on files within it.  You can
*clone* a transport to get a new Transport connected to a subdirectory or
parent directory.

Transports are not used for access to the working tree.  At present
working trees are always local and they are accessed through the regular
Python file io mechanisms.

Filenames vs URLs
-----------------

Transports work in URLs.  Take note that URLs are by definition only
ASCII - the decision of how to encode a Unicode string into a URL must be
taken at a higher level, typically in the Store.  (Note that Stores also
escape filenames which cannot be safely stored on all filesystems, but
this is a different level.)

The main reason for this is that it's not possible to safely roundtrip a
URL into Unicode and then back into the same URL.  The URL standard
gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
doesn't say how those bytes represent non-ASCII characters.  (They're not
guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)

For example if the user enters the url ``http://example/%e0`` there's no
way to tell whether that character represents "latin small letter a with
grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2
or malformed UTF-8.  So we can't convert their URL to Unicode reliably.

Equally problematic if we're given a url-like string containing non-ascii
characters (such as the accented a) we can't be sure how to convert that
to the correct URL, because we don't know what encoding the server expects
for those characters.  (Although this is not totally reliable we might still
accept these and assume they should be put into UTF-8.)

A similar edge case is that the url ``http://foo/sweet%2Fsour`` contains
one directory component whose name is "sweet/sour".  The escaped slash is
not a directory separator.  If we try to convert URLs to regular Unicode
paths this information will be lost.

This implies that Transports must natively deal with URLs; for simplicity
they *only* deal with URLs and conversion of other strings to URLs is done
elsewhere.  Information they return, such as from ``list_dir``, is also in
the form of URL components.


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

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

We have a commitment to 6 months API stability - any supported symbol in a
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'. 

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.


Coding Style Guidelines
=======================

Please write PEP-8__ compliant code.  

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

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


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 in some sense "private" are given
a leading underscore prefix.  This is just a hint that code outside the
implementation should probably not use that interface.

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. Never use a __del__ method without asking Martin/Robert first.

 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.


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.


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.


Modules versus Members
~~~~~~~~~~~~~~~~~~~~~~

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.


Passing to Other Variables
~~~~~~~~~~~~~~~~~~~~~~~~~~

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.


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.


Writing tests
=============

In general tests should be placed in a file named test_FOO.py where 
FOO is the logical thing under test. That file should be placed in the
tests subdirectory under the package being tested.

For example, tests for merge3 in bzrlib belong in bzrlib/tests/test_merge3.py.
See bzrlib/tests/test_sampler.py for a template test script.

Tests can be written for the UI or for individual areas of the library.
Choose whichever is appropriate: if adding a new command, or a new command 
option, then you should be writing a UI test.  If you are both adding UI
functionality and library functionality, you will want to write tests for 
both the UI and the core behaviours.  We call UI tests 'blackbox' tests
and they are found in ``bzrlib/tests/blackbox/*.py``. 

When writing blackbox tests please honour the following conventions:

 1. Place the tests for the command 'name' in
    bzrlib/tests/blackbox/test_name.py. This makes it easy for developers
    to locate the test script for a faulty command.

 2. Use the 'self.run_bzr("name")' utility function to invoke the command
    rather than running bzr in a subprocess or invoking the
    cmd_object.run() method directly. This is a lot faster than
    subprocesses and generates the same logging output as running it in a
    subprocess (which invoking the method directly does not).
 
 3. Only test the one command in a single test script. Use the bzrlib 
    library when setting up tests and when evaluating the side-effects of
    the command. We do this so that the library api has continual pressure
    on it to be as functional as the command line in a simple manner, and
    to isolate knock-on effects throughout the blackbox test suite when a
    command changes its name or signature. Ideally only the tests for a
    given command are affected when a given command is changed.

 4. If you have a test which does actually require running bzr in a
    subprocess you can use ``run_bzr_subprocess``. By default the spawned
    process will not load plugins unless ``--allow-plugins`` is supplied.


Test support
------------

We have a rich collection of tools to support writing tests. Please use
them in preference to ad-hoc solutions as they provide portability and
performance benefits.

TreeBuilder
~~~~~~~~~~~

The ``TreeBuilder`` interface allows the construction of arbitrary trees
with a declarative interface. A sample session might look like::

  tree = self.make_branch_and_tree('path')
  builder = TreeBuilder()
  builder.start_tree(tree)
  builder.build(['foo', "bar/", "bar/file"])
  tree.commit('commit the tree')
  builder.finish_tree()

Please see bzrlib.treebuilder for more details.

BranchBuilder
~~~~~~~~~~~~~

The ``BranchBuilder`` interface allows the creation of test branches in a
quick and easy manner. A sample session::

  builder = BranchBuilder(self.get_transport().clone('relpath'))
  builder.build_commit()
  builder.build_commit()
  builder.build_commit()
  branch = builder.get_branch()

Please see bzrlib.branchbuilder for more details.

Doctests
--------

We make selective use of doctests__.  In general they should provide 
*examples* within the API documentation which can incidentally be tested.  We 
don't try to test every important case using doctests -- regular Python
tests are generally a better solution.

Most of these are in ``bzrlib/doc/api``.  More additions are welcome.

  __ http://docs.python.org/lib/module-doctest.html


Running tests
=============
Currently, bzr selftest is used to invoke tests.
You can provide a pattern argument to run a subset. For example, 
to run just the blackbox tests, run::

  ./bzr selftest -v blackbox

To skip a particular test (or set of tests), use the --exclude option
(shorthand -x) like so::

  ./bzr selftest -v -x blackbox  

To list tests without running them, use the --list-only option like so::

  ./bzr selftest --list-only

This option can be combined with other selftest options (like -x) and
filter patterns to understand their effect.


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.

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.

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.


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
 * 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.

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 signal to bzr, which can be done by pressing
Ctrl-\\ on Unix, bzr will go into the debugger immediately.  You can
continue execution by typing ``c``.  This can be disabled if necessary
by setting the environment variable ``BZR_SIGQUIT_PDB=0``.


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 exampl: ``bzr ls`` is designed to be used with shell scripting. One
    use would be ``bzr ls --null --unknows | 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


..
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