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============================
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Guidelines for modifying bzr
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============================
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(The current version of this document is available in the file ``HACKING``
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in the source tree, or at http://doc.bazaar-vcs.org/current/hacking.html)
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* New functionality should have test cases. Preferably write the
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test before writing the code.
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In general, you can test at either the command-line level or the
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internal API level. See Writing Tests below for more detail.
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* Try to practice Test-Driven Development. before fixing a bug, write a
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test case so that it does not regress. Similarly for adding a new
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feature: write a test case for a small version of the new feature before
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starting on the code itself. Check the test fails on the old code, then
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add the feature or fix and check it passes.
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* Exceptions should be defined inside bzrlib.errors, so that we can
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see the whole tree at a glance.
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* Imports should be done at the top-level of the file, unless there is
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a strong reason to have them lazily loaded when a particular
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function runs. Import statements have a cost, so try to make sure
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they don't run inside hot functions.
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* Module names should always be given fully-qualified,
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i.e. ``bzrlib.hashcache`` not just ``hashcache``.
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* Commands should return non-zero when they encounter circumstances that
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the user should really pay attention to - which includes trivial shell
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Recommended values are
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1. Conflicts in merge-like operations, or changes are present in
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2. Unrepresentable diff changes (i.e. binary files that we cannot show
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3. An error or exception has occurred.
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We have a commitment to 6 months API stability - any supported symbol in a
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release of bzr MUST NOT be altered in any way that would result in
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breaking existing code that uses it. That means that method names,
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parameter ordering, parameter names, variable and attribute names etc must
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not be changed without leaving a 'deprecated forwarder' behind. This even
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applies to modules and classes.
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If you wish to change the behaviour of a supported API in an incompatible
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way, you need to change its name as well. For instance, if I add an optional keyword
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parameter to branch.commit - that's fine. On the other hand, if I add a
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keyword parameter to branch.commit which is a *required* transaction
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object, I should rename the API - i.e. to 'branch.commit_transaction'.
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When renaming such supported API's, be sure to leave a deprecated_method (or
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_function or ...) behind which forwards to the new API. See the
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bzrlib.symbol_versioning module for decorators that take care of the
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details for you - such as updating the docstring, and issuing a warning
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when the old api is used.
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For unsupported API's, it does not hurt to follow this discipline, but it's
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not required. Minimally though, please try to rename things so that
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callers will at least get an AttributeError rather than weird results.
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Standard parameter types
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------------------------
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There are some common requirements in the library: some parameters need to be
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unicode safe, some need byte strings, and so on. At the moment we have
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only codified one specific pattern: Parameters that need to be unicode
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should be checked via ``bzrlib.osutils.safe_unicode``. This will coerce the
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input into unicode in a consistent fashion, allowing trivial strings to be
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used for programmer convenience, but not performing unpredictably in the
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presence of different locales.
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The copyright policy for bzr was recently made clear in this email (edited
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for grammatical correctness)::
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The attached patch cleans up the copyright and license statements in
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the bzr source. It also adds tests to help us remember to add them
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with the correct text.
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We had the problem that lots of our files were "Copyright Canonical
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Development Ltd" which is not a real company, and some other variations
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on this theme. Also, some files were missing the GPL statements.
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I want to be clear about the intent of this patch, since copyright can
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be a little controversial.
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1) The big motivation for this is not to shut out the community, but
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just to clean up all of the invalid copyright statements.
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2) It has been the general policy for bzr that we want a single
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copyright holder for all of the core code. This is following the model
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set by the FSF, which makes it easier to update the code to a new
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license in case problems are encountered. (For example, if we want to
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upgrade the project universally to GPL v3 it is much simpler if there is
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a single copyright holder). It also makes it clearer if copyright is
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ever debated, there is a single holder, which makes it easier to defend
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in court, etc. (I think the FSF position is that if you assign them
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copyright, they can defend it in court rather than you needing to, and
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I'm sure Canonical would do the same).
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As such, Canonical has requested copyright assignments from all of the
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3) If someone wants to add code and not attribute it to Canonical, there
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is a specific list of files that are excluded from this check. And the
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test failure indicates where that is, and how to update it.
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4) If anyone feels that I changed a copyright statement incorrectly, just
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let me know, and I'll be happy to correct it. Whenever you have large
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mechanical changes like this, it is possible to make some mistakes.
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Just to reiterate, this is a community project, and it is meant to stay
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that way. Core bzr code is copyright Canonical for legal reasons, and
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the tests are just there to help us maintain that.
