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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" lang="en"><head><title>Twisted Documentation: Unit Tests in Twisted</title><link href="../../howto/stylesheet.css" type="text/css" rel="stylesheet" /></head><body bgcolor="white"><h1 class="title">Unit Tests in Twisted</h1><div class="toc"><ol><li><a href="#auto0">Unit Tests in the Twisted Philosophy</a></li><li><a href="#auto1">What to Test, What Not to Test</a></li><li><a href="#auto2">Running the Tests</a></li><ul><li><a href="#auto3">How</a></li><li><a href="#auto4">When</a></li></ul><li><a href="#auto5">Adding a Test</a></li><li><a href="#auto6">Skipping tests, TODO items</a></li><ul><li><a href="#auto7">.todo and Testing New Functionality </a></li></ul><li><a href="#auto8">Associating Test Cases With Source Files</a></li><li><a href="#auto9">Links</a></li></ol></div><div class="content">Each unit test tests one bit of functionality in the

software. Unit tests are entirely automated and complete quickly.

Unit tests for the entire system are gathered into one test suite,

and may all be run in a single batch. The result of a unit test

is simple: either it passes, or it doesn't. All this means you

can test the entire system at any time without inconvenience, and

quickly see what passes and what fails.<h2>Unit Tests in the Twisted Philosophy<a name="auto0"></a></h2>The Twisted development team

adheres to the practice of <a href="http://c2.com/cgi/wiki?ExtremeProgramming">Extreme

Programming</a> (XP), and the usage of unit tests is a cornerstone

XP practice. Unit tests are a tool to give you increased

confidence. You changed an algorithm -- did you break something?

Run the unit tests. If a test fails, you know where to look,

because each test covers only a small amount of code, and you know

it has something to do with the changes you just made. If all the

tests pass, you're good to go, and you don't need to second-guess

yourself or worry that you just accidently broke someone else's

program.<h2>What to Test, What Not to Test<a name="auto1"></a></h2><blockquote>You don't have to write a test for every single

method you write, only production methods that could possibly break.</blockquote>-- Kent Beck, <cite>Extreme Programming Explained</cite>, p. 58.<h2>Running the Tests<a name="auto2"></a></h2><h3>How<a name="auto3"></a></h3>From the root of the Twisted source tree, run:<pre class="shell">

$ bin/trial -R twisted

</pre>You'll find that having something like this in your emacs init

files is quite handy:<pre class="elisp">

(defun runtests () (interactive)

(compile "python /somepath/Twisted/bin/trial -R /somepath/Twisted"))

(global-set-key [(alt t)] 'runtests)

</pre><h3>When<a name="auto4"></a></h3>Always always always be sure <a href="http://www.xprogramming.com/xpmag/expUnitTestsAt100.htm">all the

tests pass</a> before committing any code. If someone else

checks out code at the start of a development session and finds

failing tests, they will not be happy and may decide to hunt

you down.Since this is a geographically dispersed team, the person who

can help you get your code working probably isn't in the room with

you. You may want to share your work in progress over the

network, but you want to leave the main Subversion tree in good working

order. So <a href="http://svnbook.red-bean.com/svnbook/ch04.html">use a branch</a>,

and merge your changes back in only after your problem is solved

and all the unit tests pass again.<h2>Adding a Test<a name="auto5"></a></h2>Please don't add new modules to Twisted without adding tests

for them too. Otherwise we could change something which breaks

your module and not find out until later, making it hard to know

exactly what the change that broke it was, or until after a

release, and nobody wants broken code in a release.Tests go in Twisted/twisted/test/, and are named <code>test_foo.py</code>,

where <code>foo</code> is the name of the module or package being tested.

