Home > Programming > Breaking up your tests

Breaking up your tests

I thought I’d pass on a simple tip that was given to me by a colleague last year. When writing tests, it can help to break it into sections called “Given”, “When”, and “Then”, indicated by comments:

  • “Given” – set up the criteria for your test
  • “When” – execute the code for the test
  • “Then” – inspect the results, and make assertions

Let’s say you were writing a unit test for the get() method of LWP::UserAgent (as an example, obviously you wouldn’t really do this, unless you are the maintainer of that module).

Assuming the use of Test::Class, the code might look like:

sub test_get : Tests(3) {
    my $self = shift;

    # GIVEN
    my $ua = LWP::UserAgent->new;

    # WHEN
    my $response = $ua->get('http://myurl.com/');

    # THEN
    isa_ok($response, 'HTTP::Response', "Returned object is a HTTP::Response");
    is($response->is_success, 1, "Response was successful");
    is(defined $response->content, 1, "Something in the response's content");

This is a fairly simple test; in reality you’d probably have a lot more assertions in the “Then” section, and you’d probably be doing some mocking (making the ‘Given’ section longer). But for the purposes of this example, you can see how the three parts of the test are apparent at a glance. Also, it makes it clear that we’re only testing one thing in this test method (another good practice). Otherwise we’d end up with multiple Given/When/Then blocks.

So a simple tip maybe, but one I find I now use in all my unit testing.

Categories: Programming Tags: ,
  1. rho
    June 6, 2009 at 7:15 pm

    What is exactly the difference between “GIVEN” and “WHEN”? Just that
    the “GIVEN” part lives longer than the “WHEN”?


    • Mutant
      June 6, 2009 at 7:37 pm

      ‘Given’ has the things you need to do to setup the test, but are not actually part of the test. (That might not be too clear from the example I give)… to test the get() method, you need to instantiate a new LWP::UserAgent object, but you’re not actually testing new() in this test.

      ‘When’ has the execution of the actual code needed for the test. In most cases, if your code is well-factored, you’d expect this to be only one line, i.e. the call to the method you’re testing. So when you go back and look at the code, you can quickly see exactly what it is you’re testing by glancing at the ‘When’ section.

      Hope that makes sense 🙂

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