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When you change bzrlib, please update the relevant documentation for the
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change you made: Changes to commands should update their help, and
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possibly end user tutorials; changes to the core library should be
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reflected in API documentation.
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The docstring of a command is used by ``bzr help`` to generate help output
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for the command. The list 'takes_options' attribute on a command is used by
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``bzr help`` to document the options for the command - the command
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docstring does not need to document them. Finally, the '_see_also'
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attribute on a command can be used to reference other related help topics.
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If you make a user-visible change, please add a note to the NEWS file.
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The description should be written to make sense to someone who's just
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a user of bzr, not a developer: new functions or classes shouldn't be
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mentioned, but new commands, changes in behaviour or fixed nontrivial
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bugs should be listed. See the existing entries for an idea of what
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Within each release, entries in the news file should have the most
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user-visible changes first. So the order should be approximately:
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* changes to existing behaviour - the highest priority because the
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user's existing knowledge is incorrect
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* new features - should be brought to their attention
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* bug fixes - may be of interest if the bug was affecting them, and
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should include the bug number if any
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* major documentation changes
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* changes to internal interfaces
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People who made significant contributions to each change are listed in
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parenthesis. This can include reporting bugs (particularly with good
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details or reproduction recipes), submitting patches, etc.
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Functions, methods, classes and modules should have docstrings
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describing how they are used.
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The first line of the docstring should be a self-contained sentence.
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For the special case of Command classes, this acts as the user-visible
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documentation shown by the help command.
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The docstrings should be formatted as reStructuredText_ (like this
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document), suitable for processing using the epydoc_ tool into HTML
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.. _reStructuredText: http://docutils.sourceforge.net/rst.html
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.. _epydoc: http://epydoc.sourceforge.net/
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Please write PEP-8__ compliant code.
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One often-missed requirement is that the first line of docstrings
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should be a self-contained one-sentence summary.
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__ http://www.python.org/peps/pep-0008.html
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Functions, methods or members that are in some sense "private" are given
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a leading underscore prefix. This is just a hint that code outside the
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implementation should probably not use that interface.
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We prefer class names to be concatenated capital words (``TestCase``)
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and variables, methods and functions to be lowercase words joined by
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underscores (``revision_id``, ``get_revision``).
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For the purposes of naming some names are treated as single compound
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words: "filename", "revno".
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Consider naming classes as nouns and functions/methods as verbs.
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Try to avoid using abbreviations in names, because there can be
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inconsistency if other people use the full name.
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``revision_id`` not ``rev_id`` or ``revid``
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Functions that transform one thing to another should be named ``x_to_y``
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(not ``x2y`` as occurs in some old code.)
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Python destructors (``__del__``) work differently to those of other
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languages. In particular, bear in mind that destructors may be called
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immediately when the object apparently becomes unreferenced, or at some
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later time, or possibly never at all. Therefore we have restrictions on
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what can be done inside them.
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0. Never use a __del__ method without asking Martin/Robert first.
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1. Never rely on a ``__del__`` method running. If there is code that
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must run, do it from a ``finally`` block instead.
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2. Never ``import`` from inside a ``__del__`` method, or you may crash the
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3. In some places we raise a warning from the destructor if the object
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has not been cleaned up or closed. This is considered OK: the warning
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may not catch every case but it's still useful sometimes.
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In some places we have variables which point to callables that construct
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new instances. That is to say, they can be used a lot like class objects,
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but they shouldn't be *named* like classes:
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> I think that things named FooBar should create instances of FooBar when
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> called. Its plain confusing for them to do otherwise. When we have
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> something that is going to be used as a class - that is, checked for via
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> isinstance or other such idioms, them I would call it foo_class, so that
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> it is clear that a callable is not sufficient. If it is only used as a
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> factory, then yes, foo_factory is what I would use.
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Several places in Bazaar use (or will use) a registry, which is a
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mapping from names to objects or classes. The registry allows for
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loading in registered code only when it's needed, and keeping
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associated information such as a help string or description.
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To make startup time faster, we use the ``bzrlib.lazy_import`` module to
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delay importing modules until they are actually used. ``lazy_import`` uses
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the same syntax as regular python imports. So to import a few modules in a
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from bzrlib.lazy_import import lazy_import
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lazy_import(globals(), """
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revision as _mod_revision,
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import bzrlib.transport
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At this point, all of these exist as a ``ImportReplacer`` object, ready to
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be imported once a member is accessed. Also, when importing a module into
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the local namespace, which is likely to clash with variable names, it is
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recommended to prefix it as ``_mod_<module>``. This makes it clearer that
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the variable is a module, and these object should be hidden anyway, since
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they shouldn't be imported into other namespaces.