Extensive documentation on using the PyUnit framework for writing

unit tests can be found in the <a href="#links">links

section</a> below.One deviation from the standard PyUnit documentation: To ensure

that any variations in test results are due to variations in the

code or environment and not the test process itself, Twisted ships

with its own, compatible, testing framework. That just

means that when you import the unittest module, you will <code class="python">from twisted.trial import unittest</code> instead of the

standard <code class="python">import unittest</code>.As long as you have followed the module naming and placement

conventions, <code class="shell">trial</code> will be smart

enough to pick up any new tests you write.<h2>Skipping tests, TODO items<a name="auto6"></a></h2>Trial, the Twisted unit test framework, has some extensions which are

designed to encourage developers to add new tests. One common situation is

that a test exercises some optional functionality: maybe it depends upon

certain external libraries being available, maybe it only works on certain

operating systems. The important common factor is that nobody considers

these limitations to be a bug.To make it easy to test as much as possible, some tests may be skipped in

certain situations. Individual test cases can raise the

<code>SkipTest</code> exception to indicate that they should be skipped, and

the remainder of the test is not run. In the summary (the very last thing

printed, at the bottom of the test output) the test is counted as a

<q>skip</q> instead of a <q>success</q> or <q>fail</q>. This should be used

inside a conditional which looks for the necessary prerequisites:<pre class="python">

def testSSHClient(self):

if not ssh_path:

raise unittest.SkipTest, "cannot find ssh, nothing to test"

foo() # do actual test after the SkipTest

</pre>You can also set the <code>.skip</code> attribute on the method, with a string to

indicate why the test is being skipped. This is convenient for temporarily

turning off a test case, but it can also be set conditionally (by

manipulating the class attributes after they've been defined):<pre class="python">

def testThing(self):

dotest()

testThing.skip = "disabled locally"

</pre><pre class="python">

class MyTestCase(unittest.TestCase):

def testOne(self):

...

def testThing(self):

dotest()

if not haveThing:

MyTestCase.testThing.im_func.skip = "cannot test without Thing"

# but testOne() will still run

</pre>Finally, you can turn off an entire TestCase at once by setting the .skip

attribute on the class. If you organize your tests by the functionality they

depend upon, this is a convenient way to disable just the tests which cannot

be run.<pre class="python">

class SSLTestCase(unittest.TestCase):

...

class TCPTestCase(unittest.TestCase):

...

if not haveSSL:

SSLTestCase.skip = "cannot test without SSL support"

# but TCPTestCase will still run

</pre><h3>.todo and Testing New Functionality <a name="auto7"></a></h3>Two good practices which arise from the <q>XP</q> development process are

sometimes at odds with each other:<ul><li>Unit tests are a good thing. Good developers recoil in horror when

they see a failing unit test. They should drop everything until the test

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has been fixed.</li><li>Good developers write the unit tests first. Once tests are done, they

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write implementation code until the unit tests pass. Then they stop.</li></ul>These two goals will sometimes conflict. The unit tests that are written

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first, before any implementation has been done, are certain to fail. We want

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developers to commit their code frequently, for reliability and to improve

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coordination between multiple people working on the same problem together.

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While the code is being written, other developers (those not involved in the

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new feature) should not have to pay attention to failures in the new code.

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We should not dilute our well-indoctrinated Failing Test Horror Syndrome by

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crying wolf when an incomplete module has not yet started passing its unit

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tests. To do so would either teach the module author to put off writing or

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committing their unit tests until after all the functionality is

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working, or it would teach the other developers to ignore failing test

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cases. Both are bad things.<q>.todo</q> is intended to solve this problem. When a developer first

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starts writing the unit tests for functionality that has not yet been

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implemented, they can set the <code>.todo</code> attribute on the test

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methods that are expected to fail. These methods will still be run, but

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their failure will not be counted the same as normal failures: they will go

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into an <q>expected failures</q> category. Developers should learn to treat

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this category as a second-priority queue, behind actual test failures.As the developer implements the feature, the tests will eventually start

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passing. This is surprising: after all those tests are marked as being

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expected to fail. The .todo tests which nevertheless pass are put into a

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<q>unexpected success</q> category. The developer should remove the .todo

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tag from these tests. At that point, they become normal tests, and their

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failure is once again cause for immediate action by the entire development

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team.The life cycle of a test is thus:<ol><li>Test is created, marked <code>.todo</code>. Test fails: <q>expected

