Module: Test::Unit

Defined in:


Test::Unit - Ruby Unit Testing Framework


Unit testing is making waves all over the place, largely due to the fact that it is a core practice of XP. While XP is great, unit testing has been around for a long time and has always been a good idea. One of the keys to good unit testing, though, is not just writing tests, but having tests. What’s the difference? Well, if you just write a test and throw it away, you have no guarantee that something won’t change later which breaks your code. If, on the other hand, you have tests (obviously you have to write them first), and run them as often as possible, you slowly build up a wall of things that cannot break without you immediately knowing about it. This is when unit testing hits its peak usefulness.

Enter Test::Unit, a framework for unit testing in Ruby, helping you to design, debug and evaluate your code by making it easy to write and have tests for it.


Test::Unit has grown out of and superceded Lapidary.


I like (and do my best to practice) XP, so I value early releases, user feedback, and clean, simple, expressive code. There is always room for improvement in everything I do, and Test::Unit is no exception. Please, let me know what you think of Test::Unit as it stands, and what you’d like to see expanded/changed/improved/etc. If you find a bug, let me know ASAP; one good way to let me know what the bug is is to submit a new test that catches it :-) Also, I’d love to hear about any successes you have with Test::Unit, and any documentation you might add will be greatly appreciated. My contact info is below.

Contact Information

A lot of discussion happens about Ruby in general on the ruby-talk mailing list (, and you can ask any questions you might have there. I monitor the list, as do many other helpful Rubyists, and you’re sure to get a quick answer. Of course, you’re also welcome to email me (Nathaniel Talbott) directly at [email protected], and I’ll do my best to help you out.


I’d like to thank…

Matz, for a great language!

Masaki Suketa, for his work on RubyUnit, which filled a vital need in the Ruby world for a very long time. I’m also grateful for his help in polishing Test::Unit and getting the RubyUnit compatibility layer right. His graciousness in allowing Test::Unit to supercede RubyUnit continues to be a challenge to me to be more willing to defer my own rights.

Ken McKinlay, for his interest and work on unit testing, and for his willingness to dialog about it. He was also a great help in pointing out some of the holes in the RubyUnit compatibility layer.

Dave Thomas, for the original idea that led to the extremely simple “require ‘test/unit’”, plus his code to improve it even more by allowing the selection of tests from the command-line. Also, without RDoc, the documentation for Test::Unit would stink a lot more than it does now.

Everyone who’s helped out with bug reports, feature ideas, encouragement to continue, etc. It’s a real privilege to be a part of the Ruby community.

The guys at RoleModel Software, for putting up with me repeating, “But this would be so much easier in Ruby!” whenever we’re coding in Java.

My Creator, for giving me life, and giving it more abundantly.


Test::Unit is copyright © 2000-2003 Nathaniel Talbott. It is free software, and is distributed under the Ruby license. See the COPYING file in the standard Ruby distribution for details.


This software is provided “as is” and without any express or implied warranties, including, without limitation, the implied warranties of merchantibility and fitness for a particular purpose.


Nathaniel Talbott. Copyright © 2000-2003, Nathaniel Talbott


The general idea behind unit testing is that you write a test method that makes certain assertions about your code, working against a test fixture. A bunch of these test methods are bundled up into a test suite and can be run any time the developer wants. The results of a run are gathered in a test result and displayed to the user through some UI. So, lets break this down and see how Test::Unit provides each of these necessary pieces.


These are the heart of the framework. Think of an assertion as a statement of expected outcome, i.e. “I assert that x should be equal to y”. If, when the assertion is executed, it turns out to be correct, nothing happens, and life is good. If, on the other hand, your assertion turns out to be false, an error is propagated with pertinent information so that you can go back and make your assertion succeed, and, once again, life is good. For an explanation of the current assertions, see Test::Unit::Assertions.

Test Method & Test Fixture

Obviously, these assertions have to be called within a context that knows about them and can do something meaningful with their pass/fail value. Also, it’s handy to collect a bunch of related tests, each test represented by a method, into a common test class that knows how to run them. The tests will be in a separate class from the code they’re testing for a couple of reasons. First of all, it allows your code to stay uncluttered with test code, making it easier to maintain. Second, it allows the tests to be stripped out for deployment, since they’re really there for you, the developer, and your users don’t need them. Third, and most importantly, it allows you to set up a common test fixture for your tests to run against.

