~bzr-pqm/bzr/bzr.dev

***********************

Bazaar Code Style Guide

***********************

Code layout

===========

Please write PEP-8__ compliant code.

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

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

should be a self-contained one-sentence summary.

We use 4 space indents for blocks, and never use tab characters.  (In vim,

``set expandtab``.)

Trailing white space should be avoided, but is allowed.

You should however not make lots of unrelated white space changes.

Unix style newlines (LF) are used.

Each file must have a newline at the end of it.

Lines should be no more than 79 characters if at all possible.

Lines that continue a long statement may be indented in either of

two ways:

within the parenthesis or other character that opens the block, e.g.::

    my_long_method(arg1,

                   arg2,

                   arg3)

or indented by four spaces::

    my_long_method(arg1,

        arg2,

        arg3)

The first is considered clearer by some people; however it can be a bit

harder to maintain (e.g. when the method name changes), and it does not

work well if the relevant parenthesis is already far to the right.  Avoid

this::

     self.legbone.kneebone.shinbone.toebone.shake_it(one,

                                                     two,

                                                     three)

but rather ::

     self.legbone.kneebone.shinbone.toebone.shake_it(one,

         two,

         three)

or ::

     self.legbone.kneebone.shinbone.toebone.shake_it(

         one, two, three)

For long lists, we like to add a trailing comma and put the closing

character on the following line.  This makes it easier to add new items in

future::

    from bzrlib.goo import (

        jam,

        jelly,

        marmalade,

        )

There should be spaces between function parameters, but not between the

keyword name and the value::

    call(1, 3, cheese=quark)

Python versions

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

Bazaar supports Python from 2.6 through 2.7, and in the future we want to

support Python 3.  Avoid using language features added in

2.7, or features deprecated in Python 3.0.  (You can check v3

compatibility using the ``-3`` option of Python2.6.)

hasattr and getattr

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

``hasattr`` should not be used because it swallows exceptions including

``KeyboardInterrupt``.  Instead, say something like ::

  if getattr(thing, 'name', None) is None

kwargs

======

``**kwargs`` in the prototype of a function should be used sparingly.

It can be good on higher-order functions that decorate other functions,

such as ``addCleanup`` or ``assertRaises``, or on functions that take only

(or almost only) kwargs, where any kwargs can be passed.  

Otherwise, be careful: if the parameters to a function are a bit complex

and might vary over time (e.g.  the ``commit`` API) then we prefer to pass an

object rather than a bag of positional and/or keyword args.  If you have

an arbitrary set of keys and values that are different with each use (e.g.

string interpolation inputs) then again that should not be mixed in with

the regular positional/keyword args, it seems like a different category of

thing.

Imitating standard objects

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

Don't provide methods that imitate built-in classes (eg ``__in__``,

``__call__``, ``__int__``, ``__getitem__``) unless the class you're

implementing really does act like the builtin class, in semantics and

performance.

For example, old code lets you say ``file_id in inv`` but we no longer

consider this good style.  Instead, say more explicitly

``inv.has_id(file_id)``.

``__repr__``, ``__cmp__``, ``__str__`` are usually fine.

Module Imports

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

* Imports should be done at the top-level of the file, unless there is

  a strong reason to have them lazily loaded when a particular

  function runs.  Import statements have a cost, so try to make sure

  they don't run inside hot functions.

* Module names should always be given fully-qualified,

  i.e. ``bzrlib.hashcache`` not just ``hashcache``.

Naming

======

Functions, methods or members that are relatively private are given

a leading underscore prefix.  Names without a leading underscore are

public not just across modules but to programmers using bzrlib as an

API.

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

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

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

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

words: "filename", "revno".

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

Try to avoid using abbreviations in names, because there can be

inconsistency if other people use the full name.

Standard Names

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

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

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

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

Destructors

===========

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

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

immediately when the object apparently becomes unreferenced, or at some

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

what can be done inside them.

0. If you think you need to use a ``__del__`` method ask another

   developer for alternatives.  If you do need to use one, explain

   why in a comment.

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

   must run, instead have a ``finally`` block or an ``addCleanup`` call an

   explicit ``close`` method.

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

   interpreter!!

3. Prior to bzr 2.4, we sometimes used to raise warnings from del methods

   that the object was not cleaned up or closed.  We no longer do this:

   failure to close the object doesn't cause a test failure; the warning

   appears an arbitrary long time after the problem occurred (the object

   being leaked); merely having a del method inhibits Python gc; the

   warnings appear to users and upset them; they can also break tests that

   are checking what appears on stderr.

In short, just don't use ``__del__``.

Cleanup methods

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

Often when something has failed later code will fail too, including

cleanups invoked from ``finally`` blocks.  These secondary failures are

generally uninteresting compared to the original exception.  ``bzrlib``

has some facilities you can use to mitigate this.

