rspec-given

Covering rspec-given, version 2.2.0.

rspec-given is an RSpec extension to allow Given/When/Then notation in RSpec specifications. It is a natural extension of the experimental work done on the Given framework. It turns out that 90% of the Given framework can be trivially implemented on top of RSpec.

Why Given/When/Then

RSpec has done a great job of making specifications more readable for humans. However, I really like the given / when / then nature of Cucumber stories and would like to follow the same structure in my unit tests. rspec-given allows a simple given/when/then structure RSpec specifications.

Status

rspec-given is ready for production use.

Example

Here is a specification written in the rspec-given framework:

require 'rspec/given'
require 'spec_helper'
require 'stack'

describe Stack do
  def stack_with(initial_contents)
    stack = Stack.new
    initial_contents.each do |item| stack.push(item) end
    stack
  end

  Given(:stack) { stack_with(initial_contents) }
  Invariant { stack.empty?.should == (stack.depth == 0) }

  context "when empty" do
    Given(:initial_contents) { [] }
    Then { stack.depth.should == 0 }

    context "when pushing" do
      When { stack.push(:an_item) }

      Then { stack.depth.should == 1 }
      Then { stack.top.should == :an_item }
    end

    context "when popping" do
      When(:result) { stack.pop }
      Then { result.should have_failed(Stack::UnderflowError, /empty/) }
    end
  end

  context "with one item" do
    Given(:initial_contents) { [:an_item] }

    context "when popping" do
      When(:pop_result) { stack.pop }

      Then { pop_result.should == :an_item }
      Then { stack.depth.should == 0 }
    end
  end

  context "with several items" do
    Given(:initial_contents) { [:second_item, :top_item] }
    Given!(:original_depth) { stack.depth }

    context "when pushing" do
      When { stack.push(:new_item) }

      Then { stack.top.should == :new_item }
      Then { stack.depth.should == original_depth + 1 }
    end

    context "when popping" do
      When(:pop_result) { stack.pop }

      Then { pop_result.should == :top_item }
      Then { stack.top.should == :second_item }
      Then { stack.depth.should == original_depth - 1 }
    end
  end
end

Let's talk about the individual statements used in the Given framework.

Given

The Given section specifies a starting point, a set of preconditions that must be true before the code under test is allowed to be run. In standard test frameworks the preconditions are established with a combination of setup methods (or :before actions in RSpec) and code in the test.

In the example code above the preconditions are started with Given statements. A top level Given (that applies to the entire describe block) says that one of the preconditions is that there is a stack with some initial contents.

Note that initial contents are not specified in the top level describe block, but are given in each of the nested contexts. By pushing the definition of "initial_contents" into the nested contexts, we can vary them as needed for that particular context.

A precondition in the form "Given(:var) ..." creates an accessor method named "var". The accessor is lazily initialized by the code block. If you want a non-lazy given, use "Given!(:var) ...".

A precondition in the form "Given ..." just executes the code block for side effects. Since there is no accessor, the code block is executed immediately (i.e. no lazy evaluation).

The preconditions are run in order of definition. Nested contexts will inherit the preconditions from the enclosing context, with out preconditions running before inner preconditions.

Given examples:

    Given(:stack) { Stack.new }

The block for the given clause is lazily run if 'stack' is ever referenced in the test and the value of the block is bound to 'stack'. The first reference to 'stack' in the specification will cause the code block to execute. Futher references to 'stack' will reuse the previously generated value.

    Given!(:original_size) { stack.size }

The code block is run unconditionally once before each test and the value of the block is bound to 'original_size'. This form is useful when you want to record the value of something that might be affected by the When code.

    Given { stack.clear }

The block for the given clause is run unconditionally once before each test. This form of given is used for code that is executed for side effects.

When

The When clause specifies the code to be tested ... oops, excuse me ... specified. After the preconditions in the given section are met, the when code block is run.

In general there should not be more than one When clause for a given direct context. However, a When in an outer context will be run after all the Givens but before the inner When. You can think of an outer When as setting up additional given state for the inner When.

E.g.

    context "outer context" do
      When { code specified in the outer context }
      Then { assert something about the outer context }

      context "inner context" do

        # At this point, the _When_ of the outer context
        # should be treated as a _Given_ of the inner context

        When { code specified in the inner context }
        Then { assert something about the inner context }
      end
    end

When examples:

    When { stack.push(:item) }

The code block is executed once per test. The effect of the When{} block is very similar to Given{}. However, When is used to identify the particular code that is being specified in the current context or describe block.

