Buildbot Coding Style
=====================
Twisted Idioms
--------------
Programming with Twisted Python can be daunting. But sticking to a few
well-defined patterns can help avoid surprises.
Helpful Twisted Classes
~~~~~~~~~~~~~~~~~~~~~~~
Twisted has some useful, but little-known classes. They are listed here with
brief descriptions, but you should consult the API documentation or source code
for the full details.
:class:`twisted.internet.task.LoopingCall`
Calls an asynchronous function repeatedly at set intervals.
:class:`twisted.application.internet.TimerService`
Similar to ``t.i.t.LoopingCall``, but implemented as a service that will
automatically start and stop the function calls when the service is started and
stopped.
deferredGenerator
~~~~~~~~~~~~~~~~~
:class:`twisted.internet.defer.deferredGenerator` is a great help to writing
code that makes a lot of asynchronous calls. Refer to the Twisted
documentation for the details, but the style within Buildbot is as follows::
from twisted.internet import defer
@defer.deferredGenerator
def mymethod(self, x, y):
wfd = defer.waitForDeferred(
getSomething(x))
yield wfd
xval = wfd.getResult()
yield xval + y # return value
The key points to notice here:
* Always import ``defer`` as a module, not the names within it.
* Use the decorator form of ``deferredGenerator``
* For each ``waitForDeferred`` call, use the variable ``wfd``, and assign to it
on one line, with the operation returning the Deferred on the next.
* While ``wfd.getResult()`` can be used in an expression, if that expression is
complex, pull it out into a simple assignment. This helps reviewers scanning
the code for missing ``getResult`` calls.
* When ``yield`` is used to return a value, add a comment to that effect, since
this can often be missed.
As a reminder, Builtbot is compatible with Python-2.4, which does not support
try/finally blocks in generators.
Writing Buildbot Tests
----------------------
In general, we are trying to ensure that new tests are *good*. So what makes
a good test?
.. _Tests-Independent-of-Time:
Independent of Time
~~~~~~~~~~~~~~~~~~~
Tests that depend on wall time will fail. As a bonus, they run very slowly. Do
not use :meth:`reactor.callLater` to wait "long enough" for something to happen.
For testing things that themselves depend on time, consider using
:class:`twisted.internet.tasks.Clock`. This may mean passing a clock instance to
the code under test, and propagating that instance as necessary to ensure that
all of the code using :meth:`callLater` uses it. Refactoring code for
testability is difficult, but wortwhile.
For testing things that do not depend on time, but for which you cannot detect
the "end" of an operation: add a way to detect the end of the operation!
Clean Code
~~~~~~~~~~
Make your tests readable. This is no place to skimp on comments! Others will
attempt to learn about the expected behavior of your class by reading the
tests. As a side note, if you use a :class:`Deferred` chain in your test, write
the callbacks as nested functions, rather than using object methods with funny
names::
def testSomething(self):
d = doThisFirst()
def andThisNext(res):
pass # ...
d.addCallback(andThisNext)
return d
This isolates the entire test into one indented block. It is OK to add methods
for common functionality, but give them real names and explain in detail what
they do.
Good Name
~~~~~~~~~
Your test module should be named after the package or class it tests, replacing
``.`` with ``_`` and omitting the ``buildbot_``. For example,
:file:`test_status_web_authz_Authz.py` tests the :class:`Authz` class in
:file:`buildbot/status/web/authz.py`. Modules with only one class, or a few
trivial classes, can be tested in a single test module. For more complex
situations, prefer to use multiple test modules.
Test method names should follow the pattern :samp:`test_{METHOD}_{CONDITION}`
where *METHOD* is the method being tested, and *CONDITION* is the
condition under which it's tested. Since we can't always test a single
method, this is not a hard-and-fast rule.
Assert Only One Thing
~~~~~~~~~~~~~~~~~~~~~
Each test should have a single assertion. This may require a little bit of work
to get several related pieces of information into a single Python object for
comparison. The problem with multiple assertions is that, if the first
assertion fails, the remainder are not tested. The test results then do not
tell the entire story.
If you need to make two unrelated assertions, you should be running two tests.
Use Mocks and Stubs
~~~~~~~~~~~~~~~~~~~
Mocks assert that they are called correctly. Stubs provide a predictable base
on which to run the code under test. See
`Mock Object <http://en.wikipedia.org/wiki/Mock_object>`_ and
`Method Stub <http://en.wikipedia.org/wiki/Method_stub>`_.
Mock objects can be constructed easily using the aptly-named
`mock <http://www.voidspace.org.uk/python/mock/>`_ module, which is a
requirement for Buildbot's tests.
One of the difficulties with Buildbot is that interfaces are unstable and
poorly documented, which makes it difficult to design stubs. A common
repository for stubs, however, will allow any interface changes to be reflected
in only one place in the test code.
Small Tests
~~~~~~~~~~~
The shorter each test is, the better. Test as little code as possible in each test.
It is fine, and in fact encouraged, to write the code under test in such a way
as to facilitate this. As an illustrative example, if you are testing a new
Step subclass, but your tests require instantiating a BuildMaster, you're
probably doing something wrong! (Note that this rule is almost universally
violated in the existing buildbot tests).
This also applies to test modules. Several short, easily-digested test modules
are preferred over a 1000-line monster.
Isolation
~~~~~~~~~
Each test should be maximally independent of other tests. Do not leave files
laying around after your test has finished, and do not assume that some other
test has run beforehand. It's fine to use caching techniques to avoid repeated,
lengthy setup times.
Be Correct
~~~~~~~~~~
Tests should be as robust as possible, which at a basic level means using the
available frameworks correctly. All deferreds should have callbacks and be
chained properly. Error conditions should be checked properly. Race conditions
should not exist (see :ref:`Tests-Independent-of-Time`, above).
Be Helpful
~~~~~~~~~~
Note that tests will pass most of the time, but the moment when they are most
useful is when they fail.
When the test fails, it should produce output that is helpful to the person
chasing it down. This is particularly important when the tests are run
remotely, in which case the person chasing down the bug does not have access to
the system on which the test fails. A test which fails sporadically with no
more information than "AssertionFailed?" is a prime candidate for deletion if
the error isn't obvious. Making the error obvious also includes adding comments
describing the ways a test might fail.
Mixins
~~~~~~
Do not define setUp and tearDown directly in a mixin. This is the path to
madness. Instead, define a :func:`myMixinNameSetUp` and
:func:`myMixinNameTearDown`, and call them explicitly from the subclass's
:meth:`setUp` and :meth:`tearDown`. This makes it perfectly clear what is being
set up and torn down from a simple analysis of the test case.
Keeping State
~~~~~~~~~~~~~
Python does not allow assignment to anything but the innermost local scope or
the global scope with the ``global`` keyword. This presents a problem when
creating nested functions::
def test_localVariable(self):
cb_called = False
def cb():
cb_called = True
cb()
self.assertTrue(cb_called) # will fail!
The ``cb_called = True`` assigns to a *different variable* than
``cb_called = False``. In production code, it's usually best to work around
such problems, but in tests this is often the clearest way to express the
behavior under test.
The solution is to change something in a common mutable object. While a simple
list can serve as such a mutable object, this leads to code that is hard to
read. Instead, use :class:`State`::
from buildbot.test.state import State
def test_localVariable(self):
state = State(cb_called=False)
def cb():
state.cb_called = True
cb()
self.assertTrue(state.cb_called) # passes
This is almost as readable as the first example, but it actually works.
