RSpec ships with a number of useful Expression Matchers. An Expression Matcher is any object that responds to the following methods:
matches?(actual) failure_message_for_should
These methods are also part of the matcher protocol, but are optional:
does_not_match?(actual) failure_message_for_should_not description #optional
These methods are from older versions of the protocol. They are still supported, but are not recommended:
failure_message (use failure_message_for_should instead) negative_failure_message (use failure_message_for_should_not instead)
See Spec::Expectations to learn how to use these as Expectation Matchers.
In addition to those Expression Matchers that are defined explicitly, RSpec will create custom Matchers on the fly for any arbitrary predicate, giving your specs a much more natural language feel.
A Ruby predicate is a method that ends with a “?” and returns true or false. Common examples are empty?, nil?, and instance_of?.
All you need to do is write +should be_+ followed by the predicate without the question mark, and RSpec will figure it out from there. For example:
[].should be_empty => [].empty? #passes [].should_not be_empty => [].empty? #fails
In addtion to prefixing the predicate matchers with “be_”, you can also use “be_a_” and “be_an_”, making your specs read much more naturally:
"a string".should be_an_instance_of(String) =>"a string".instance_of?(String) #passes 3.should be_a_kind_of(Fixnum) => 3.kind_of?(Numeric) #passes 3.should be_a_kind_of(Numeric) => 3.kind_of?(Numeric) #passes 3.should be_an_instance_of(Fixnum) => 3.instance_of?(Fixnum) #passes 3.should_not be_instance_of(Numeric) => 3.instance_of?(Numeric) #fails
RSpec will also create custom matchers for predicates like has_key?. To use this feature, just state that the object should have_key(:key) and RSpec will call has_key?(:key) on the target. For example:
{:a => "A"}.should have_key(:a) => {:a => "A"}.has_key?(:a) #passes {:a => "A"}.should have_key(:b) => {:a => "A"}.has_key?(:b) #fails
You can use this feature to invoke any predicate that begins with “has_”, whether it is part of the Ruby libraries (like +Hash#has_key?+) or a method you wrote on your own class.
When you find that none of the stock Expectation Matchers provide a natural feeling expectation, you can very easily write your own using RSpec’s matcher DSL or writing one from scratch.
Imagine that you are writing a game in which players can be in various zones on a virtual board. To specify that bob should be in zone 4, you could say:
bob.current_zone.should eql(Zone.new("4"))
But you might find it more expressive to say:
bob.should be_in_zone("4")
and/or
bob.should_not be_in_zone("3")
You can create such a matcher like so:
Spec::Matchers.define :be_in_zone do |zone| match do |player| player.in_zone?(zone) end end
This will generate a be_in_zone method that returns a matcher with logical default messages for failures. You can override the failure messages and the generated description as follows:
Spec::Matchers.define :be_in_zone do |zone| match do |player| player.in_zone?(zone) end failure_message_for_should do |player| # generate and return the appropriate string. end failure_message_for_should_not do |player| # generate and return the appropriate string. end description do # generate and return the appropriate string. end end
Each of the message-generation methods has access to the block arguments passed to the create method (in this case, zone). The failure message methods (failure_message_for_should and failure_message_for_should_not) are passed the actual value (the receiver of should or should_not).
You can also create matchers that obey a fluent interface using the chain method:
Spec::Matchers.define :tip do |expected_tip| chain :on do |bill| @bill = bill end match do |person| person.tip_for(bill) == expected_tip end end
This matcher can be used as follows:
describe Customer do it { should tip(10).on(50 } end
You could also write a custom matcher from scratch, as follows:
class BeInZone def initialize(expected) @expected = expected end def matches?(target) @target = target @target.current_zone.eql?(Zone.new(@expected)) end def failure_message_for_should "expected #{@target.inspect} to be in Zone #{@expected}" end def failure_message_for_should_not "expected #{@target.inspect} not to be in Zone #{@expected}" end end
… and a method like this:
def be_in_zone(expected) BeInZone.new(expected) end
And then expose the method to your specs. This is normally done by including the method and the class in a module, which is then included in your spec:
module CustomGameMatchers class BeInZone ... end def be_in_zone(expected) ... end end describe "Player behaviour" do include CustomGameMatchers ... end
or you can include in globally in a spec_helper.rb file required from your spec file(s):
Spec::Runner.configure do |config| config.include(CustomGameMatchers) end
Given true, false, or nil, will pass if actual value is true, false or nil (respectively). Given no args means the caller should satisfy an if condition (to be or not to be).
Predicates are any Ruby method that ends in a “?” and returns true or false. Given be_ followed by arbitrary_predicate (without the “?”), RSpec will match convert that into a query against the target object.
