1
============================
2
Guidelines for modifying bzr
3
============================
7
(The current version of this document is available in the file ``HACKING``
8
in the source tree, or at http://bazaar-ng.org/hacking.html)
13
* New functionality should have test cases. Preferably write the
14
test before writing the code.
16
In general, you can test at either the command-line level or the
17
internal API level. See Writing Tests below for more detail.
19
* Try to practice Test-Driven Development. before fixing a bug, write a
20
test case so that it does not regress. Similarly for adding a new
21
feature: write a test case for a small version of the new feature before
22
starting on the code itself. Check the test fails on the old code, then
23
add the feature or fix and check it passes.
25
* Exceptions should be defined inside bzrlib.errors, so that we can
26
see the whole tree at a glance.
28
* Imports should be done at the top-level of the file, unless there is
29
a strong reason to have them lazily loaded when a particular
30
function runs. Import statements have a cost, so try to make sure
31
they don't run inside hot functions.
33
* Module names should always be given fully-qualified,
34
i.e. ``bzrlib.hashcache`` not just ``hashcache``.
36
* Commands should return non-zero when they encounter circumstances that
37
the user should really pay attention to - which includes trivial shell
40
Recommended values are
42
1- Conflicts in merge-like operations, or changes are present in
44
2- Unrepresentable diff changes (i.e. binary files that we cannot show
46
3- An error or exception has occurred.
51
We have a commitment to 6 months API stability - any supported symbol in a
52
release of bzr MUST NOT be altered in any way that would result in
53
breaking existing code that uses it. That means that method names,
54
parameter ordering, parameter names, variable and attribute names etc must
55
not be changed without leaving a 'deprecated forwarder' behind. This even
56
applies to modules and classes.
58
If you wish to change the behaviour of a supported API in an incompatible
59
way, you need to change its name as well. For instance, if I add a optional keyword
60
parameter to branch.commit - that's fine. On the other hand, if I add a
61
keyword parameter to branch.commit which is a *required* transaction
62
object, I should rename the API - i.e. to 'branch.commit_transaction'.
64
When renaming such supported API's, be sure to leave a deprecated_method (or
65
_function or ...) behind which forwards to the new API. See the
66
bzrlib.symbol_versioning module for decorators that take care of the
67
details for you - such as updating the docstring, and issuing a warning
68
when the old api is used.
70
For unsupported API's, it does not hurt to follow this discipline, but its
71
not required. Minimally though, please try to rename things so that
72
callers will at least get an AttributeError rather than weird results.
75
Standard parameter types
76
------------------------
78
There are some common requirements in the library: some parameters need to be
79
unicode safe, some need byte strings, and so on. At the moment we have
80
only codified one specific pattern: Parameters that need to be unicode
81
should be check via 'bzrlib.osutils.safe_unicode'. This will coerce the
82
input into unicode in a consistent fashion, allowing trivial strings to be
83
used for programmer convenience, but not performing unpredictably in the
84
presence of different locales.
89
If you change the behaviour of a command, please update its docstring
90
in bzrlib/commands.py. This is displayed by the 'bzr help' command.
95
If you make a user-visible change, please add a note to the NEWS file.
96
The description should be written to make sense to someone who's just
97
a user of bzr, not a developer: new functions or classes shouldn't be
98
mentioned, but new commands, changes in behaviour or fixed nontrivial
99
bugs should be listed. See the existing entries for an idea of what
102
Within each release, entries in the news file should have the most
103
user-visible changes first. So the order should be approximately:
105
* changes to existing behaviour - the highest priority because the
106
user's existing knowledge is incorrect
107
* new features - should be brought to their attention
108
* bug fixes - may be of interest if the bug was affecting them, and
109
should include the bug number if any
110
* major documentation changes
111
* changes to internal interfaces
113
People who made significant contributions to each change are listed in
114
parenthesis. This can include reporting bugs (particularly with good
115
details or reproduction recipes), submitting patches, etc.
120
Functions, methods, classes and modules should have docstrings
121
describing how they are used.
123
The first line of the docstring should be a self-contained sentence.
125
For the special case of Command classes, this acts as the user-visible
126
documentation shown by the help command.
128
The docstrings should be formatted as reStructuredText_ (like this
129
document), suitable for processing using the epydoc_ tool into HTML
132
.. _reStructuredText: http://docutils.sourceforge.net/rst.html
133
.. _epydoc: http://epydoc.sourceforge.net/
140
Please write PEP-8__ compliant code.
142
One often-missed requirement is that the first line of docstrings
143
should be a self-contained one-sentence summary.
