Bazaar Developer Guide

Contents

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

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:

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.

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.

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 ensure that all tests are being run and succeeding, you can use the --strict option which will fail if there are any missing features or known failures, like so:

./bzr selftest --strict

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.

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.

Testing exceptions and errors

It's important to test handling of errors and exceptions. Because this code is often not hit in ad-hoc testing it can often have hidden bugs -- it's particularly common to get NameError because the exception code references a variable that has since been renamed.

In general we want to test errors at two levels:

  1. A test in test_errors.py checking that when the exception object is constructed with known parameters it produces an expected string form. This guards against mistakes in writing the format string, or in the str representations of its parameters. There should be one for each exception class.
  2. Tests that when an api is called in a particular situation, it raises an error of the expected class. You should typically use assertRaises, which in the Bazaar test suite returns the exception object to allow you to examine its parameters.

In some cases blackbox tests will also want to check error reporting. But it can be difficult to provoke every error through the commandline interface, so those tests are only done as needed -- eg in response to a particular bug or if the error is reported in an unusual way(?) Blackbox tests should mostly be testing how the command-line interface works, so should only test errors if there is something particular to the cli in how they're displayed or handled.

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.

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 "private" to bzrlib 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. As a consequence, a leading underscore is appropriate for names exposed across modules but that are not to be exposed to bzrlib API programmers.

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.

  1. Never use a __del__ method without asking Martin/Robert first.
  2. Never rely on a __del__ method running. If there is code that must run, do it from a finally block instead.
  3. Never import from inside a __del__ method, or you may crash the interpreter!!
  4. 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.

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.

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.

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.

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.

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:

  1. OK.
  2. Conflicts in merge-like operations, or changes are present in diff-like operations.
  3. Unrepresentable diff changes (i.e. binary files that we cannot show a diff of).
  4. 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.

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.

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