The arbitrary_predicate feature will handle any predicate prefixed with “be_an_” (e.g. be_an_instance_of), “be_a_” (e.g. be_a_kind_of) or “be_” (e.g. be_empty), letting you choose the prefix that best suits the predicate.
target.should be_true target.should be_false target.should be_nil target.should_not be_nil collection.should be_empty #passes if target.empty? target.should_not be_empty #passes unless target.empty? target.should_not be_old_enough(16) #passes unless target.old_enough?(16)
# File lib/spec/matchers/be.rb, line 236 def be(*args) args.empty? ? Matchers::Be.new : Matchers::BeSameAs.new(*args) end
passes if target.kind_of?(klass)
# File lib/spec/matchers/be.rb, line 243 def be_a(klass) be_a_kind_of(klass) end
Passes if actual.kind_of?(expected)
5.should be_kind_of(Fixnum) 5.should be_kind_of(Numeric) 5.should_not be_kind_of(Float)
# File lib/spec/matchers/be_kind_of.rb, line 16 def be_a_kind_of(expected) Matcher.new :be_a_kind_of, expected do |_expected_| match do |actual| actual.kind_of?(_expected_) end end end
Passes if actual.instance_of?(expected)
5.should be_instance_of(Fixnum) 5.should_not be_instance_of(Numeric) 5.should_not be_instance_of(Float)
# File lib/spec/matchers/be_instance_of.rb, line 16 def be_an_instance_of(expected) Matcher.new :be_an_instance_of, expected do |_expected_| match do |actual| actual.instance_of?(_expected_) end end end
Passes if actual == expected +/- delta
result.should be_close(3.0, 0.5)
# File lib/spec/matchers/be_close.rb, line 12 def be_close(expected, delta) Matcher.new :be_close, expected, delta do |_expected_, _delta_| match do |actual| (actual - _expected_).abs < _delta_ end failure_message_for_should do |actual| "expected #{_expected_} +/- (< #{_delta_}), got #{actual}" end failure_message_for_should_not do |actual| "expected #{_expected_} +/- (< #{_delta_}), got #{actual}" end description do "be close to #{_expected_} (within +- #{_delta_})" end end end
Allows you to specify that a Proc will cause some value to change.
lambda { team.add_player(player) }.should change(roster, :count) lambda { team.add_player(player) }.should change(roster, :count).by(1) lambda { team.add_player(player) }.should change(roster, :count).by_at_least(1) lambda { team.add_player(player) }.should change(roster, :count).by_at_most(1) string = "string" lambda { string.reverse! }.should change { string }.from("string").to("gnirts") lambda { person.happy_birthday }.should change(person, :birthday).from(32).to(33) lambda { employee.develop_great_new_social_networking_app }.should change(employee, :title).from("Mail Clerk").to("CEO")
Evaluates receiver.message or block before and after it evaluates the c object (generated by the lambdas in the examples above).
Then compares the values before and after the receiver.message and evaluates the difference compared to the expected difference.
should_not change only supports the form with no subsequent calls to by, by_at_least, by_at_most, to or from.
blocks passed to should change and should_not change must use the {} form (do/end is not supported).
# File lib/spec/matchers/change.rb, line 147 def change(receiver=nil, message=nil, &block) Matchers::Change.new(receiver, message, &block) end
Passes if actual and expected are of equal value, but not necessarily the same object.
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
5.should eql(5) 5.should_not eql(3)
# File lib/spec/matchers/eql.rb, line 15 def eql(expected) Matcher.new :eql, expected do |_expected_| match do |actual| actual.eql?(_expected_) end failure_message_for_should do |actual| expected #{_expected_.inspect} got #{actual.inspect}(compared using eql?) end failure_message_for_should_not do |actual| expected #{actual.inspect} not to equal #{_expected_.inspect}(compared using eql?) end end end
Passes if actual and expected are the same object (object identity).
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
5.should equal(5) #Fixnums are equal "5".should_not equal("5") #Strings that look the same are not the same object
# File lib/spec/matchers/equal.rb, line 16 def equal(expected) Matcher.new :equal, expected do |_expected_| match do |actual| actual.equal?(_expected_) end def inspect_object(o) "#<#{o.class}:#{o.object_id}> => #{o.inspect}" end failure_message_for_should do |actual| expected #{inspect_object(_expected_)} got #{inspect_object(actual)}Compared using equal?, which compares object identity,but expected and actual are not the same object. Use'actual.should == expected' if you don't care aboutobject identity in this example. end failure_message_for_should_not do |actual| expected not #{inspect_object(actual)} got #{inspect_object(_expected_)}Compared using equal?, which compares object identity. end end end
Passes if actual.exist?
