~bzr-pqm/bzr/bzr.dev

============================

Guidelines for modifying bzr

============================

.. contents::

(The current version of this document is available in the file ``HACKING``

in the source tree, or at http://bazaar-ng.org/hacking.html)

Overall

=======

* 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.  Choose whichever is appropriate: if adding a

  new command, or a new command option, then call through run_bzr().

  It is not necessary to do both. Tests that test the command line level

  are appropriate for checking the UI behaves well - bug fixes and

  core improvements should be tested closer to the code that is doing the

  work. Command line level tests should be placed in 'blackbox.py'.

* Before fixing a bug, write a test case so that it does not regress.

* Exceptions should be defined inside bzrlib.errors, so that we can

  see the whole tree at a glance.

* 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``.

* Commands should return Non-Zero when they encounter circumstances that

  the user should really pay attention to - which includes trivial shell

  pipelines.

  Recommanded values are 

    0- OK, 

    1- Conflicts in merge-like operations, or changes are present in

       diff-like operations. 

    2- Unrepresentable diff changes (i.e. binary files that we cannot show 

       a diff of).

    3- An error or exception has occured.

Evolving interfaces

-------------------

If you change the behaviour of an API in an incompatible way, please

be sure to change its name as well. For instance, if I add a 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'.

This will prevent users of the old api getting surprising results. 

Instead, they will get an Attribute error as the api is missing, and

will know to update their code. If in doubt, just ask on #bzr.

Documentation

=============

If you change the behaviour of a command, please update its docstring

in bzrlib/commands.py.  This is displayed by the 'bzr help' command.

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.

API documentation

-----------------

Functions, methods, classes and modules should have docstrings

describing how they are used. 

The first line of the docstring should be a self-contained sentence.

For the special case of Command classes, this acts as the user-visible

documentation shown by the help command.

The docstrings should be formatted as reStructuredText_ (like this

document), suitable for processing using the epydoc_ tool into HTML

documentation.

.. _reStructuredText: http://docutils.sourceforge.net/rst.html

.. _epydoc: http://epydoc.sourceforge.net/

Coding style

============

Please write PEP-8__ compliant code.  

One often-missed requirement is that the first line of docstrings

should be a self-contained one-sentence summary.

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

Naming

------

Functions, methods or members that are in some sense "private" are given

a leading underscore prefix.  This is just a hint that code outside the

implementation should probably not use that interface.

We prefer class names to be concatenated capital words (``TestCase``)

and variables, methods and functions to be lowercase words joined by

underscores (``revision_id``, ``get_revision``).

For the purposes of naming some names are treated as single compound

words: "filename", "revno".

Consider naming classes as nouns and functions/methods as verbs.

Standard names

--------------

``revision_id`` not ``rev_id`` or ``revid``

Functions that transform one thing to another should be named ``x_to_y``

(not ``x2y`` as occurs in some old code.)

Destructors

-----------

Python destructors (``__del__``) work differently to those of other

languages.  In particular, bear in mind that destructors may be called

immediately when the object apparently becomes unreferenced, or at some

later time, or possibly never at all.  Therefore we have restrictions on

what can be done inside them.

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

 1. Never rely on a ``__del__`` method running.  If there is code that

    must run, do it from a ``finally`` block instead.

 2. Never ``import`` from inside a ``__del__`` method, or you may crash the

    interpreter!!

 3. In some places we raise a warning from the destructor if the object

    has not been cleaned up or closed.  This is considered OK: the warning

    may not catch every case but it's still useful sometimes.

Writing output

==============

(The strategy described here is what we want to get to, but it's not

consistently followed in the code at the moment.)

bzrlib is intended to be a generically reusable library.  It shouldn't

write messages to stdout or stderr, because some programs that use it

might want to display that information through a GUI or some other

mechanism.

We can distinguish two types of output from the library:

 1. Structured data representing the progress or result of an

    operation.  For example, for a commit command this will be a list

    of the modified files and the finally committed revision number

    and id.

    These should be exposed either through the return code or by calls

    to a callback parameter.

    A special case of this is progress indicators for long-lived

    operations, where the caller should pass a ProgressBar object.

 2. Unstructured log/debug messages, mostly for the benefit of the

    developers or users trying to debug problems.  This should always

    be sent through ``bzrlib.trace`` and Python ``logging``, so that

    it can be redirected by the client.

The distinction between the two is a bit subjective, but in general if

there is any chance that a library would want to see something as

structured data, we should make it so.

The policy about how output is presented in the text-mode client

should be only in the command-line tool.

Writing tests

=============

In general tests should be placed in a file named testFOO.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/testmerge3.py.

See bzrlib/selftest/testsampler.py for a template test script.

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 whitebox tests, run::

  bzr selftest -v whitebox

Errors and exceptions

=====================

Errors are handled through Python exceptions.  They can represent user

errors, environmental errors or program bugs.  Sometimes we can't be sure

at the time it's raised which case applies.  See bzrlib/errors.py for 

details on the error-handling practices.

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.

:: vim: tw=74 ai