RSpec::Matchers provides a number of useful matchers we use to define expectations. A matcher is any object that responds to the following:
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
## Predicates
In addition to 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 `expect(..).to be_` followed by the predicate without the question mark, and RSpec will figure it out from there. For example:
expect([]).to be_empty # => [].empty?() | passes expect([]).not_to 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:
expect("a string").to be_an_instance_of(String) # =>"a string".instance_of?(String) # passes expect(3).to be_a_kind_of(Fixnum) # => 3.kind_of?(Numeric) | passes expect(3).to be_a_kind_of(Numeric) # => 3.kind_of?(Numeric) | passes expect(3).to be_an_instance_of(Fixnum) # => 3.instance_of?(Fixnum) | passes expect(3).not_to be_an_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:
expect(:a => "A").to have_key(:a) expect(:a => "A").to have_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.
## Custom Matchers
When you find that none of the stock matchers provide a natural feeling expectation, you can very easily write your own using RSpec's matcher DSL or writing one from scratch.
### Matcher DSL
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:
expect(bob.current_zone).to eql(Zone.new("4"))
But you might find it more expressive to say:
expect(bob).to be_in_zone("4")
and/or
expect(bob).not_to be_in_zone("3")
You can create such a matcher like so:
RSpec::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:
RSpec::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 expect(..)
or
expect(..).not_to
).
### Custom Matcher from scratch
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
require
d from your spec file(s):
RSpec::configure do |config| config.include(CustomGameMatchers) end
# File lib/rspec/matchers/generated_descriptions.rb, line 7 def self.clear_generated_description self.last_matcher = nil self.last_should = nil end
The configuration object @return [RSpec::Matchers::Configuration] the configuration object
# File lib/rspec/matchers/configuration.rb, line 100 def self.configuration @configuration ||= Configuration.new end
# File lib/rspec/matchers/generated_descriptions.rb, line 12 def self.generated_description return nil if last_should.nil? "#{last_should.to_s.gsub('_',' ')} #{last_description}" end
@api private
# File lib/rspec/matchers.rb, line 178 def self.is_a_matcher?(obj) return true if ::RSpec::Matchers::BuiltIn::BaseMatcher === obj return false if obj.respond_to?(:i_respond_to_everything_so_im_not_really_a_matcher) return false unless obj.respond_to?(:matches?) obj.respond_to?(:failure_message_for_should) || obj.respond_to?(:failure_message) end
# File lib/rspec/matchers/generated_descriptions.rb, line 19 def self.last_description last_matcher.respond_to?(:description) ? last_matcher.description : <<-MESSAGE When you call a matcher in an example without a String, like this: specify { object.should matcher } or this: it { should matcher } RSpec expects the matcher to have a #description method. You should either add a String to the example this matcher is being used in, or give it a description method. Then you won't have to suffer this lengthy warning again. MESSAGE end
@example
expect(actual).to be_true expect(actual).to be_false expect(actual).to be_nil expect(actual).to be_[arbitrary_predicate](*args) expect(actual).not_to be_nil expect(actual).not_to be_[arbitrary_predicate](*args)
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.
# File lib/rspec/matchers.rb, line 225 def be(*args) args.empty? ? Matchers::BuiltIn::Be.new : equal(*args) end
passes if target.kind_of?(klass)
# File lib/rspec/matchers.rb, line 231 def be_a(klass) be_a_kind_of(klass) end
Passes if actual.kind_of?(expected)
@example
expect(5).to be_a_kind_of(Fixnum) expect(5).to be_a_kind_of(Numeric) expect(5).not_to be_a_kind_of(Float)
# File lib/rspec/matchers.rb, line 257 def be_a_kind_of(expected) BuiltIn::BeAKindOf.new(expected) end
Passes if actual.instance_of?(expected)
@example
expect(5).to be_an_instance_of(Fixnum) expect(5).not_to be_an_instance_of(Numeric) expect(5).not_to be_an_instance_of(Float)
# File lib/rspec/matchers.rb, line 244 def be_an_instance_of(expected) BuiltIn::BeAnInstanceOf.new(expected) end
@deprecated use be_within
instead.
