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Practical PHP Testing Patterns: Chained Tests

01.24.2011
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Sometimes you have tests where the fixtures can be recyled, instead of being recreated; but that's not all: sometimes, even the state reset we preach for Shared Fixture gets in the way instead of being mandatory.

For example, think of testing addition and removal of values over a collection object: to test the removal, you need a collection containing something. But this collection has to be populated somewhere, involving other methods which would have their own independent tests. If you look closer, however, you notice that the collection you need for testing values removal is just like the one that testAddsElements() produces when it succeeds, and simply throws away when the Test Methods returns...

The pattern

In these cases, the Chained Tests pattern can help you design isolated tests, by expressing the dependencies they implicitly have.

The application of this pattern results in a form of Api encapsulation: a test that tests Collection::remove() does not need to use Collection::add(), and won't be influenced by a change in the name or the signature of add(), which it does not focus on. The test just needs just a collection which has been already filled with values. Before "spolverare" unserialization and other strange techniques, just consider using the leftovers from a previous test.

Let's suppose we have two tests A and B, where B uses a variable produced in A. Of course, if A fails, the dependent Test Method B cannot (and shouldn't) be run; it will only add noise with an obvious failure which derives not from the behavior really under test there, but just from the fixture setup, which happens to be the previous test. If A instead runs correctly, B can be run.

Implementation

In PHPUnit, Chained Tests are supported when they are in the same Testcase Class. In this case, the Testcase Objects are not totally isolated anymore: you can return variables and objects from one test, in order for PHPUnit to pass them to other tests, that simply asks for them as dependencies.

You can do so by adding @depends annotation to dependent tests. Note that this annotation can also be used without passing fixtures around, just for the sake of not executing complex tests where the base ones have already failed.

@depends working by stopping the execution of tests down in the chain when the dependencies fail: they will be marked as skipped and signaled by an S in PHPUnit's ordinary output.

  • The tests which produce a fixture as a leftover must return that fixture at the end of the test.
  • The tests accepting fixtures, tagged with @depends, must accept parameters in the Test Method signatures.
  • In case of multiple parameters to return, you can return an array() and use list() in the dependent method to extract variables from it.

There is a known issue in running dependent tests with --filter: dependent tests would not run alone, so if you plan to use filter you may want to pick similar test names for couples of dependent/dependency in order to execute both of them when needed.

Example

Here we have a sample Testcase Class where the second and third tests are dependent on the first. Long chains of tests are not encouraged as it becomes increasingly difficult to tell what is the state of the fixture in the tests at the end of chain.

<?php
class ArrayTest extends PHPUnit_Framework_TestCase
{
public function testArrayAcceptsNewValues()
{
$array = array(4, 8);
// this won't work: if you chain two tests to this one
// the object won't be cloned
// $array = new ArrayObject(array(4, 8));
$array[] = 15;

$this->assertEquals(15, $array[2], 'The third element was not 15.');

return $array;
}

/**
* @depends testArrayAcceptsNewValues
*/
public function testArrayCanDeleteValues($array)
{
unset($array[2]);

$this->assertEquals(2, count($array), 'There are more than 2 elements in the array.');
}

/**
* @depends testArrayAcceptsNewValues
*/
public function testArrayAcceptsDuplicateValues($array)
{
$array[] = 15;

$this->assertEquals(4, count($array), 'There are less than 4 elements in the array.');
}
}

If we execute the whole test case, we obtain:

[10:40:55][giorgio@Desmond:~]$ phpunit txt/articles/chainedtest.php 
PHPUnit 3.5.5 by Sebastian Bergmann.

...

Time: 0 seconds, Memory: 5.00Mb

OK (3 tests, 3 assertions)

If we put a $this->fail() in the first test, instead, we get:

[10:41:40][giorgio@Desmond:~]$ phpunit txt/articles/chainedtest.php 
PHPUnit 3.5.5 by Sebastian Bergmann.

FSS

Time: 0 seconds, Memory: 5.00Mb

There was 1 failure:

1) ArrayTest::testArrayAcceptsNewValues

/home/giorgio/Dropbox/txt/articles/chainedtest.php:13

FAILURES!
Tests: 1, Assertions: 1, Failures: 1, Skipped: 2.

If we filter the second test, we cannot run it due to the missing dependency:

[10:49:25][giorgio@Desmond:~]$ phpunit --filter testArrayCan txt/articles/chainedtest.php 
PHPUnit 3.5.5 by Sebastian Bergmann.

S

Time: 0 seconds, Memory: 4.75Mb

OK, but incomplete or skipped tests!
Tests: 0, Assertions: 0, Skipped: 1.

But if we filter the third test, thanks to the well-chosen name, we get:

[10:49:16][giorgio@Desmond:~]$ phpunit --filter testArrayAccepts txt/articles/chainedtest.php 
PHPUnit 3.5.5 by Sebastian Bergmann.

..

Time: 0 seconds, Memory: 5.00Mb

OK (2 tests, 2 assertions)
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