Is it possible to instantiate a Ruby class without calling initialize?

Question

Some times when I write unit tests I need to instantiate a class without the initialize method being invoked. For instance when the constructor instantiates other classes that I will replace with stubs anyway. For instance:

class SomeClassThatIWillTest
  def initialize
    @client = GoogleAnalyticsClient.new
    @cache = SuperAdvancedCacheSystem.new
  end

  # ...
end

In a test I will probably replace both @client and @cache with stubs, so I'd rather the constructor was never invoked. Is there any black magic that can help me out with that?

Answer 1

Sure you can. Class#new is nothing more than a convenience method that saves you from having to allocate and initialize an object manually. Its implementation looks roughly like this:

class Class
  def new(*args, **kwargs, &blk)
    obj = allocate
    obj.send(:initialize, *args, **kwargs, &blk)
    obj
  end
end

You can just call Class#allocate manually instead, and not call initialize.

Answer 2

You should not change behavior of tested class in order to unit test it. If your class will have more actions in constructor you will have to mimic it every time. Your test will get tedious to maintain. Replace objects (or even classes) with doubles.

Maybe you could provide already created objects as an arguments to the constructor? It would allow you to use doubles without stubbing new method on classes.

If you are using rspec, you can:

GoogleAnalyticsClient.stub(new: double)
SuperAdvancedCacheSystem.stub(new: double)

Define your doubles to match expected interface, and voila! No dirty tricks needed.

Answer 3

What about subclassing SomeClassThatIWillTest and overwriting initialize in the subclass? No black magic involved ;)

This way you could even call the super initializer (to test its code, if its more than you showed us), and then alter @client and @cache afterwards.

Example of a MiniTest spec using this method:

describe MyTestClass do

  subject {
    Class.new(MyTestClass) {
      def initialize; end
    }
  }

  it "must do something" do
    subject.new.do_something.must_equal something
    # ...
  end

end