# File lib/rspec/matchers/be_close.rb, line 4 def be_close(expected, delta) RSpec.deprecate("be_close(#{expected}, #{delta})", :replacement => "be_within(#{delta}).of(#{expected})") be_within(delta).of(expected) end
Passes if actual is falsy (false or nil)
# File lib/rspec/matchers.rb, line 192 def be_false BuiltIn::BeFalse.new end
Passes if actual is nil
# File lib/rspec/matchers.rb, line 197 def be_nil BuiltIn::BeNil.new end
Passes if actual is truthy (anything but false or nil)
# File lib/rspec/matchers.rb, line 187 def be_true BuiltIn::BeTrue.new end
Passes if actual == expected +/- delta
@example
expect(result).to be_within(0.5).of(3.0) expect(result).not_to be_within(0.5).of(3.0)
# File lib/rspec/matchers.rb, line 269 def be_within(delta) BuiltIn::BeWithin.new(delta) end
Applied to a proc, specifies that its execution will cause some value to change.
@param [Object] receiver @param [Symbol] message the message to send the receiver
You can either pass receiver
and message
, or a
block, but not both.
When passing a block, it must use the { ... }
format, not
do/end, as { ... }
binds to the change
method,
whereas do/end would errantly bind to the +expect(..)+ or
+expect(..).not_to+ method.
@example
expect { team.add_player(player) }.to change(roster, :count) expect { team.add_player(player) }.to change(roster, :count).by(1) expect { team.add_player(player) }.to change(roster, :count).by_at_least(1) expect { team.add_player(player) }.to change(roster, :count).by_at_most(1) string = "string" expect { string.reverse! }.to change { string }.from("string").to("gnirts") string = "string" expect { string }.not_to change { string } expect { person.happy_birthday }.to change(person, :birthday).from(32).to(33) expect { employee.develop_great_new_social_networking_app }.to change(employee, :title).from("Mail Clerk").to("CEO") expect { doctor.leave_office }.to change(doctor, :sign).from(/is in/).to(/is out/) user = User.new(:type => "admin") expect { user.symbolize_type }.to change(user, :type).from(String).to(Symbol)
Evaluates receiver.message
or block
before and
after it evaluates the block passed to expect
.
expect( ... ).not_to change
only supports the form with no
subsequent calls to by
, by_at_least
,
by_at_most
, to
or from
.
# File lib/rspec/matchers.rb, line 339 def change(receiver=nil, message=nil, &block) BuiltIn::Change.new(receiver, message, &block) end
Passes if actual covers expected. This works for Ranges. You can also pass in multiple args and it will only pass if all args are found in Range.
@example
expect(1..10).to cover(5) expect(1..10).to cover(4, 6) expect(1..10).to cover(4, 6, 11) # fails expect(1..10).not_to cover(11) expect(1..10).not_to cover(5) # fails
Ruby >= 1.9 only
# File lib/rspec/matchers.rb, line 355 def cover(*values) BuiltIn::Cover.new(*values) end
Matches if the actual value ends with the expected value(s). In the case of a string, matches against the last `expected.length` characters of the actual string. In the case of an array, matches against the last `expected.length` elements of the actual array.
@example
expect("this string").to end_with "string" expect([0, 1, 2, 3, 4]).to end_with 4 expect([0, 2, 3, 4, 4]).to end_with 3, 4
# File lib/rspec/matchers.rb, line 369 def end_with(*expected) BuiltIn::EndWith.new(*expected) end
Passes if actual == expected
.
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
@example
expect(5).to eq(5) expect(5).not_to eq(3)
# File lib/rspec/matchers.rb, line 382 def eq(expected) BuiltIn::Eq.new(expected) end
Passes if +actual.eql?(expected)+
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
@example
expect(5).to eql(5) expect(5).not_to eql(3)
# File lib/rspec/matchers.rb, line 395 def eql(expected) BuiltIn::Eql.new(expected) end
Passes if actual.equal?(expected)
(object identity).