    When(:result) { stack.pop }

The code block is executed once per test and the value of the code block is bound to 'result'. Use this form when the code under test returns a value that you wish to interrogate in the Then code.

If an exception occurs during the execution of the block for the When clause, the exception is caught and a failure object is bound to 'result'. The failure can be checked in a then block with the 'have_failed' matcher.

The failure object will rethrow the captured exception if anything other than have_failed matcher is used on the failure object.

For example, if the stack is empty when it is popped, then it is reasonable for pop to raise an UnderflowError. This is how you might specify that behavior:

    When(:result) { stack.pop }
    Then { result.should have_failed(UnderflowError, /empty/) }

Note that the arguments to the 'have_failed' matcher are the same as those given to the standard RSpec matcher 'raise_error'.

Then

The Then clauses are the postconditions of the specification. These then conditions must be true after the code under test (the When clause) is run.

The code in the block of a Then clause should be a single should assertion. Code in Then clauses should not have any side effects.

Let me repeat that: Then clauses should not have any side effects! Then clauses with side effects are erroneous. Then clauses need to be idempotent, so that running them once, twice, a hundred times, or never does not change the state of the program. (The same is true of And clauses).

In RSpec terms, a Then clause forms a RSpec Example that runs in the context of an Example Group (defined by a describe or context clause).

Each Example Group must have at least one Then clause, otherwise there will be no examples to be run for that group. If all the assertions in an example group are done via Invariants, then the group should use an empty Then clause, like this:

    Then { }

Then examples:

    Then { stack.should be_empty }

After the related block for the When clause is run, the stack should be empty. If it is not empty, the test will fail.

And

The And clause is similar to Then, but does not form its own RSpec example. This means that And clauses reuse the setup from a sibling Then clause. Using a single Then an multiple And clauses in an example group means the setup for that group is run only once (for the Then clause) and reused for all the _And_s. This can be a significant speed savings where the setup for an example group is expensive.

Some things to keep in mind about And clauses:

There must be at least one Then in the example group and it must be declared before the And clauses. Forgetting the Then clause is an error.
The code in the And clause is run immediately after the first (executed) Then of an example group.
And assertion failures in a Then clause or a And clause will cause all the subsequent And clauses to be skipped.
Since And clauses do not form their own RSpec examples, they are not represented in the formatted output of RSpec. That means And clauses do not produce dots in the Progress format, nor do they appear in the documentation, html or textmate formats (options -fhtml, -fdoc, or -ftextmate).
Like Then clauses, And clauses must be idempotent. That means they should not execute any code that changes global program state. (See the section on the Then clause).

The choice to use an And clause is primarily a speed consideration. If an example group has expensive setup and there are a lot of Then clauses, then choosing to make some of the Then clauses into And clause will speed up the spec. Otherwise it is probably better to stick with Then clauses.

Then/And examples:

  Then { pop_result.should == :top_item }           # Required
  And  { stack.top.should == :second_item }         # No Setup rerun
  And  { stack.depth.should == original_depth - 1 } # ... for these

Invariant

The Invariant clause is a new idea that doesn't have an analog in RSpec or Test::Unit. The invariant allows you specify things that must always be true in the scope of the invariant. In the stack example, empty? is defined in term of size. Whenever size is 0, empty? should be true. Whenever size is non-zero, empty? should be false.

You can conceptually think of an Invariant clause as a Then block that automatically gets added to every Then within its scope.

Invariants nested within a context only apply to the Then clauses that are in the scope of that context.

Invariants that reference a Given precondition accessor must only be used in contexts that define that accessor.

Notes:

Since Invariants do not form their own RSpec example, they are not represented in the RSpec formatted output (e.g. the '--format html' option).

Configuration

Just require 'rspec/given' in the spec helper of your project and it is ready to go.

If the RSpec format option document, html or textmate are chosen, RSpec/Given will automatically add addition source code information to the examples to produce better looking output. If you don't care about the pretty output and wish to disable source code caching unconditionally, then add the following line to your spec helper file:

    RSpec::Given.source_caching_disabled = true

Future Directions

I really like the way the Given framework is working out. I feel my tests are much more like specifications when I use it. However, I'm not entirely happy with it.

I would like to remove the need for the ".should" in all the Then clauses. In other words, instead of saying:

    Then { x.should == y }

we could say:

    Then { x == y }

I think the wrong assertion library has laid some groundwork in this area.