RSpec Expectations 
RSpec::Expectations lets you express expected outcomes on an object in an example.
expect(account.balance).to 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:
RSpec.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, Item and Money 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
expect(actual).to eq(expected) # passes if actual == expected
expect(actual).to eql(expected) # passes if actual.eql?(expected)
Note: The new expect syntax no longer supports the == matcher.
Identity
expect(actual).to be(expected) # passes if actual.equal?(expected)
expect(actual).to equal(expected) # passes if actual.equal?(expected)
Comparisons
expect(actual).to be > expected
expect(actual).to be >= expected
expect(actual).to be <= expected
expect(actual).to be < expected
expect(actual).to be_within(delta).of(expected)
Regular expressions
expect(actual).to match(/expression/)
Note: The new expect syntax no longer supports the =~ matcher.
Types/classes
expect(actual).to be_an_instance_of(expected) # passes if actual.class == expected
expect(actual).to be_a(expected) # passes if actual.is_a?(expected)
expect(actual).to be_an(expected) # an alias for be_a
expect(actual).to be_a_kind_of(expected) # another alias
Truthiness
expect(actual).to be_truthy # passes if actual is truthy (not nil or false)
expect(actual).to be true # passes if actual == true
expect(actual).to be_falsy # passes if actual is falsy (nil or false)
expect(actual).to be false # passes if actual == false
expect(actual).to 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
expect(actual).to be_xxx # passes if actual.xxx?
expect(actual).to have_xxx(:arg) # passes if actual.has_xxx?(:arg)
Ranges (Ruby >= 1.9 only)
expect(1..10).to cover(3)
Collection membership
expect(actual).to include(expected)
expect(actual).to start_with(expected)
expect(actual).to end_with(expected)
expect(actual).to contain_exactly(individual, items)
# ...which is the same as:
expect(actual).to match_array(expected_array)
Examples
expect([1, 2, 3]).to include(1)
expect([1, 2, 3]).to include(1, 2)
expect([1, 2, 3]).to start_with(1)
expect([1, 2, 3]).to start_with(1, 2)
expect([1, 2, 3]).to end_with(3)
expect([1, 2, 3]).to end_with(2, 3)
expect({:a => 'b'}).to include(:a => 'b')
expect("this string").to include("is str")
expect("this string").to start_with("this")
expect("this string").to end_with("ring")
expect([1, 2, 3]).to contain_exactly(2, 3, 1)
expect([1, 2, 3]).to match_array([3, 2, 1])
should syntax
In addition to the expect syntax, rspec-expectations continues to support the
should syntax:
actual.should eq expected
actual.should be > 3
[1, 2, 3].should_not include 4
See detailed information on the should syntax and its usage.
Compound Matcher Expressions
You can also create compound matcher expressions using and or or:
expect(alphabet).to start_with("a").and end_with("z")
expect(stoplight.color).to eq("red").or eq("green").or eq("yellow")
Composing Matchers
Many of the built-in matchers are designed to take matchers as arguments, to allow you to flexibly specify only the essential aspects of an object or data structure. In addition, all of the built-in matchers have one or more aliases that provide better phrasing for when they are used as arguments to another matcher.
Examples
expect { k += 1.05 }.to change { k }.by( a_value_within(0.1).of(1.0) )
expect { s = "barn" }.to change { s }
.from( a_string_matching(/foo/) )
.to( a_string_matching(/bar/) )
expect(["barn", 2.45]).to contain_exactly(
a_value_within(0.1).of(2.5),
a_string_starting_with("bar")
)
expect(["barn", "food", 2.45]).to end_with(
a_string_matching("foo"),
a_value > 2
)
expect(["barn", 2.45]).to include( a_string_starting_with("bar") )
expect(:a => "food", :b => "good").to include(:a => a_string_matching(/foo/))
hash = {
:a => {
:b => ["foo", 5],
:c => { :d => 2.05 }
}
}
expect(hash).to match(
:a => {
:b => a_collection_containing_exactly(
a_string_starting_with("f"),
an_instance_of(Fixnum)
),
:c => { :d => (a_value < 3) }
}
)
expect { |probe|
[1, 2, 3].each(&probe)
}.to yield_successive_args( a_value < 2, 2, a_value > 2 )
Usage outside rspec-core
You always need to load rspec/expectations even if you only want to use one part of the library:
require 'rspec/expectations'
Then simply include RSpec::Matchers in any class:
class MyClass
include RSpec::Matchers
def do_something(arg)
expect(arg).to be > 0
# do other stuff
end
end