# File lib/spec/matchers/exist.rb, line 8 def exist(arg=nil) Matcher.new :exist do match do |actual| arg ? actual.exist?(arg) : actual.exist? end end end
Passes if receiver is a collection with the submitted number of items OR if the receiver OWNS a collection with the submitted number of items.
If the receiver OWNS the collection, you must use the name of the collection. So if a Team instance has a collection named #players, you must use that name to set the expectation.
If the receiver IS the collection, you can use any name you like for named_collection. We’d recommend using either “elements”, “members”, or “items” as these are all standard ways of describing the things IN a collection.
This also works for Strings, letting you set an expectation about its length
# Passes if team.players.size == 11 team.should have(11).players # Passes if [1,2,3].length == 3 [1,2,3].should have(3).items #"items" is pure sugar # Passes if "this string".length == 11 "this string".should have(11).characters #"characters" is pure sugar
# File lib/spec/matchers/have.rb, line 123 def have(n) Matchers::Have.new(n) end
Exactly like have() with >=.
should_not have_at_least is not supported
# File lib/spec/matchers/have.rb, line 136 def have_at_least(n) Matchers::Have.new(n, :at_least) end
Exactly like have() with <=.
should_not have_at_most is not supported
# File lib/spec/matchers/have.rb, line 148 def have_at_most(n) Matchers::Have.new(n, :at_most) end
# File lib/spec/matchers/include.rb, line 25 def helper(actual, *_expected_) _expected_.each do |expected| if actual.is_a?(Hash) if expected.is_a?(Hash) expected.each_pair do |k,v| return false unless actual[k] == v end else return false unless actual.has_key?(expected) end else return false unless actual.include?(expected) end end true end
Passes if actual includes expected. This works for collections and Strings. You can also pass in multiple args and it will only pass if all args are found in collection.
[1,2,3].should include(3) [1,2,3].should include(2,3) #would pass [1,2,3].should include(2,3,4) #would fail [1,2,3].should_not include(4) "spread".should include("read") "spread".should_not include("red")
# File lib/spec/matchers/include.rb, line 19 def include(*expected) Matcher.new :include, *expected do |*_expected_| match do |actual| helper(actual, *_expected_) end def helper(actual, *_expected_) _expected_.each do |expected| if actual.is_a?(Hash) if expected.is_a?(Hash) expected.each_pair do |k,v| return false unless actual[k] == v end else return false unless actual.has_key?(expected) end else return false unless actual.include?(expected) end end true end end end
# File lib/spec/matchers/equal.rb, line 22 def inspect_object(o) "#<#{o.class}:#{o.object_id}> => #{o.inspect}" end
Given a Regexp or String, passes if actual.match(pattern)
email.should match(/^([^\s]+)((?:[-a-z0-9]+\.)+[a-z]{2,})$/i) email.should match("@example.com")
# File lib/spec/matchers/match.rb, line 13 def match(expected) Matcher.new :match, expected do |_expected_| match do |actual| actual.match(_expected_) end end end
With no args, matches if any exception is raised. With a named exception, matches only if that specific exception is raised. With a named exception and messsage specified as a String, matches only if both match. With a named exception and messsage specified as a Regexp, matches only if both match. Pass an optional block to perform extra verifications on the exception matched
lambda { do_something_risky }.should raise_exception lambda { do_something_risky }.should raise_exception(PoorRiskDecisionError) lambda { do_something_risky }.should raise_exception(PoorRiskDecisionError) { |exception| exception.data.should == 42 } lambda { do_something_risky }.should raise_exception(PoorRiskDecisionError, "that was too risky") lambda { do_something_risky }.should raise_exception(PoorRiskDecisionError, /oo ri/) lambda { do_something_risky }.should_not raise_exception lambda { do_something_risky }.should_not raise_exception(PoorRiskDecisionError) lambda { do_something_risky }.should_not raise_exception(PoorRiskDecisionError, "that was too risky") lambda { do_something_risky }.should_not raise_exception(PoorRiskDecisionError, /oo ri/)
# File lib/spec/matchers/raise_exception.rb, line 125 def raise_exception(exception=Exception, message=nil, &block) Matchers::RaiseException.new(exception, message, &block) end
Matches if the target object responds to all of the names provided. Names can be Strings or Symbols.
# File lib/spec/matchers/respond_to.rb, line 67 def respond_to(*names) Matchers::RespondTo.new(*names) end
Passes if the submitted block returns true. Yields target to the block.
Generally speaking, this should be thought of as a last resort when you can’t find any other way to specify the behaviour you wish to specify.
If you do find yourself in such a situation, you could always write a custom matcher, which would likely make your specs more expressive.
