Breezy Developer Guide

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

If you have any questions or something seems to be incorrect, unclear or missing, please talk to us in irc://, or write to the Breezy 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

Getting Started

Exploring the Breezy 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 Breezy Platform. Here are some links to browse:

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 brz -Devil commit -m "test" will log a backtrace to the Breezy log file for every method call which triggers a slow or non-scalable part of the breezy 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 Breezy. Mostly we meet on IRC (#breezy on and on the mailing list. To join the Breezy community, see

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.

Breezy Development in a Nutshell

One of the fun things about working on a version control system like Breezy 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 Breezy. 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

$ brz init-shared-repo ~/bzr
$ cd ~/bzr
$ brz branch lp:brz

Now make your own branch:

$ brz branch 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 Breezy project. The documentation is in the doc/ subdirectory of the Breezy 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 Breezy developers use Launchpad to further enable a truly distributed style of development. Anyone can propose a branch for merging into the Breezy 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 Breezy:

$ brz 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 Breezy trunk. Go to <<your_lp_username>/bzr/meaningful_name_here> and choose “Propose for merging into another branch”. Select “lp:bzr” to hand your changes off to the Breezy developers for review and merging.

Alternatively, after pushing you can use the propose command to create the merge proposal.

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).

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.

Why make a local copy of

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

  • You can use that copy of as your main brz executable, and keep it up-to-date using brz pull.

  • Certain operations are faster, and can be done when offline. For example:

    • brz bundle

    • brz diff -r ancestor:...

    • brz 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
    $ brz branch 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:

For further information, see <>.

Preparing a Sandbox for Making Changes to Breezy

Breezy supports many ways of organising your work. See 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 ( by using this command:

    brz branch lp:brz
  • keep your copy of pristine (by not developing in it) and keep it up to date (by using brz pull)

  • create a new branch off your local 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.

Core Topics

Evolving Interfaces

We don’t change APIs in stable branches: any supported symbol in a stable release of Breezy 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:

@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.

General Guidelines

Miscellaneous Topics


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

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

If you send a SIGQUIT or SIGBREAK signal to brz 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 BRZ_SIGQUIT_PDB=0.

All tests inheriting from bzrlib.tests.TestCase can use self.debug() instead of the longer import pdb; pdb.set_trace(). The former also works when stdin/stdout are redirected (by using the original stdin/stdout file handles at the start of the bzr script) while the later doesn’t. bzrlib.debug.set_trace() also uses the original stdin/stdout file handles.

Debug Flags

Breezy 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 brz 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. ~/.config/breezy/breezy.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



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 Breezy uses to handle characters that are outside the ASCII set.


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:


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: brz log should not fail if one of the entries has text that cannot be displayed.


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: brz ls is designed to be used with shell scripting. One use would be brz 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.


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


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.

C Extension Modules

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

  • User with no C compiler

  • User with C compiler

  • Developers

The recommended way to install Breezy 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 Cython 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 for building it with Cython , and with distutils. Now start with an empty .pyx file. At the top add “include ‘’”. This will import the contents of 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.

Making Installers for OS Windows

To build a win32 installer, see the instructions on the wiki page:

Core Developer Tasks


What is a Core Developer?

While everyone in the Breezy 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:


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:

  1. Assign bugs to yourself in Launchpad

  2. Mention it on the mailing list

  3. 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. ~/.config/breezy/breezy.conf):

email = Joe Smith <>
smtp_server =

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

post_commit_to =
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.

Planning Releases

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.


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