Does the self-shunt testing pattern violate the Single Responsibility Principle?

Question

I've used the self-shunt unit testing pattern a few times over the years. As I was explaining it to someone recently they argued that it violated the SRP. The argument is that the test class can now be changed for one of two reasons: when the test changes, or when the method signature on the interface that the test is implementing changes. After thinking about it for a while, it seems like this is a correct assessment, but I wanted to get other peoples' opinions. Thoughts?

Reference: http://www.objectmentor.com/resources/articles/SelfShunPtrn.pdf

Answer 1

My take on this is that the test class technically violates the SRP, but it doesn't violate the spirit of SRP. The alternative to self-shunting is to have a mock class separate from the test class.

With the separate mock class you might think that it's all self contained and satisfies the SRP, however the semantic coupling to the mock class's attributes is still there. So, really, we didn't achieve any meaningful separation.

Taking the example out of the PDF:

public class ScannerTest extends TestCase implements Display
{
  public ScannerTest (String name) {
    super (name);
  }
  public void testScan () {
    // pass self as a display
    Scanner scanner = new Scanner (this);
    // scan calls displayItem on its display
    scanner.scan ();
    assertEquals (new Item (“Cornflakes”), lastItem);
  }
  // impl. of Display.displayItem ()
  void displayItem (Item item) {
    lastItem = item;
  }
  private Item lastItem;
}

Now we make a Mock:

public class DisplayMock implements Display
{
  // impl. of Display.displayItem ()
  void displayItem (Item item) {
    lastItem = item;
  }

  public Item getItem() {
     return lastItem;
  }
  private Item lastItem;
}

public class ScannerTest extends TestCase
{
  public ScannerTest (String name) {
    super (name);
  }
  public void testScan () {
    // pass self as a display
    DisplayMock dispMock = new DisplayMock();
    Scanner scanner = new Scanner (dispMock );
    // scan calls displayItem on its display
    scanner.scan ();
    assertEquals (new Item (“Cornflakes”), dispMock.GetItem());
  }
}

In practical terms (IMHO) the higher coupling of TestClass to DisplayMock is a greater evil than the violation of the SRP for TestClass. Besides, with the use mocking frameworks, this problem goes away completely.

EDIT I have just encountered a brief mention of self-shunt pattern in Robert C. Martin's excellent book Agile Principles, Patterns, and Practices in C#. Here is the snippet out of the book:

We can accomplish this by using an abstract interface for the database. One implementation of this abstract interface uses the real database. Another implementation is test code written to simulate the behavior of the database and to check that the database calls are being made correctly. Figure 29-5 shows the structure. The PayrollTest module tests the PayrollModule by making calls to it and also implements the Database interface so that it can trap the calls that Payroll makes to the database. This allows PayrollTest to ensure that Payroll is behaving properly. It also allows PayrollTest to simulate many kinds of database failures and problems that are otherwise difficult to create. This is a testing pattern known as SELF-SHUNT, also sometimes known as mocking or spoofing.

So, the guy who coined the SRP (which is talked about in great detail in the same book) has no qualms using self-shunt pattern. In light of that, I'd say you are pretty safe from the OOP (Objected Orientated Police) when using this pattern.

Answer 2

In my opinion it is a violation, but a very minor one.

You test class is now a test class and a dependency for whatever you are testing.

However, is that a bad thing? For a couple of simple tests, probably not. As your number of test cases grows, you'll probably want to refactor and use a mock class to separate some of the concerns. (As the link you pasted says, self-shunt is a stepping stone to mocking). But if the number of test cases remains static and low, what's the problem?

I think a little pragmatism is required. Does it violate the SRP? Yes, but I'm guessing probably not as much as some of the code in the system you're testing. Do you need to do anything about it? No, not as long as the code is clear and maintainable, which for me is always the bottom line. SRP is a guideline, not a rule.

Answer 3

If the interface being implemented or shunted is changed, it's relatively likely that the test suite will have to change as well. So I don't really see it as a violation of SRP.

Answer 4

I prefer to have a little more control over the mocks/stubs that I'm creating. When I tried using the self shunt pattern I ended up making my test class more complex. By creating the mocks as locals within a test method I ended up having cleaner code.

FWIW unless you're using something powerful like C# (or python or an equivalent) your test code will change when you change an interface.