5.should satisfy { |n| n > 3 }
# File lib/spec/matchers/satisfy.rb, line 43 def satisfy(&block) Matchers::Satisfy.new(&block) end
simple_matcher makes it easy for you to create your own custom matchers in just a few lines of code when you don't need all the power of a completely custom matcher object.
The description argument will appear as part of any failure message, and is also the source for auto-generated descriptions.
The match_block can have an arity of 1 or 2. The first block argument will be the given value. The second, if the block accepts it will be the matcher itself, giving you access to set custom failure messages in favor of the defaults.
The match_block should return a boolean: true indicates a match, which will pass if you use should and fail if you use should_not. false (or nil) indicates no match, which will do the reverse: fail if you use should and pass if you use should_not.
An error in the match_block will bubble up, resulting in a failure.
def be_even simple_matcher("an even number") { |given| given % 2 == 0 } end describe 2 do it "should be even" do 2.should be_even end end
Given an odd number, this example would produce an error message stating: expected “an even number”, got 3.
Unfortunately, if you’re a fan of auto-generated descriptions, this will produce “should an even number.” Not the most desirable result. You can control that using custom messages:
def rhyme_with(expected) simple_matcher("rhyme with #{expected.inspect}") do |given, matcher| matcher.failure_message = "expected #{given.inspect} to rhyme with #{expected.inspect}" matcher.negative_failure_message = "expected #{given.inspect} not to rhyme with #{expected.inspect}" given.rhymes_with? expected end end # OR def rhyme_with(expected) simple_matcher do |given, matcher| matcher.description = "rhyme with #{expected.inspect}" matcher.failure_message = "expected #{given.inspect} to rhyme with #{expected.inspect}" matcher.negative_failure_message = "expected #{given.inspect} not to rhyme with #{expected.inspect}" given.rhymes_with? expected end end describe "pecan" do it "should rhyme with 'be gone'" do nut = "pecan" nut.extend Rhymer nut.should rhyme_with("be gone") end end
The resulting messages would be:
description: rhyme with "be gone" failure_message: expected "pecan" to rhyme with "be gone" negative failure_message: expected "pecan" not to rhyme with "be gone"
Because errors will bubble up, it is possible to wrap other expectations in a SimpleMatcher.
def be_even simple_matcher("an even number") { |given| (given % 2).should == 0 } end
BE VERY CAREFUL when you do this. Only use wrapped expectations for matchers that will always be used in only the positive (should) or negative (should_not), but not both. The reason is that is you wrap a should and call the wrapper with should_not, the correct result (the should failing), will fail when you want it to pass.
# File lib/spec/matchers/simple_matcher.rb, line 129 def simple_matcher(description=nil, &match_block) SimpleMatcher.new(description, &match_block) end
Given no argument, matches if a proc throws any Symbol.
Given a Symbol, matches if the given proc throws the specified Symbol.
Given a Symbol and an arg, matches if the given proc throws the specified Symbol with the specified arg.
lambda { do_something_risky }.should throw_symbol lambda { do_something_risky }.should throw_symbol(:that_was_risky) lambda { do_something_risky }.should throw_symbol(:that_was_risky, culprit) lambda { do_something_risky }.should_not throw_symbol lambda { do_something_risky }.should_not throw_symbol(:that_was_risky) lambda { do_something_risky }.should_not throw_symbol(:that_was_risky, culprit)
# File lib/spec/matchers/throw_symbol.rb, line 96 def throw_symbol(sym=nil) Matchers::ThrowSymbol.new(sym) end
wraps an expectation in a block that will return true if the expectation passes and false if it fails (without bubbling up the failure).
This is intended to be used in the context of a simple matcher, and is especially useful for wrapping multiple expectations or one or more assertions from test/unit extensions when running with test/unit.
def eat_cheese(cheese) simple_matcher do |mouse, matcher| matcher.failure_message = "expected #{mouse} to eat cheese" wrap_expectation do |matcher| assert_eats_cheese(mouse) end end end describe Mouse do it "eats cheese" do Mouse.new.should eat_cheese end end
You might be wondering “why would I do this if I could just say” assert_eats_cheese?“, a fair question, indeed. You might prefer to replace the word assert with something more aligned with the rest of your code examples. You are using rspec, after all.
The other benefit you get is that you can use the negative version of the matcher:
describe Cat do it "does not eat cheese" do Cat.new.should_not eat_cheese end end
So in the event there is no assert_does_not_eat_cheese available, you’re all set!
# File lib/spec/matchers/wrap_expectation.rb, line 45 def wrap_expectation(matcher, &block) begin block.call(matcher) return true rescue Exception => e matcher.failure_message = e.message return false end end
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