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Given/When/Then keywords for RSpec Specifications
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Covering rspec-given, version 2.0.0.

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.


rspec-given is ready for production use.


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 =
    initial_contents.each do |item| stack.push(item) 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 { == :an_item }

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

  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 }

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

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

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

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


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) { }

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


The When block 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.

There should only be one When block for a given context. However, a When in an outer context shoud be treated as a Given in an inner context. 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 }

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 When block, 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'.


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

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

Then examples:

    Then { stack.should be_empty }

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


The Invariant block 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 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 block as a Then block that automatically gets added to every Then within its scope.

Invariants nested within a context only apply to the Then blocks in that context.

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

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 blocks. 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.


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