~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/meaningful_name_here

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/meaningful_name_here> 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.

Using a meaningful name for your branch will help you and the reviewer(s)

better track the submission. Use a very succint description of your submission

and prefix it with bug number if needed (lp:~mbp/bzr/484558-merge-directory

for example). Alternatively, you can suffix with the bug number

(lp:~jameinel/bzr/export-file-511987).

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.

https://wiki.ubuntu.com/UnitsPolicy provides a good explanation about

which unit should be used when. Roughly speaking, IEC standard applies

for base-2 units and SI standard applies for base-10 units::

* for network bandwidth an disk sizes, use base-10 (Mbits/s, kB/s, GB),

* for RAM sizes, use base-2 (GiB, TiB).

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