RSpec Expectations

RSpec::Expectations lets you express expected outcomes on an object in an example.

account.balance.should eq(Money.new(37.42, :USD))

Install

If you want to use rspec-expectations with rspec, just install the rspec gem and RubyGems will also install rspec-expectations for you (along with rspec-core and rspec-mocks):

gem install rspec

If you want to use rspec-expectations with another tool, like Test::Unit, Minitest, or Cucumber, you can install it directly:

gem install rspec-expectations

Basic usage

Here's an example using rspec-core:

describe Order do
  it "sums the prices of the items in its line items" do
    order = Order.new
    order.add_entry(LineItem.new(:item => Item.new(
      :price => Money.new(1.11, :USD)
    )))
    order.add_entry(LineItem.new(:item => Item.new(
      :price => Money.new(2.22, :USD),
      :quantity => 2
    )))
    expect(order.total).to eq(Money.new(5.55, :USD))
  end
end

The describe and it methods come from rspec-core. The Order, LineItem, and Item classes would be from your code. The last line of the example expresses an expected outcome. If order.total == Money.new(5.55, :USD), then the example passes. If not, it fails with a message like:

expected: #<Money @value=5.55 @currency=:USD>
     got: #<Money @value=1.11 @currency=:USD>

Built-in matchers

Equivalence

actual.should eq(expected)  # passes if actual == expected
actual.should == expected   # passes if actual == expected
actual.should eql(expected) # passes if actual.eql?(expected)

Note: we recommend the eq matcher over == to avoid Ruby's "== in a useless context" warning when the == matcher is used anywhere but the last statement of an example.

Identity

actual.should be(expected)    # passes if actual.equal?(expected)
actual.should equal(expected) # passes if actual.equal?(expected)

Comparisons

actual.should be >  expected
actual.should be >= expected
actual.should be <= expected
actual.should be <  expected
actual.should be_within(delta).of(expected)

Regular expressions

actual.should match(/expression/)
actual.should =~ /expression/

Types/classes

actual.should be_an_instance_of(expected)
actual.should be_a_kind_of(expected)

Truthiness

actual.should be_true  # passes if actual is truthy (not nil or false)
actual.should be_false # passes if actual is falsy (nil or false)
actual.should be_nil   # passes if actual is nil

Expecting errors

expect { ... }.to raise_error
expect { ... }.to raise_error(ErrorClass)
expect { ... }.to raise_error("message")
expect { ... }.to raise_error(ErrorClass, "message")

Expecting throws

expect { ... }.to throw_symbol
expect { ... }.to throw_symbol(:symbol)
expect { ... }.to throw_symbol(:symbol, 'value')

Yielding

expect { |b| 5.tap(&b) }.to yield_control # passes regardless of yielded args

expect { |b| yield_if_true(true, &b) }.to yield_with_no_args # passes only if no args are yielded

expect { |b| 5.tap(&b) }.to yield_with_args(5)
expect { |b| 5.tap(&b) }.to yield_with_args(Fixnum)
expect { |b| "a string".tap(&b) }.to yield_with_args(/str/)

expect { |b| [1, 2, 3].each(&b) }.to yield_successive_args(1, 2, 3)
expect { |b| { :a => 1, :b => 2 }.each(&b) }.to yield_successive_args([:a, 1], [:b, 2])

Predicate matchers

actual.should be_xxx         # passes if actual.xxx?
actual.should have_xxx(:arg) # passes if actual.has_xxx?(:arg)

Ranges (Ruby >= 1.9 only)

(1..10).should cover(3)

Collection membership

actual.should include(expected)
actual.should start_with(expected)
actual.should end_with(expected)

Examples

[1,2,3].should include(1)
[1,2,3].should include(1, 2)
[1,2,3].should start_with(1)
[1,2,3].should start_with(1,2)
[1,2,3].should end_with(3)
[1,2,3].should end_with(2,3)
{:a => 'b'}.should include(:a => 'b')
"this string".should include("is str")
"this string".should start_with("this")
"this string".should end_with("ring")

expect syntax

In addition to the should syntax, rspec-expectations supports a new expect syntax as of version 2.11.0:

expect(actual).to eq expected
expect(actual).to be > 3
expect([1, 2, 3]).to_not include 4

If you want your project to only use one of these syntaxes, you can configure it:

RSpec.configure do |config|
  config.expect_with :rspec do |c|
    c.syntax = :expect
    # or
    c.syntax = :should
    # or
    c.syntax = [:should, :expect]
  end
end

See RSpec::Expectations::Syntax#expect for more information.

Motivation for expect

We added the expect syntax to resolve some edge case issues, most notably that objects whose definitions wipe out all but a few methods were throwing should and should_not away. expect solves that by not monkey patching those methods onto Kernel (or any global object).

See http://myronmars.to/n/dev-blog/2012/06/rspecs-new-expectation-syntax for a detailed explanation.

One-liners

The one-liner syntax supported by rspec-core uses should even when config.syntax = :expect. It reads better than the alternative, and does not require a global monkey patch:

describe User do
  it { should validate_presence_of :email }
end

Also see

• http://github.com/rspec/rspec
• http://github.com/rspec/rspec-core
• http://github.com/rspec/rspec-mocks