What happens when RSpec runs, or, what I think about testing with blocks

Written on February 18 2013 at 21:41

Welcome to part two of the the post series which will hopefully cauterize the bleeding stump that is my Ruby Testing Quest.

This time, we will take a not-too-deep dive into how RSpec works. Last time we looked at MiniTest; if you haven’t already read that, it might be a better place to start than this.

Let’s get going.

A simple RSpec example

Here’s a simple RSpec example.

require "rspec/autorun"

describe "an object" do
  before :all do
    @shared_thing = Object.new
  end

  before :each do
    @something = Object.new
  end

  it "should be an Object" do
    @something.should be_an(Object)
  end

  describe "compared to another object" do
    before :each do
      @other = Object.new
    end

    it "should not be equal" do
      @something.should_not == @other
    end
  end

  after do
    @something = nil
  end
end

This is obviously extremely dull and pointless – just like the minitest one – but it contains just enough to exercise the major parts of RSpec that I care about. It’s actually slightly more sophisticated than the example that I used for MiniTest, because RSpec provides a couple of notable features that MiniTest doesn’t provide. Specifically, these are before :all setup blocks, and nested groups of tests12.

I’m not particularly interested in looking at the other obvious distinguishing features of RSpec, like matchers and the BDD-style “should” language, as these aren’t actually a part of the core RSpec implementation3.

The two hallmark attributes here that I am interested in are:

Running the test spec

The simplest way of running this spec would be to save as something_spec.rb and run it from the command-line.

$ ruby something_spec.rb
..

Finished in 0.00198 seconds
2 examples, 0 failures
[Finished in 0.5s]

So – what’s actually happening here?

Autorun

As with the minitest example, the first line loads a special file within the test library that not only loads the library, but also installs an at_exit hook for Ruby to run when the interpreter exists.

In RSpec’s case, this is defined in RSpec::Core::Runner.autorun. This calls RSpec::Core::Runner.run with ARGV and the stderr and stdout streams.

In contrast with MiniTest, RSpec parses the options at this point, and will try to determine whether or not to launch using DRb. In most cases it will create an instance of RSpec::Core::CommandLine with the parsed options, and then calls run on that instance.

Within the run method, some setup happens (mostly preamble to be output by the reporter, which is set via the configuration). Then we iterate through all of the “example groups”, returned by RSpec::world.example_groups4.

Let’s take a diversion to see how things actually get into RSpec::world.example_groups.

Your example groups

Consider our example spec again. At the top we have a call to describe:

describe "an object" do

The describe method is actually defined within the module RSpec::Code::DSL, but this module is extended into self at the top level of the running Ruby interpreter (which is main, a singleton instance of Object), making the methods in that module available to call in your spec files. You can actually see all of the modules that have been extended into this instance:

require "rspec/core"

self.class.ancestors
# => [Object, Kernel, BasicObject]

class << self
  ancestors
  # => [RSpec::Core::SharedExampleGroup, RSpec::Core::DSL, Object, Kernel, BasicObject]
end

# also, self.singleton_class.ancestors in Ruby 1.9

From this we can tell that the ancestors of Object are still just Kernel and BasicObject, but the ancestors of the specific instance main includes a few extra modules from RSpec. Anyway, moving on…

describe and RSpec::Core::ExampleGroup

The describe method in RSpec::Core::DSL passes its arguments straight through to RSpec::Core::ExampleGroup.describe. This is where things get a little interesting. Within this inner describe method, a subclass of RSpec::Code::ExampleGroup is created, and given a generated name.

describe "a thing" do
  # your tests, um, I mean specs
end
RSpec::Core::ExampleGroup.constants
# => [:Nested_1, :Extensions, :Pretty, :BuiltIn, :DSL, :OperatorMatcher, :Configuration]

The class that was created is there: Nested_1. For each describe at the top level, you’ll have a new generated class:

describe "a thing" do
  # your specs
end
describe "another thing" do
  # more specs
end
RSpec::Core::ExampleGroup.constants
# => [:Nested_1, :Nested_2, :Extensions, :Pretty, :BuiltIn, :DSL, :OperatorMatcher, :Configuration]

After each subclass is created, it is “set up” via the set_it_up method, which roughly speaking adds a set of metadata about the group (such as which file and line it was defined upon, and perhaps some information about the class if it was called in the form describe SomeClass do ...), and stashes that within the created subclass.

module_eval

More importantly, however, the block which was passed to describe is evaluated against this new subclass using module_eval.