* In ``Command`` subclasses, prefer the ``add_cleanup`` method to using

  ``try``/``finally`` blocks.  E.g. to acquire a lock and ensure it will

  always be released when the command is done::

    self.add_cleanup(branch.lock_read().unlock)

  This also avoids heavily indented code. It also makes it easier to notice

  mismatched lock/unlock pairs (and other kinds of resource

  acquire/release) because there isn't a large block of code separating

  them.

* Use the ``only_raises`` decorator (from ``bzrlib.decorators``) when

  defining methods that are typically called in ``finally`` blocks, such

  as ``unlock`` methods.  For example, ``@only_raises(LockNotHeld,

  LockBroken)``.  All errors that are unlikely to be a knock-on failure

  from a previous failure should be allowed.

* Consider using the ``OperationWithCleanups`` helper from

  ``bzrlib.cleanup`` anywhere else you have a ``finally`` block that

  might fail.

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.  Things called ``FooBar`` should

create an instance of ``FooBar``.  A factory method that might create a

``FooBar`` or might make something else should be called ``foo_factory``.

Registries

==========

Several places in Bazaar use (or will use) a registry, which is a

mapping from names to objects or classes.  The registry allows for

loading in registered code only when it's needed, and keeping

associated information such as a help string or description.

InterObject and multiple dispatch

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

The ``InterObject`` provides for two-way `multiple dispatch`__: matching

up for example a source and destination repository to find the right way

to transfer data between them.

.. __: http://en.wikipedia.org/wiki/Multiple_dispatch

There is a subclass ``InterObject`` classes for each type of object that is

dispatched this way, e.g. ``InterRepository``.  Calling ``.get()`` on this

class will return an ``InterObject`` instance providing the best match for

those parameters, and this instance then has methods for operations

between the objects.

::

  inter = InterRepository.get(source_repo, target_repo)

  inter.fetch(revision_id)

``InterRepository`` also acts as a registry-like object for its

subclasses, and they can be added through ``.register_optimizer``.  The

right one to run is selected by asking each class, in reverse order of

registration, whether it ``.is_compatible`` with the relevant objects.

Lazy Imports

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

To make startup time faster, we use the ``bzrlib.lazy_import`` module to

delay importing modules until they are actually used. ``lazy_import`` uses

the same syntax as regular python imports. So to import a few modules in a

lazy fashion do::

  from bzrlib.lazy_import import lazy_import

  lazy_import(globals(), """

  import os

  import subprocess

  import sys

  import time

  from bzrlib import (

     errors,

     transport,

     revision as _mod_revision,

     )

  import bzrlib.transport

  import bzrlib.xml5

  """)

At this point, all of these exist as a ``ImportReplacer`` object, ready to

be imported once a member is accessed. Also, when importing a module into

the local namespace, which is likely to clash with variable names, it is

recommended to prefix it as ``_mod_<module>``. This makes it clearer that

the variable is a module, and these object should be hidden anyway, since

they shouldn't be imported into other namespaces.

While it is possible for ``lazy_import()`` to import members of a module

when using the ``from module import member`` syntax, it is recommended to

only use that syntax to load sub modules ``from module import submodule``.

This is because variables and classes can frequently be used without

needing a sub-member for example::

  lazy_import(globals(), """

  from module import MyClass

  """)

  def test(x):

      return isinstance(x, MyClass)

This will incorrectly fail, because ``MyClass`` is a ``ImportReplacer``

object, rather than the real class.

It also is incorrect to assign ``ImportReplacer`` objects to other variables.

Because the replacer only knows about the original name, it is unable to

replace other variables. The ``ImportReplacer`` class will raise an

``IllegalUseOfScopeReplacer`` exception if it can figure out that this

happened. But it requires accessing a member more than once from the new

variable, so some bugs are not detected right away.

The Null revision

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

The null revision is the ancestor of all revisions.  Its revno is 0, its

revision-id is ``null:``, and its tree is the empty tree.  When referring

to the null revision, please use ``bzrlib.revision.NULL_REVISION``.  Old

code sometimes uses ``None`` for the null revision, but this practice is

being phased out.

Object string representations

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

Python prints objects using their ``__repr__`` method when they are

written to logs, exception tracebacks, or the debugger.  We want

objects to have useful representations to help in determining what went

wrong.

If you add a new class you should generally add a ``__repr__`` method

unless there is an adequate method in a parent class.  There should be a

test for the repr.

Representations should typically look like Python constructor syntax, but

they don't need to include every value in the object and they don't need

to be able to actually execute.  They're to be read by humans, not

machines.  Don't hardcode the classname in the format, so that we get the

correct value if the method is inherited by a subclass.  If you're

printing attributes of the object, including strings, you should normally

use ``%r`` syntax (to call their repr in turn).

