Unit testing by simplifying the problem: memoization

Jamie Hill December 18, 2008

I see unit testing as a way to test each possible snippet of functionality and route the code in question can take. With Ruby being such a dynamic language and allowing shortcuts to common problems, sometimes it can seem somewhat of a mystery, how to test these snippets of functionality.
Using Memoization as an example:

class MyClass
  def lazy_initialized_value
    @lazy_initialized_value ||= Expensive.request
  end
end

There are actually 3 separate snippets of functionality that need testing here, however it is not immediately obvious from the example. Lets be slightly more verbose about what is actually happening:

class MyClass
  def lazy_initialized_value
    @lazy_initialized_value = Expensive.request unless @lazy_initialized_value
    @lazy_initialized_value
  end
end

Now it is much easier to see the 3 steps the code should take:

Store result of expensive request in instance variable
Leave instance variable alone when it is already set
Return the value of the instance variable
Now we have this information, our tests become (using Mocha to mock external methods):

class Expensive; end
module Tests::MyClass
  # lazy_initialized_value
  # ----------------------
  class LazyInitializedValueTest &lt; Test::Unit::TestCase
    def test_should_respond
      assert_respond_to MyClass.new, :lazy_initialized_value
    end
    def test_should_store_result_of_expensive_request_in_instance_variable
      instance = MyClass.new
      Expensive.stubs(:request).with().returns('expensive value')
      instance.lazy_initialized_value
      assert_equal 'expensive value', instance.instance_variable_get('@lazy_initialized_value')
    end
    def test_should_return_value_of_instance_varable
      instance = MyClass.new
      instance.instance_variable_set '@lazy_initialized_value', 'the value'
      Expensive.stubs(:request)
      assert_equal 'the value', instance.lazy_initialized_value
    end
    def test_should_maintain_existing_instance_variable_value_when_already_set
      instance = MyClass.new
      instance.instance_variable_set '@lazy_initialized_value', 'existing value'
      Expensive.stubs(:request)
      instance.lazy_initialized_value
      assert_equal 'existing value', instance.instance_variable_get('@lazy_initialized_value')
    end
  end
end

Now we have these tests in place, we can go back and refractor the code ’til our heart’s content using all the tricks in the book but by simplifying the problem in the first place, it gives us a solid test suite and the confidence to make changes without breaking functionality.
If you were solving this problem test-first then you wouldn’t (but more likely, shouldn’t) have written the first example until re-factoring stage anyway, however when these shortcuts become engrained in your brain, it’s all too easy to forget what they are _actually_ doing.
So there we go, simplify the initial implementation, get a solid test suite in order, _then_ re-factor.