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Modules versus Members
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~~~~~~~~~~~~~~~~~~~~~~
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While it is possible for ``lazy_import()`` to import members of a module
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when using the ``from module import member`` syntax, it is recommended to
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only use that syntax to load sub modules ``from module import submodule``.
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This is because variables and classes can frequently be used without
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needing a sub-member for example::
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lazy_import(globals(), """
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from module import MyClass
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return isinstance(x, MyClass)
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This will incorrectly fail, because ``MyClass`` is a ``ImportReplacer``
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object, rather than the real class.
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Passing to other variables
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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It also is incorrect to assign ``ImportReplacer`` objects to other variables.
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Because the replacer only knows about the original name, it is unable to
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replace other variables. The ``ImportReplacer`` class will raise an
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``IllegalUseOfScopeReplacer`` exception if it can figure out that this
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happened. But it requires accessing a member more than once from the new
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variable, so some bugs are not detected right away.
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(The strategy described here is what we want to get to, but it's not
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consistently followed in the code at the moment.)
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bzrlib is intended to be a generically reusable library. It shouldn't
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write messages to stdout or stderr, because some programs that use it
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might want to display that information through a GUI or some other
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We can distinguish two types of output from the library:
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1. Structured data representing the progress or result of an
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operation. For example, for a commit command this will be a list
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of the modified files and the finally committed revision number
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These should be exposed either through the return code or by calls
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to a callback parameter.
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A special case of this is progress indicators for long-lived
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operations, where the caller should pass a ProgressBar object.
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2. Unstructured log/debug messages, mostly for the benefit of the
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developers or users trying to debug problems. This should always
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be sent through ``bzrlib.trace`` and Python ``logging``, so that
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it can be redirected by the client.
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The distinction between the two is a bit subjective, but in general if
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there is any chance that a library would want to see something as
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structured data, we should make it so.
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The policy about how output is presented in the text-mode client
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should be only in the command-line tool.
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In general tests should be placed in a file named test_FOO.py where
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FOO is the logical thing under test. That file should be placed in the
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tests subdirectory under the package being tested.
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For example, tests for merge3 in bzrlib belong in bzrlib/tests/test_merge3.py.
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See bzrlib/tests/test_sampler.py for a template test script.
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Tests can be written for the UI or for individual areas of the library.
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Choose whichever is appropriate: if adding a new command, or a new command
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option, then you should be writing a UI test. If you are both adding UI
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functionality and library functionality, you will want to write tests for
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both the UI and the core behaviours. We call UI tests 'blackbox' tests
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and they are found in ``bzrlib/tests/blackbox/*.py``.
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When writing blackbox tests please honour the following conventions:
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1. Place the tests for the command 'name' in
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bzrlib/tests/blackbox/test_name.py. This makes it easy for developers
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to locate the test script for a faulty command.
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2. Use the 'self.run_bzr("name")' utility function to invoke the command
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rather than running bzr in a subprocess or invoking the
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cmd_object.run() method directly. This is a lot faster than
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subprocesses and generates the same logging output as running it in a
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subprocess (which invoking the method directly does not).
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3. Only test the one command in a single test script. Use the bzrlib
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library when setting up tests and when evaluating the side-effects of
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the command. We do this so that the library api has continual pressure
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on it to be as functional as the command line in a simple manner, and
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to isolate knock-on effects throughout the blackbox test suite when a
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command changes its name or signature. Ideally only the tests for a
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given command are affected when a given command is changed.
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4. If you have a test which does actually require running bzr in a
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subprocess you can use ``run_bzr_subprocess``. By default the spawned
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process will not load plugins unless ``--allow-plugins`` is supplied.
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We make selective use of doctests__. In general they should provide
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*examples* within the API documentation which can incidentally be tested. We
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don't try to test every important case using doctests -- regular Python
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tests are generally a better solution.
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Most of these are in ``bzrlib/doc/api``. More additions are welcome.
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__ http://docs.python.org/lib/module-doctest.html
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Currently, bzr selftest is used to invoke tests.
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You can provide a pattern argument to run a subset. For example,
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to run just the blackbox tests, run::
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./bzr selftest -v blackbox
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To skip a particular test (or set of tests), use the --exclude option
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(shorthand -x) like so::
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./bzr selftest -v -x blackbox
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To list tests without running them, use the --list-only option like so::
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./bzr selftest --list-only
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This option can be combined with other selftest options (like -x) and
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filter patterns to understand their effect.