145
__ http://www.python.org/peps/pep-0008.html
152
Functions, methods or members that are in some sense "private" are given
153
a leading underscore prefix. This is just a hint that code outside the
154
implementation should probably not use that interface.
156
We prefer class names to be concatenated capital words (``TestCase``)
157
and variables, methods and functions to be lowercase words joined by
158
underscores (``revision_id``, ``get_revision``).
160
For the purposes of naming some names are treated as single compound
161
words: "filename", "revno".
163
Consider naming classes as nouns and functions/methods as verbs.
169
``revision_id`` not ``rev_id`` or ``revid``
171
Functions that transform one thing to another should be named ``x_to_y``
172
(not ``x2y`` as occurs in some old code.)
178
Python destructors (``__del__``) work differently to those of other
179
languages. In particular, bear in mind that destructors may be called
180
immediately when the object apparently becomes unreferenced, or at some
181
later time, or possibly never at all. Therefore we have restrictions on
182
what can be done inside them.
184
0. Never use a __del__ method without asking Martin/Robert first.
186
1. Never rely on a ``__del__`` method running. If there is code that
187
must run, do it from a ``finally`` block instead.
189
2. Never ``import`` from inside a ``__del__`` method, or you may crash the
192
3. In some places we raise a warning from the destructor if the object
193
has not been cleaned up or closed. This is considered OK: the warning
194
may not catch every case but it's still useful sometimes.
200
In some places we have variables which point to callables that construct
201
new instances. That is to say, they can be used a lot like class objects,
202
but they shouldn't be *named* like classes:
204
> I think that things named FooBar should create instances of FooBar when
205
> called. Its plain confusing for them to do otherwise. When we have
206
> something that is going to be used as a class - that is, checked for via
207
> isinstance or other such idioms, them I would call it foo_class, so that
208
> it is clear that a callable is not sufficient. If it is only used as a
209
> factory, then yes, foo_factory is what I would use.
215
(The strategy described here is what we want to get to, but it's not
216
consistently followed in the code at the moment.)
218
bzrlib is intended to be a generically reusable library. It shouldn't
219
write messages to stdout or stderr, because some programs that use it
220
might want to display that information through a GUI or some other
223
We can distinguish two types of output from the library:
225
1. Structured data representing the progress or result of an
226
operation. For example, for a commit command this will be a list
227
of the modified files and the finally committed revision number
230
These should be exposed either through the return code or by calls
231
to a callback parameter.
233
A special case of this is progress indicators for long-lived
234
operations, where the caller should pass a ProgressBar object.
236
2. Unstructured log/debug messages, mostly for the benefit of the
237
developers or users trying to debug problems. This should always
238
be sent through ``bzrlib.trace`` and Python ``logging``, so that
239
it can be redirected by the client.
241
The distinction between the two is a bit subjective, but in general if
242
there is any chance that a library would want to see something as
243
structured data, we should make it so.
245
The policy about how output is presented in the text-mode client
246
should be only in the command-line tool.
251
In general tests should be placed in a file named test_FOO.py where
252
FOO is the logical thing under test. That file should be placed in the
253
tests subdirectory under the package being tested.
255
For example, tests for merge3 in bzrlib belong in bzrlib/tests/test_merge3.py.
256
See bzrlib/selftest/test_sampler.py for a template test script.
258
Tests can be written for the UI or for individual areas of the library.
259
Choose whichever is appropriate: if adding a new command, or a new command
260
option, then you should be writing a UI test. If you are both adding UI
261
functionality and library functionality, you will want to write tests for
262
both the UI and the core behaviours. We call UI tests 'blackbox' tests
263
and they are found in bzrlib/tests/blackbox/*.py.
265
When writing blackbox tests please honour the following conventions:
267
1. Place the tests for the command 'name' in
268
bzrlib/tests/blackbox/test_name.py. This makes it easy for developers
269
to locate the test script for a faulty command.
271
2. Use the 'self.run_bzr("name")' utility function to invoke the command
272
rather than running bzr in a subprocess or invoking the
273
cmd_object.run() method directly. This is a lot faster than
274
subprocesses and generates the same logging output as running it in a
275
subprocess (which invoking the method directly does not).
277
3. Only test the one command in a single test script. Use the bzrlib
278
library when setting up tests and when evaluating the side-effects of
279
the command. We do this so that the library api has continual pressure
280
on it to be as functional as the command line in a simple manner, and
281
to isolate knock-on effects throughout the blackbox test suite when a
282
command changes it name or signature. Ideally only the tests for a
283
given command are affected when a given command is changed.