The effect of using module_eval against a class is that the contents of the passed block are evaluated essentially as if they were within the definition of that class itself:

class Lionel; end

Lionel.module_eval do
  def hello?
    "is it me you're looking for?"
  end
end

Lionel.new.hello?
# => "is it me you're looking for?"

You can see above that the behaviour is effectively the same as if we’d defined the hello? method within the Lionel class without any “metaprogramming magic”5.

It’s because of module_eval that you can define methods within example groups:

describe "a thing" do
  def invert_phase_polarity
    # waggle the flux capacitor or something
  end
end

RSpec::Core::ExampleGroup::Nested_1.instance_methods(false) # false means don't include methods from ancestors
# => [:invert_phase_polarity]

These methods are then effectively defined as part of the Nested_1 class that we are implicitly creating. This means that methods defined in this way can be called from within your specs:

describe "a method in an example group" do
  def the_method_in_question
    :result
  end

  it "can be called from within a spec" do
    the_method_in_question.should == :result
  end
end

We’ll see how this actually works a bit later. Knowing that the contents of the describe block are effectively evaluated within a class definition also explains what’s happening when the before methods are called:

  before :all do
    @shared_thing = Object.new
  end

  before :each do
    @something = Object.new
  end

Because this is evaluated as if it was written in a class definition, then before must be a method available on the ExampleGroup class. And indeed it is – RSpec::Code::ExampleGroup.before.

Well, almost.

Hooks

The before method actually comes from the module RSpec::Core::Hooks, which is extended into ExampleGroup. RSpec has a very complicated behind-the-scenes hook registry, which for the purposes of brevity I’m not going to inspect here..

The before method registers its block within that registry, to be retrieved later when the specs actually run.

Because I’m not going to really look too deeply at hooks, the call to the after method works in pretty much the same way. Here it is though, just because:

  after do
    @something = nil
  end

The spec itself

The next method that’s module_eval‘d within our ExampleGroup subclass is the it:

  it "should be an Object" do
    @something.should be_an(Object)
  end

Users of RSpec will know that you can call a number of methods to define a single spec: it, specify example, and others with additional meaning like pending or focus. These methods are actually all generated while RSpec is being loaded, by calls to define_example_method within the class definition of ExampleGroup. For simplicity’s sake (pending and focussed specs are somewhat outwith the remit of this exploration), we’ll only look at the simplest case.

When it is called, more metadata is assembled about the spec (again, including the line and file), and then both this metadata and the block are passed to RSpec::Core::Example.new, which stashes them for later.

Nesting

Within our outer example group, we’ve nested another group:

  describe "compared to another object" do
    before :each do
      @other = Object.new
    end

    it "should not be equal" do
      @something.should_not == @other
    end
  end

Just as the top-level call to describe invokes a class method on RSpec::Core::ExampleGroup, this call will be invoked against the subclass of ExampleGroup (i.e. Nested_1) that our outer group defined. Accordingly, each call to describe defines a new subclass6, stored as a constant within the top-level class: Nested_1::Nested_1. This subclass is stored within an array of children in the outer Nested_1 class.

Within the definition, our before and it calls evaluate as before.

Your spec, as objects

So, for every describe, a new subclass of ExampleGroup is created, with calls to before and after registering hooks within that subclass, and then each it call defines a new instance of RSpec::Core::Example, and these are stored in an array called examples within that subclass.

We can even take a look at these now, for a simplified example:

group = describe "a thing" do
  it "should work" do
    (1 + 1).should_not equal(2)
  end
end