Try to avoid the representation becoming more than one or two lines long.

(But balance this against including useful information, and simplicity of

implementation.)

Because repr methods are often called when something has already gone

wrong, they should be written somewhat more defensively than most code.

They shouldn't have side effects like doing network or disk

IO.

The object may be half-initialized or in some other way in an illegal

state.  The repr method shouldn't raise an exception, or it may hide the

(probably more useful) underlying exception.

Example::

    def __repr__(self):

        return '%s(%r)' % (self.__class__.__name__,

                           self._transport)

Exception handling

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

A bare ``except`` statement will catch all exceptions, including ones that

really should terminate the program such as ``MemoryError`` and

``KeyboardInterrupt``.  They should rarely be used unless the exception is

later re-raised.  Even then, think about whether catching just

``Exception`` (which excludes system errors in Python2.5 and later) would

be better.

The ``__str__`` method on exceptions should be small and have no side

effects, following the rules given for `Object string representations`_.

In particular it should not do any network IO, or complicated

introspection of other objects.  All the state needed to present the

exception to the user should be gathered before the error is raised.

In other words, exceptions should basically be value objects.

Test coverage

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

All code should be exercised by the test suite.  See the `Bazaar Testing

Guide <http://doc.bazaar.canonical.com/developers/testing.html>`_ for detailed

information about writing tests.

Assertions

==========

Do not use the Python ``assert`` statement, either in tests or elsewhere.

A source test checks that it is not used.  It is ok to explicitly raise

AssertionError.

Rationale:

* It makes the behaviour vary depending on whether bzr is run with -O

  or not, therefore giving a chance for bugs that occur in one case or

  the other, several of which have already occurred: assertions with

  side effects, code which can't continue unless the assertion passes,

  cases where we should give the user a proper message rather than an

  assertion failure.

* It's not that much shorter than an explicit if/raise.

* It tends to lead to fuzzy thinking about whether the check is

  actually needed or not, and whether it's an internal error or not

* It tends to cause look-before-you-leap patterns.

* It's unsafe if the check is needed to protect the integrity of the

  user's data.

* It tends to give poor messages since the developer can get by with

  no explanatory text at all.

* We can't rely on people always running with -O in normal use, so we

  can't use it for tests that are actually expensive.

* Expensive checks that help developers are better turned on from the

  test suite or a -D flag.

* If used instead of ``self.assert*()`` in tests it makes them falsely

  pass with -O.

emacs setup

===========

In emacs::

    ;(defface my-invalid-face

    ;  '((t (:background "Red" :underline t)))

    ;  "Face used to highlight invalid constructs or other uglyties"

    ;  )

    (defun my-python-mode-hook ()

     ;; setup preferred indentation style.

     (setq fill-column 79)

     (setq indent-tabs-mode nil) ; no tabs, never, I will not repeat

    ;  (font-lock-add-keywords 'python-mode

    ;                         '(("^\\s *\t" . 'my-invalid-face) ; Leading tabs

    ;                            ("[ \t]+$" . 'my-invalid-face)  ; Trailing spaces

    ;                            ("^[ \t]+$" . 'my-invalid-face)); Spaces only

    ;                          )

     )

    (add-hook 'python-mode-hook 'my-python-mode-hook)

The lines beginning with ';' are comments. They can be activated

if one want to have a strong notice of some tab/space usage

violations.

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.

Using the ``open(..).read(..)`` or ``open(..).write(..)`` style chaining

of methods for reading or writing file content relies on garbage collection

to close the file which may keep the file open for an undefined period of

time. This may break some follow up operations like rename on Windows.

Use ``try/finally`` to explictly close the file. E.g.::

    f = open('foo.txt', 'w')

    try:

        f.write(s)

    finally:

        f.close()

Terminology

===========

Bazaar is a GNU project and uses standard GNU terminology, especially:

 * Use the word "Linux" to refer to the Linux kernel, not as a synechoche

   for the entire operating system.  (See `bug 528253

   <https://bugs.launchpad.net/bzr/+bug/528253>`_).

 * Don't say "open source" when you mean "free software".

Dynamic imports

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

If you need to import a module (or attribute of a module) named in a

variable:

 * If importing a module, not an attribute, and the module is a top-level

   module (i.e. has no dots in the name), then it's ok to use the builtin

   ``__import__``, e.g. ``__import__(module_name)``.

 * In all other cases, prefer ``bzrlib.pyutils.get_named_object`` to the

   built-in ``__import__``.  ``__import__`` has some subtleties and

   unintuitive behaviours that make it hard to use correctly.

..

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