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Errors and exceptions
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=====================
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Errors are handled through Python exceptions.
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We broadly classify errors as either being either internal or not,
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depending on whether ``user_error`` is set or not. If we think it's our
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fault, we show a backtrace, an invitation to report the bug, and possibly
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other details. This is the default for errors that aren't specifically
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recognized as being caused by a user error. Otherwise we show a briefer
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message, unless -Derror was given.
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Many errors originate as "environmental errors" which are raised by Python
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or builtin libraries -- for example IOError. These are treated as being
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our fault, unless they're caught in a particular tight scope where we know
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that they indicate a user errors. For example if the repository format
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is not found, the user probably gave the wrong path or URL. But if one of
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the files inside the repository is not found, then it's our fault --
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either there's a bug in bzr, or something complicated has gone wrong in
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the environment that means one internal file was deleted.
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Many errors are defined in ``bzrlib/errors.py`` but it's OK for new errors
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to be added near the place where they are used.
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Exceptions are formatted for the user by conversion to a string
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(eventually calling their ``__str__`` method.) As a convenience the
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``._fmt`` member can be used as a template which will be mapped to the
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error's instance dict.
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New exception classes should be defined when callers might want to catch
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that exception specifically, or when it needs a substantially different
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Exception strings should start with a capital letter and should not have a
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final fullstop. If long, they may contain newlines to break the text.
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Integer identifier for a revision on the main line of a branch.
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Revision 0 is always the null revision; others are 1-based
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indexes into the branch's revision history.
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The ``Transport`` layer handles access to local or remote directories.
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Each Transport object acts like a logical connection to a particular
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directory, and it allows various operations on files within it. You can
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*clone* a transport to get a new Transport connected to a subdirectory or
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Transports are not used for access to the working tree. At present
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working trees are always local and they are accessed through the regular
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Python file io mechanisms.
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Transports work in URLs. Take note that URLs are by definition only
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ASCII - the decision of how to encode a Unicode string into a URL must be
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taken at a higher level, typically in the Store. (Note that Stores also
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escape filenames which cannot be safely stored on all filesystems, but
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this is a different level.)
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The main reason for this is that it's not possible to safely roundtrip a
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URL into Unicode and then back into the same URL. The URL standard
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gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
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doesn't say how those bytes represent non-ASCII characters. (They're not
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guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)
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For example if the user enters the url ``http://example/%e0`` there's no
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way to tell whether that character represents "latin small letter a with
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grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2
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or malformed UTF-8. So we can't convert their URL to Unicode reliably.
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Equally problematic if we're given a url-like string containing non-ascii
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characters (such as the accented a) we can't be sure how to convert that
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to the correct URL, because we don't know what encoding the server expects
541
for those characters. (Although this is not totally reliable we might still
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accept these and assume they should be put into UTF-8.)
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A similar edge case is that the url ``http://foo/sweet%2Fsour`` contains
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one directory component whose name is "sweet/sour". The escaped slash is
546
not a directory separator. If we try to convert URLs to regular Unicode
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paths this information will be lost.
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This implies that Transports must natively deal with URLs; for simplicity
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they *only* deal with URLs and conversion of other strings to URLs is done
551
elsewhere. Information they return, such as from ``list_dir``, is also in
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the form of URL components.
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Unicode and Encoding Support
556
============================
558
This section discusses various techniques that Bazaar uses to handle
559
characters that are outside the ASCII set.
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When a ``Command`` object is created, it is given a member variable
565
accessible by ``self.outf``. This is a file-like object, which is bound to
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``sys.stdout``, and should be used to write information to the screen,
567
rather than directly writing to ``sys.stdout`` or calling ``print``.
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This file has the ability to translate Unicode objects into the correct
569
representation, based on the console encoding. Also, the class attribute
570
``encoding_type`` will effect how unprintable characters will be
571
handled. This parameter can take one of 3 values:
574
Unprintable characters will be represented with a suitable replacement
575
marker (typically '?'), and no exception will be raised. This is for
576
any command which generates text for the user to review, rather than
577
for automated processing.
578
For example: ``bzr log`` should not fail if one of the entries has text
579
that cannot be displayed.
582
Attempting to print an unprintable character will cause a UnicodeError.
583
This is for commands that are intended more as scripting support, rather
584
than plain user review.