group
# => RSpec::Core::ExampleGroup::Nested_1

group.examples
# => [#<RSpec::Core::Example:0x007ff2523db048
#      @example_block=#<Proc:0x007ff2523db110@example_spec.rb:7>,
#      @options={},
#      @example_group_class=RSpec::Core::ExampleGroup::Nested_1,
#      @metadata={
#        :example_group=>{
#          :description_args=>["a thing"],
#          :caller=>["/Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-212.#   2/lib/rspec/core/example_group.rb:291:in `set_it_up'", "/Users/james/Code/experiments/rspec-ivestigation/.#   bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:243:in `ubclass'", #   "/Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.#  2/lib/rspec/core/example_group.rb:230:in `describe'", "/Users/james/Code/experiments/rspec-investigation/.#  bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.2/lib/rspec/core/dsl.rb:18:in `describe'", "example_spec.#   r:6:in `<main>'"]
#        },
#        :example_group_block=>#<Proc:0x007ff255c11430@example_spec.rb:6>,
#        :description_args=>["should work"],
#        :caller=>["/Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-2.1.#   2/lib/rspec/core/metadata.rb:181:in `for_example'", "/Users/james/Code/experiments/rspec-investigation/.#  bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.2/lib/rspec/core/example.rb:81:in `initialize'", #   "Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.#   2lib/rspec/core/example_group.rb:67:in `new'", "/Users/james/Code/experiments/rspec-investigation/.#   bndle/gems/ruby/1.9.1/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:67:in `it'", "example_spec.#   r:7:in `block in <main>'", "/Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.#   1gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:244:in `module_eval'", #   "Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.#   2lib/rspec/core/example_group.rb:244:in `subclass'", "/Users/james/Code/experiments/rspec-investigation/.#   bndle/gems/ruby/1.9.1/gems/rspec-core-2.12.2/lib/rspec/core/example_group.rb:230:in `describe'", #   "Users/james/Code/experiments/rspec-investigation/.bundle/gems/ruby/1.9.1/gems/rspec-core-2.12.#   2lib/rspec/core/dsl.rb:18:in `describe'", "example_spec.rb:6:in `<main>'"]
#      },
#      @exception=nil,
#      @pending_declared_in_example=false>
#    ]

Where example groups are nested, further subclasses are created, and stored in an array of children within their respective parent groups.

Almost there!

Phew. The detour we took when looking at this aspect of minitest was much shorter, but now that we understand what happened when our actual spec definition was evaluated, we can return to RSpec running and see how it’s actually exercised.

As we saw above, the describe method returns the created subclass of RSpec::Core::ExampleGroup, and when that is returned back in RSpec::Code::DSL#describe, the register method is called on it. This calls world.register with that class as an argument, where world is returned by RSpec.world and is an instance of RSpec::Core::World, which acts as a kind of global object to contain example groups, configuration and that sort of thing.

Calling register on the World instance stashes our Nested_1 class in an example_groups array within that world.

Our diversion is complete! You deserve a break. Go fetch a cup of your preferred delicious beverage, you have earned it!

Back in RSpec

OK, pop your brain-stack back until we’re in RSpec::Core::Commandline#run again. Our reporter did its preamble stuff, and we were iterating through @world.example_groups, whose origin we now understand.

For each example group, the run method is called on that class, with the reporter instance passed as an argument.

This gets a bit intricate, so I’m going to step through the method definition itself (for version 2.12.2) to help anchor things.

def self.run(reporter)
  if RSpec.wants_to_quit
    RSpec.clear_remaining_example_groups if top_level?
    return
  end

RSpec has a “fail fast” mode, where any single example failure will cause the execution of specs to finish as quickly as possible. Here, RSpec is checking whether anything has triggered this.

  reporter.example_group_started(self)

Next, the reporter is notified that an example group is about to start. The reporter can use this information to print out the name of the group, for example.

  begin
    run_before_all_hooks(new)

The run of the examples is wrapped in a block so it can catch any exceptions and handle them gracefully as you might expect.

The before :all hooks

The call to run_before_all_hooks is very interesting though, and worth exploring. A new instance of the example group is created. It is then passed into this method, where any “before all” blocks are evaluated against that instance, and then the values of any instance variables are stashed.

Consider our original example:

  before :all do
    @shared_thing = Object.new
  end

Given this, we’ll stash the value of @shared_thing (and the fact that it was called @shared_thing) for later use.

It’s actually quite easy to inspect the instance variables of an object in Ruby; try calling instance_variables, instance_variable_get and instance_variable_set on some objects in an IRB session:

class Thing
  def initialize
    @value = Object.new
  end
end

class OtherThing
end

thing = Thing.new
thing.instance_variables # => [:@value]
ivar = thing.instance_variable_get(:@value) # => #<Object:0x007fe43a050e30>

other_thing = OtherThing.new
other_thing.instance_variables # => []

other_thing.instance_variable_set(:@transplanted_value, ivar)
other_thing.instance_variables # => [:@transplanted_value]
other_thing.instance_variable_get(:@transplanted_value) # => #<Object:0x007fe43a050e30>

As you can see above, we can poke around with the innards of objects to our heart’s content. Who needs encapsulation, eh?