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failure</q>.</li><li>Code is written, test starts to pass. <q>unexpected success</q>.</li><li><code>.todo</code> tag is removed. Test passes. <q>success</q>.</li><li>Code is broken, test starts to fail. <q>failure</q>. Developers spring

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into action.</li><li>Code is fixed, test passes once more. <q>success</q>.</li></ol>Any test which remains marked with <code>.todo</code> for too long should

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be examined. Either it represents functionality which nobody is working on,

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or the test is broken in some fashion and needs to be fixed.<h2>Associating Test Cases With Source Files<a name="auto8"></a></h2>Please add a <code>test-case-name</code> tag to the source file that is

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covered by your new test. This is a comment at the beginning of the file

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which looks like one of the following:<pre class="python">

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# -*- test-case-name: twisted.test.test_defer -*-

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</pre>or<pre class="python">

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#!/usr/bin/python

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# -*- test-case-name: twisted.test.test_defer -*-

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</pre>This format is understood by emacs to mark <q>File Variables</q>. The

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intention is to accept <code>test-case-name</code> anywhere emacs would on

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the first or second line of the file (but not in the <code>File

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Variables:</code> block that emacs accepts at the end of the file). If you

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need to define other emacs file variables, you can either put them in the

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<code>File Variables:</code> block or use a semicolon-separated list of

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variable definitions:<pre class="python">

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# -*- test-case-name: twisted.test.test_defer; fill-column: 75; -*-

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</pre>If the code is exercised by multiple test cases, those may be marked by

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using a comma-separated list of tests, as follows: (NOTE: not all tools can

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handle this yet.. <code>trial --testmodule</code> does, though)<pre class="python">

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# -*- test-case-name: twisted.test.test_defer,twisted.test.test_tcp -*-

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</pre>The <code>test-case-name</code> tag will allow <code class="shell">trial

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--testmodule twisted/dir/myfile.py</code> to determine which test cases need

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to be run to exercise the code in <code>myfile.py</code>. Several tools (as

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well as <code>twisted-dev.el</code>'s F9 command) use this to automatically

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run the right tests.<h2 id="links">Links<a name="auto9"></a></h2><a name="links"></a><ul><li>A chapter on <a href="http://diveintopython.org/roman_divein.html">Unit Testing</a>

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in Mark Pilgrim's <a href="http://diveintopython.org">Dive Into

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Python</a>.</li><li><a href="http://www.python.org/doc/current/lib/module-unittest.html"><code>unittest</code></a> module documentation, in the <a href="http://www.python.org/doc/current/lib/">Python Library

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Reference</a>.</li><li><a href="http://c2.com/cgi/wiki?UnitTests">UnitTests</a> on

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the <a href="http://c2.com/cgi/wiki">PortlandPatternRepository

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Wiki</a>, where all the cool <a href="http://c2.com/cgi/wiki?ExtremeProgramming">ExtremeProgramming</a> kids hang out.</li><li><a href="http://www.extremeprogramming.org/rules/unittests.html">Unit

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Tests</a> in <a href="http://www.extremeprogramming.org">Extreme Programming: A Gentle Introduction</a>.</li><li>Ron Jeffries espouses on the importance of <a href="http://www.xprogramming.com/xpmag/expUnitTestsAt100.htm">Unit

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Tests at 100%</a>.</li><li>Ron Jeffries writes about the <a href="http://www.xprogramming.com/Practices/PracUnitTest.html">Unit

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Test</a> in the <a href="http://www.xprogramming.com/Practices/xpractices.htm">Extreme

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Programming practices of C3</a>.</li><li><a href="http://pyunit.sourceforge.net">PyUnit's homepage</a>.</li><li>The <a href="http://svn.twistedmatrix.com/cvs/trunk/twisted/?root=Twisted">twisted/test directory</a> in Subversion.</li></ul>See also <a href="../testing.html">Tips for writing tests for Twisted

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code</a>.</div><a href="../../howto/index.html">Index</a>Version: 2.4.0</body></html>

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