Testing Your Privates

This article is part of a series on testing untestable code:

No, not those privates. If you need help with those, this book might help.

One question I get over and over again when talking about Unit Testing is this:

"How do I test the private attributes and methods of my objects?"

Lets assume we have a class Foo:

<?php
class Foo
{
    private $bar = 'baz';
 
    public function doSomething()
    {
        return $this->bar = $this->doSomethingPrivate();
    }
 
    private function doSomethingPrivate()
    {
        return 'blah';
    }
}
?>

Before we explore how protected and private attributes and methods can be tested directly, lets have a look at how they can be tested indirectly.

The following test calls the testDoSomething() method which in turn calls the doSomethingPrivate() method:

<?php
class FooTest extends PHPUnit_Framework_TestCase
{
    /**
     * @covers Foo::doSomething
     * @covers Foo::doSomethingPrivate
     */
    public function testDoSomething()
    {
        $foo = new Foo;
        $this->assertEquals('blah', $foo->doSomething());
    }
}
?>

The test above assumes that testDoSomething() only works correctly when testDoSomethingPrivate() works correctly. This means that we have indirectly tested testDoSomethingPrivate(). The problem with this approach is that when the test fails we do not know directly where the root cause for the failure is. It could be in either testDoSomething() or testDoSomethingPrivate(). This makes the test less valuable.

PHPUnit supports reading protected and private attributes through the PHPUnit_Framework_Assert::readAttribute() method. Convenience wrappers such as PHPUnit_Framework_TestCase::assertAttributeEquals() exist to express assertions on protected and private attributes:

<?php
class FooTest extends PHPUnit_Framework_TestCase
{
    public function testPrivateAttribute()
    {
        $this->assertAttributeEquals(
          'baz',  /* expected value */
          'bar',  /* attribute name */
          new Foo /* object         */
        );
    }
}
?>

PHP 5.3.2 introduces the ReflectionMethod::setAccessible() method to allow the invocation of protected and private methods through the Reflection API:

<?php
class FooTest extends PHPUnit_Framework_TestCase
{
    /**
     * @covers Foo::doSomethingPrivate
     */
    public function testPrivateMethod()
    {
        $method = new ReflectionMethod(
          'Foo', 'doSomethingPrivate'
        );
 
        $method->setAccessible(TRUE);
 
        $this->assertEquals(
          'blah', $method->invoke(new Foo)
        );
    }
}
?>

In the test above we directly test testDoSomethingPrivate(). When it fails we immediately know where to look for the root cause.

I agree with Dave Thomas and Andy Hunt, who write in their book "Pragmatic Unit Testing":

"In general, you don't want to break any encapsulation for the sake of testing (or as Mom used to say, "don't expose your privates!"). Most of the time, you should be able to test a class by exercising its public methods. If there is significant functionality that is hidden behind private or protected access, that might be a warning sign that there's another class in there struggling to get out."

So: Just because the testing of protected and private attributes and methods is possible does not mean that this is a "good thing".