Why did RSpec have to create an instance of the example group class, only to throw it away after the before :all blocks have been evaluated? Because RSpec needs to evaluate the block against an instance of the example group so that it has access to the same scope (e.g. can call the same methods) as any of the specs themselves.

Running the example

Now we’re finally ready to run the examples:

    result_for_this_group = run_examples(reporter)

To understand this, we need to look at the definition of run_examples:

def self.run_examples(reporter)
  filtered_examples.ordered.map do |example|
    next if RSpec.wants_to_quit
    instance = new
    set_ivars(instance, before_all_ivars)
    succeeded = example.run(instance, reporter)
    RSpec.wants_to_quit = true if fail_fast? && !succeeded
    succeeded
  end.all?
end

This method iterates over each Example that was stored in the examples array earlier, filtering them according to any command-line parameters (though we are ignoring that here). The most relevant part for us lies in the middle:

    instance = new
    set_ivars(instance, before_all_ivars)
    succeeded = example.run(instance, reporter)

A striking parallel with MiniTest

Another new instance of the ExampleGroup subclass is created. Remember, RSpec created one instance of the class for the before :all blocks, but now it’s creating a fresh instance for this specific spec to be evaluated against.

Thinking back to how MiniTest works, there’s a striking parallel: where MiniTest would instantiate a new instance of the MiniTest::Unit::TestCase for each test method, RSpec is creating a new instance of the ExampleGroup subclass to evaluate each Example block against.

Instances of this class are used so that any methods defined as part of the spec definition are implicitly available as methods to be called in the “setup” and “test” bodies (see the module_eval section above). Not so different after all, eh?

Next, the instance variables that we stashed after evaluating the before :all blocks are injected (effectively using instance_variable_set as we saw above) into this new instance, which will allow the spec to interact with any objects those blocks created. It also means that these values are shared between every spec, and so interactions within one spec that changed the state of one of these instance variables will be present when the next spec runs.

Finally, the #run method on the Example subclass is called, passing the ExampleGroup instance and the reporter. Down one level we go, into Example#run

The spec finally runs

Here’s the full definition of RSpec::Core::Example#run:

def run(example_group_instance, reporter)
  @example_group_instance = example_group_instance
  @example_group_instance.example = self

  start(reporter)

  begin
    unless pending
      with_around_each_hooks do
        begin
          run_before_each
          @example_group_instance.instance_eval(&@example_block)
        rescue Pending::PendingDeclaredInExample => e
          @pending_declared_in_example = e.message
        rescue Exception => e
          set_exception(e)
        ensure
          run_after_each
        end
      end
    end
  rescue Exception => e
    set_exception(e)
  ensure
    @example_group_instance.instance_variables.each do |ivar|
      @example_group_instance.instance_variable_set(ivar, nil)
    end
    @example_group_instance = nil

    begin
      assign_generated_description
    rescue Exception => e
      set_exception(e, "while assigning the example description")
    end
  end

  finish(reporter)
end

For our purposes, we again only need to consider a small part. Once all the reporter and “around” block housekeeping has taken place, the essential core of the example is run:

          run_before_each
          @example_group_instance.instance_eval(&@example_block)
        rescue Pending::PendingDeclaredInExample => e
          @pending_declared_in_example = e.message
        rescue Exception => e
          set_exception(e)
        ensure
          run_after_each

The call to run_before_each introspects the hook registry and evaluates every relevant before hook against the ExampleGroup instance. In effect, this will find any before blocks registered in this example group, and then any blocks registered in any parent groups, and evaluate them all in order, so that each nested before block runs.

Then, the spec block (stored in @example_block) is evaluated against the ExampleGroup instance. This is where your assertions, or matchers, are finally – finally! – evaluated.

If there was a problem, such as a matcher failing or an exception being raised, then the exception is stored against this Example for later reporting. Just as MiniTest assertions raise an exception when they fail, RSpec matchers raise an RSpec::Expectations::ExpectationNotMetError exception. It seems this is the universal way of halting execution when a test fails7. Another hidden similarity between RSpec and MiniTest!

As in MiniTest, whether or not the spec failed or an exception occured, an ensure section is used to guarantee that run_after_hooks is called, and any teardown is performed.