585
For exampl: ``bzr ls`` is designed to be used with shell scripting. One
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use would be ``bzr ls --null --unknows | xargs -0 rm``. If ``bzr``
587
printed a filename with a '?', the wrong file could be deleted. (At the
588
very least, the correct file would not be deleted). An error is used to
589
indicate that the requested action could not be performed.
592
Do not attempt to automatically convert Unicode strings. This is used
593
for commands that must handle conversion themselves.
594
For example: ``bzr diff`` needs to translate Unicode paths, but should
595
not change the exact text of the contents of the files.
598
``bzrlib.urlutils.unescape_for_display``
599
----------------------------------------
601
Because Transports work in URLs (as defined earlier), printing the raw URL
602
to the user is usually less than optimal. Characters outside the standard
603
set are printed as escapes, rather than the real character, and local
604
paths would be printed as ``file://`` urls. The function
605
``unescape_for_display`` attempts to unescape a URL, such that anything
606
that cannot be printed in the current encoding stays an escaped URL, but
607
valid characters are generated where possible.
613
The ``bzrlib.osutils`` module has many useful helper functions, including
614
some more portable variants of functions in the standard library.
616
In particular, don't use ``shutil.rmtree`` unless it's acceptable for it
617
to fail on Windows if some files are readonly or still open elsewhere.
618
Use ``bzrlib.osutils.rmtree`` instead.
624
If you'd like to propose a change, please post to the
625
bazaar@lists.canonical.com list with a patch, bzr changeset, or link to a
626
branch. Please put '[patch]' in the subject so we can pick them out, and
627
include some text explaining the change. Remember to put an update to the NEWS
628
file in your diff, if it makes any changes visible to users or plugin
629
developers. Please include a diff against mainline if you're giving a link to
632
Please indicate if you think the code is ready to merge, or if it's just a
633
draft or for discussion. If you want comments from many developers rather than
634
to be merged, you can put '[rfc]' in the subject lines.
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Anyone is welcome to review code. There are broadly three gates for
639
* Doesn't reduce test coverage: if it adds new methods or commands,
640
there should be tests for them. There is a good test framework
641
and plenty of examples to crib from, but if you are having trouble
642
working out how to test something feel free to post a draft patch
645
* Doesn't reduce design clarity, such as by entangling objects
646
we're trying to separate. This is mostly something the more
647
experienced reviewers need to help check.
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* Improves bugs, features, speed, or code simplicity.
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Code that goes in should pass all three.
653
If you read a patch please reply and say so. We can use a numeric scale
654
of -1, -0, +0, +1, meaning respectively "really don't want it in current
655
form", "somewhat uncomfortable", "ok with me", and "please put it in".
656
Anyone can "vote". (It's not really voting, just a terse expression.)
658
If something gets say two +1 votes from core reviewers, and no
659
vetos, then it's OK to come in. Any of the core developers can bring it
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into their integration branch, which I'll merge regularly. (If you do
661
so, please reply and say so.)
667
We write some extensions in C using pyrex. We design these to work in
670
* User with no C compiler
671
* User with C compiler
674
The recommended way to install bzr is to have a C compiler so that the
675
extensions can be built, but if no C compiler is present, the pure python
676
versions we supply will work, though more slowly.
678
For developers we recommend that pyrex be installed, so that the C
679
extensions can be changed if needed.
681
For the C extensions, the extension module should always match the
682
original python one in all respects (modulo speed). This should be
683
maintained over time.
685
To create an extension, add rules to setup.py for building it with pyrex,
686
and with distutils. Now start with an empty .pyx file. At the top add
687
"include 'yourmodule.py'". This will import the contents of foo.py into this
688
file at build time - remember that only one module will be loaded at
689
runtime. Now you can subclass classes, or replace functions, and only your
690
changes need to be present in the .pyx file.
692
Note that pyrex does not support all 2.4 programming idioms, so some
693
syntax changes may be required. I.e.
695
- 'from foo import (bar, gam)' needs to change to not use the brackets.
696
- 'import foo.bar as bar' needs to be 'import foo.bar; bar = foo.bar'
698
If the changes are too dramatic, consider
699
maintaining the python code twice - once in the .pyx, and once in the .py,
700
and no longer including the .py file.
702
Making installers for OS Windows
703
================================
704
To build a win32 installer, see the instructions on the wiki page:
705
http://bazaar-vcs.org/BzrWin32Installer
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:: vim: ft=rst tw=74 ai
added
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HACKING
