Stubbing :new Considered Harmful

Stubbing :new Considered Harmful

Eric Smith

February 08, 2010

Hi my name is Eric, and I have made a mess. There I said it. I’m not proud of it, but I believe a couple very loosely related things:

  • You only learn from your mistakes
  • Everybody makes mistakes, a craftsman cleans them up.

It is in the process of cleaning up the mess that I’ve been trying to evaluate the messy code in the first place. How is the code messy? Was it Test-Driven?

It was—or at least it attempted to be (we’ll get back to that). Was I lazy? Certainly not. Am I stupid? Crap I hope not. So how did we end up in this mess?

What’s the mess nimrod?

Recently Justin, our latest apprentice, wrote about attending a code review here. The feature he is referring to is part of one of our best client’s systems, where we integrated with several webservices, making them look seamless in our application.

This was an extremely difficult problem, and our code bent in a dozen painful ways for a solution, and has since become rigid. Changes have stopped being done in the nice Test-Code-Refactor mode, and instead are being done by trying a change, trying the product, then backing out the change and making the test pass.

While we identified several ways to improve the code, such as better naming and adding a facade between some of our interfaces, one stated goal was to “make the tests readable.” See the problem isn’t that it’s not tested—it’s that you can’t follow the tests at all. Let’s look at an example, modified to protect the guilty:

it "should call a webservice when its complete" do
		typedObj = mock("Obj", :no_follow_ups? => false, :null_object => true)
		@product.page_cache = [@page]
		@widget.follow_ups({}, {:application_id => "123", :group => "group"})

This probably isn’t the worst example—if you look closely you can see that we’re testing that call_webservice needs to get called when ‘its complete.’ But what complete? I don’t see a stubbed object that returns complete.

Also I have no idea what @page, @widget, @productor :create_typed_obj are doing here. When I want to write the next test I have to guess at intent, and start playing with variables.

What this is trying to test is that a webservice caller object receives call_webservice when the widget (an object on the screen) is called with certain variables. I’d tell you what those variables are, but I truly don’t know.

It doesn’t get better. Let’s look at where some of those variables are coming from. These are excerpts from a setup method that is too long.

@caller = mock('caller', :call_webservice => nil)
@page_factory = mock_model(PageFactory, :acquire => @page)

So by my count we’ve stubbed new three times, two finders, and an acquire all of which inject yet another dependency into the code. Thus the premise of the article stubbing new considered harmful.

Is it really harmful?

Dependencies in static languages are generally more onerous than what we’ve got here, sometimes extremely painful although usually it can be accomplished with simple setters and constructors.

In Ruby this isn’t the case-you can just stub new and create the object the way the user intended, thereby continuing to test. This can be a very bad thing because it’s a moment where you need to pause.

Do you need this dependency? Should you be wrapping dependencies? Do you need a facade or wrapper object for a couple dependencies? The difficulty of DI in other languages enforces that pause, but Ruby does not. It lets you stub anything, only to one day wonder why you have such a mess on your hands.

It’s important to remember that Dependency Inversion existed before Unit Testing, and isn’t just a Unit Testing technique. Indeed we often sell TDD as a way to make you have better designs, and yet here it failed me because I can so simply call new and continue testing. This is a mistake.

To paraphrase Jurassic Park, just because you can easily stub new doesn’t mean you should. Would you create a constructor that took five objects? Then why would you stub :new (or finders, or factory methods) five times.

Would you pass in an object to the constructor that’s only used in one method? No—that’s not cohesive. By stubbing :new you can slowly, and easily, introduce dependency after dependency creating an indecipherable mess despite writing tests.

Worse you can do this easily, without suffering the traditional pain of tightly coupled code. It’s only later that you’ll feel the awful, horrible, indescribable pain. Trust me it’s bad.

Am I really advocating stopping stubbing :new? No not really. It makes perfect since to just call :new when you can, and in many cases factories are a way to get around the limitations of a static language. What I am saying is that each time you do stub!(:new) think! You have a dependency to manage, so manage it. It’s your job you know.

Eric Smith

Principal Crafter

Eric Smith is a fan of the Chicago Bears, Chicago Cubs, and Bruce Springsteen; and he’s the recent author of Game Development with Rust and WebAssembly, published by Packt. Eric is a consummate polyglot, with more than a decade of experience leading development teams and delivering software for global enterprise systems. He has also delivered native Android and iOS apps at every stage of their lifecycle.