After the specs have run

Once all the specs in this example group have run, all the examples in any subclasses are run (recall that the inner describe stashed the nested ExampleGroup subclass in an array called children). We map each ExampleGroup subclass to the result of calling run on it, which starts this whole process again, for every nested example group. Whether or not this group passed or failed overall is then determined using simple boolean logic:

    results_for_descendants = children.ordered.map {|child| child.run(reporter)}.all?
    result_for_this_group && results_for_descendants

As we leave the call to ExampleGroup#run, we run any corresponding after :all blocks, and also clear out our stash of before :all instance variables, because they are no longer necessary.

  ensure
    run_after_all_hooks(new)
    before_all_ivars.clear
    reporter.example_group_finished(self)
  end
end

Finishing up

You can once again pop your brain-stack back until we’re in RSpec::Core::Commandline#run.

Having run all of the example groups, RSpec will do a little bit of tidy up, and finally return back up through the stack. Along the way printing the results of the run to the console is performed, before interpreter finally, properly quits.

Phew. You deserve another rest.

Testing with blocks

In contrast to the class-based implementation with MiniTest, we’ve now seen how a block-based test framework can work. In a nutshell, it can be characterised in a couple of key ways:

I would say these two aspects are the hallmark attributes of an RSpec-style test framework. The other notable aspect is the ability to nest example groups, and the subsequent necessity to be able to gather the implicit chain of setup blocks and evaluate them against the test environment instance, but this could be considered another example of using instance_eval.

Supporting method definition in example groups

One thing I’ve found particularly interesting is that RSpec ends up generating classes and subclasses behind the scenes. I believe this is almost entirely a consequence of wanting to support the kind of “natural” method definition within group bodies (see the module_eval section again).

If any test framework chose to not support this, there’s almost certainly no reason to create classes that map to example groups at all, and the setup and test blocks could be evaluated against a bare instance of Object.

Supporting nesting and dynamic composition

It’s clear that RSpec has more “features” (e.g. nesting, before :all and so on) than MiniTest (ignoring the many extensions available for MiniTest, the most sophisticated of which end up significantly modifying or replacing the MiniTest::Unit.run behaviour). I’m deliberately ignoring features like matchers, or a built-in mocking framework, because what I’m most interested in here are the features that affect the structure of the tests.

It’s certainly possible to implement features like nesting using subclasses and explicit calls to super, but this is the kind of plumbing work that Ruby programmers are not accustomed to accepting. By separating creation of tests from Ruby’s class implementation, the implicit relationships between groups of tests can take this burden instead, and behaviours like before :all, which have no natural analogue in class-based testing, are possible.

Now, you may believe that nesting is fundamentally undesirable, and it is not my present intention to disabuse you of that opinion. It’s useful (I think) to understand the constraints we accept by our choice of framework, and I’ve certainly found my explorations of MiniTest and RSpec have helped clarify my own opinions about which approach is ultimately more aligned with my own preferences. While I wouldn’t say that I’m ready to jump wholesale into the RSpec ecosystem, I think it’s fair to say that my advocacy of class-style testing frameworks is at an end.

RSpec and Kintama

I started this exploration because I wanted to understand the relationship between the software I have accidentally produced and what’s already available. I already had strong suspicions that any block-based testing implementation would converge on a few common implementation decisions, and while I have now identified a few interesting (to me) ways in which RSpec and Kintama diverge, the essential approach is the same.

In the final article in this triptych (coming soon, I hope), I’ll walk through Kintama and point those out.

  1. There’s no built-in way to ‘nest’ test groups with MiniTest, or test-unit; the closest simulation would be to create subclasses, and explicitly ensure that super is called within every setup method.

  2. There are other RSpec features like shared examples and global before/after hooks that are definitely interesting, but I need to keep the scope of this article down…

  3. They are actually within a separate gem (rspec-expectations), and it’s quite possible to use rspec-core with test-unit’s assertions (for the curious, hunt for config.expect_with :stdlib).

  4. I’m not sure why some people prefer the syntax Module::method rather than Module.method; as I understand it they are exactly the same, but the former seems more confusing to me, since if you don’t notice the lower-case w in world then you’d assume it was refering to a constant.

  5. It’s not really magic, and it’s not really “metaprogramming”, because it’s all just programming. It just so happens that it’s quite sophisticated programming.

  6. The nested class is a subclass of the outer subclass of ExampleGroup (sorry, I realise that’s confusing), precisely such that any methods defined in the outer class are also available in nested subclasses via the regular mechanisms of inheritance.

  7. Raising an exception might not be the only way to stop a test executing at the point it fails; it could be possible to use fibers/continuations to “pause” failing tests…


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