288
We make selective use of doctests__. In general they should provide
289
*examples* within the API documentation which can incidentally be tested. We
290
don't try to test every important case using doctests -- regular Python
291
tests are generally a better solution.
293
Most of these are in ``bzrlib/doc/api``. More additions are welcome.
295
__ http://docs.python.org/lib/module-doctest.html
300
Currently, bzr selftest is used to invoke tests.
301
You can provide a pattern argument to run a subset. For example,
302
to run just the blackbox tests, run::
304
./bzr selftest -v blackbox
306
To skip a particular test (or set of tests), you need to use a negative
308
./bzr selftest '^(?!.*blackbox)'
311
Errors and exceptions
312
=====================
314
Errors are handled through Python exceptions. They can represent user
315
errors, environmental errors or program bugs. Sometimes we can't be sure
316
at the time it's raised which case applies. See bzrlib/errors.py for
317
details on the error-handling practices.
324
Integer identifier for a revision on the main line of a branch.
325
Revision 0 is always the null revision; others are 1-based
326
indexes into the branch's revision history.
332
The ``Transport`` layer handles access to local or remote directories.
333
Each Transport object acts like a logical connection to a particular
334
directory, and it allows various operations on files within it. You can
335
*clone* a transport to get a new Transport connected to a subdirectory or
338
Transports are not used for access to the working tree. At present
339
working trees are always local and they are accessed through the regular
340
Python file io mechanisms.
345
Transports work in URLs. Take note that URLs are by definition only
346
ASCII - the decision of how to encode a Unicode string into a URL must be
347
taken at a higher level, typically in the Store. (Note that Stores also
348
escape filenames which cannot be safely stored on all filesystems, but
349
this is a different level.)
351
The main reason for this is that it's not possible to safely roundtrip a
352
URL into Unicode and then back into the same URL. The URL standard
353
gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
354
doesn't say how those bytes represent non-ASCII characters. (They're not
355
guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)
357
For example if the user enters the url ``http://example/%e0`` there's no
358
way to tell whether that character represents "latin small letter a with
359
grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2
360
or malformed UTF-8. So we can't convert their URL to Unicode reliably.
362
Equally problematic if we're given a url-like string containing non-ascii
363
characters (such as the accented a) we can't be sure how to convert that
364
to the correct URL, because we don't know what encoding the server expects
365
for those characters. (Although this is not totally reliable we might still
366
accept these and assume they should be put into UTF-8.)
368
A similar edge case is that the url ``http://foo/sweet%2Fsour" contains
369
one directory component whose name is "sweet/sour". The escaped slash is
370
not a directory separator. If we try to convert URLs to regular Unicode
371
paths this information will be lost.
373
This implies that Transports must natively deal with URLs; for simplicity
374
they *only* deal with URLs and conversion of other strings to URLs is done
375
elsewhere. Information they return, such as from ``list_dir``, is also in
376
the form of URL components.
382
If you'd like to propose a change, please post to the
383
bazaar-ng@lists.canonical.com list with a patch, bzr changeset, or link to a
384
branch. Please put '[patch]' in the subject so we can pick them out, and
385
include some text explaining the change. Remember to put an update to the NEWS
386
file in your diff, if it makes any changes visible to users or plugin
387
developers. Please include a diff against mainline if you're giving a link to
390
Please indicate if you think the code is ready to merge, or if it's just a
391
draft or for discussion. If you want comments from many developers rather than
392
to be merged, you can put '[rfc]' in the subject lines.
394
Anyone is welcome to review code. There are broadly three gates for
397
* Doesn't reduce test coverage: if it adds new methods or commands,
398
there should be tests for them. There is a good test framework
399
and plenty of examples to crib from, but if you are having trouble
400
working out how to test something feel free to post a draft patch
403
* Doesn't reduce design clarity, such as by entangling objects
404
we're trying to separate. This is mostly something the more
405
experienced reviewers need to help check.
407
* Improves bugs, features, speed, or code simplicity.
409
Code that goes in should pass all three.
411
If you read a patch please reply and say so. We can use a numeric scale
412
of -1, -0, +0, +1, meaning respectively "really don't want it in current
413
form", "somewhat uncomfortable", "ok with me", and "please put it in".
414
Anyone can "vote". (It's not really voting, just a terse expression.)
416
If something gets say two +1 votes from core reviewers, and no
417
vetos, then it's OK to come in. Any of the core developers can bring it
418
into their integration branch, which I'll merge regularly. (If you do
419
so, please reply and say so.)
422
:: vim: ft=rst tw=74 ai
added
removed
HACKING