What’s a test fixture? Well, tests do not live in a vacuum; rather, they’re run against the code they are testing. Often, a collection of tests will run against a common set of data, also called a fixture. If they’re all bundled into the same test class, they can all share the setting up and tearing down of that data, eliminating unnecessary duplication and making it much easier to add related tests.

Test::Unit::TestCase wraps up a collection of test methods together and allows you to easily set up and tear down the same test fixture for each test. This is done by overriding #setup and/or #teardown, which will be called before and after each test method that is run. The TestCase also knows how to collect the results of your assertions into a Test::Unit::TestResult, which can then be reported back to you… but I’m getting ahead of myself. To write a test, follow these steps:

  • Make sure Test::Unit is in your library path.

  • require ‘test/unit’ in your test script.

  • Create a class that subclasses Test::Unit::TestCase.

  • Add a method that begins with “test” to your class.

  • Make assertions in your test method.

  • Optionally define #setup and/or #teardown to set up and/or tear down your common test fixture.

  • You can now run your test as you would any other Ruby script… try it and see!

A really simple test might look like this (#setup and #teardown are commented out to indicate that they are completely optional):

require 'test/unit'

class TC_MyTest < Test::Unit::TestCase
  # def setup
  # end

  # def teardown
  # end

  def test_fail
    assert(false, 'Assertion was false.')

Test Runners

So, now you have this great test class, but you still need a way to run it and view any failures that occur during the run. This is where Test::Unit::UI::Console::TestRunner (and others, such as Test::Unit::UI::GTK::TestRunner) comes into play. The console test runner is automatically invoked for you if you require ‘test/unit’ and simply run the file. To use another runner, or to manually invoke a runner, simply call its run class method and pass in an object that responds to the suite message with a Test::Unit::TestSuite. This can be as simple as passing in your TestCase class (which has a class suite method). It might look something like this:

require 'test/unit/ui/console/testrunner'

Test Suite

As more and more unit tests accumulate for a given project, it becomes a real drag running them one at a time, and it also introduces the potential to overlook a failing test because you forget to run it. Suddenly it becomes very handy that the TestRunners can take any object that returns a Test::Unit::TestSuite in response to a suite method. The TestSuite can, in turn, contain other TestSuites or individual tests (typically created by a TestCase). In other words, you can easily wrap up a group of TestCases and TestSuites like this:

require 'test/unit/testsuite'
require 'tc_myfirsttests'
require 'tc_moretestsbyme'
require 'ts_anothersetoftests'

class TS_MyTests
  def self.suite
    suite =
    suite << TC_MyFirstTests.suite
    suite << TC_MoreTestsByMe.suite
    suite << TS_AnotherSetOfTests.suite
    return suite

Now, this is a bit cumbersome, so Test::Unit does a little bit more for you, by wrapping these up automatically when you require ‘test/unit’. What does this mean? It means you could write the above test case like this instead:

require 'test/unit'
require 'tc_myfirsttests'
require 'tc_moretestsbyme'
require 'ts_anothersetoftests'

Test::Unit is smart enough to find all the test cases existing in the ObjectSpace and wrap them up into a suite for you. It then runs the dynamic suite using the console TestRunner.


I’d really like to get feedback from all levels of Ruby practitioners about typos, grammatical errors, unclear statements, missing points, etc., in this document (or any other).

Defined Under Namespace

Modules: Assertions, Collector, UI, Util Classes: AssertionFailedError, AutoRunner, Error, Failure, TestCase, TestResult, TestSuite

Constant Summary collapse


Class Method Summary collapse

Class Method Details

.run=(flag) ⇒ Object

If set to false Test::Unit will not automatically run at exit.

# File 'lib/test/unit.rb', line 265

  @run = flag


Automatically run tests at exit?


  • (Boolean)

# File 'lib/test/unit.rb', line 270

  @run ||= false