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
@example
expect(5).to equal(5) # Fixnums are equal expect("5").not_to equal("5") # Strings that look the same are not the same object
# File lib/rspec/matchers.rb, line 408 def equal(expected) BuiltIn::Equal.new(expected) end
Passes if `actual.exist?` or `actual.exists?`
@example
expect(File).to exist("path/to/file")
# File lib/rspec/matchers.rb, line 416 def exist(*args) BuiltIn::Exist.new(*args) 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 expectations about their lengths.
@example
# Passes if team.players.size == 11 expect(team).to have(11).players # Passes if [1,2,3].length == 3 expect([1,2,3]).to have(3).items #"items" is pure sugar # Passes if ['a', 'b', 'c'].count == 3 expect([1,2,3]).to have(3).items #"items" is pure sugar # Passes if "this string".length == 11 expect("this string").to have(11).characters #"characters" is pure sugar
# File lib/rspec/matchers.rb, line 448 def have(n) BuiltIn::Have.new(n) end
Exactly like have() with >=.
@example
expect("this").to have_at_least(3).letters
### Warning:
`expect(..).not_to #have_at_least` is not supported
# File lib/rspec/matchers.rb, line 461 def have_at_least(n) BuiltIn::Have.new(n, :at_least) end
Exactly like have() with <=.
@example
expect("this").to have_at_most(4).letters
### Warning:
`expect(..).not_to #have_at_most` is not supported
# File lib/rspec/matchers.rb, line 473 def have_at_most(n) BuiltIn::Have.new(n, :at_most) 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.
@example
expect([1,2,3]).to include(3) expect([1,2,3]).to include(2,3) expect([1,2,3]).to include(2,3,4) # fails expect([1,2,3]).not_to include(4) expect("spread").to include("read") expect("spread").not_to include("red")
# File lib/rspec/matchers.rb, line 489 def include(*expected) BuiltIn::Include.new(*expected) end
Given a Regexp or String, passes if actual.match(pattern)
@example
expect(email).to match(/^([^\s]+)((?:[-a-z0-9]+\.)+[a-z]{2,})$/i) expect(email).to match("@example.com") expect(zipcode).to match_regex(/\A\d{5}(-\d{4})?\z/) expect(zipcode).to match_regex("90210")
@note Due to Ruby's method dispatch mechanism, using the `#match` matcher within a custom matcher defined via the matcher DSL (`RSpec::Matcher.define`) will result Ruby calling the wrong `#match` method and raising an `ArgumentError`. Instead, use the aliased `#match_regex` method.
# File lib/rspec/matchers.rb, line 507 def match(expected) BuiltIn::Match.new(expected) end
Passes if actual contains all of the expected regardless of order. This works for collections. Pass in multiple args and it will only pass if all args are found in collection.
@note This is also available using the `=~` operator with `should`,
but `=~` is not supported with `expect`.
@note This matcher only supports positive expectations.
expect(..).not_to match_array(other_array) is not supported.
@example
expect([1,2,3]).to match_array([1,2,3]) expect([1,2,3]).to match_array([1,3,2])
# File lib/rspec/matchers.rb, line 692 def match_array(array) BuiltIn::MatchArray.new(array) end
With no args, matches if any error is raised. With a named error, matches only if that specific error is raised. With a named error and messsage specified as a String, matches only if both match. With a named error and messsage specified as a Regexp, matches only if both match. Pass an optional block to perform extra verifications on the exception matched
@example
expect { do_something_risky }.to raise_error expect { do_something_risky }.to raise_error(PoorRiskDecisionError) expect { do_something_risky }.to raise_error(PoorRiskDecisionError) { |error| expect(error.data).to eq 42 } expect { do_something_risky }.to raise_error(PoorRiskDecisionError, "that was too risky") expect { do_something_risky }.to raise_error(PoorRiskDecisionError, /oo ri/) expect { do_something_risky }.not_to raise_error
# File lib/rspec/matchers.rb, line 528 def raise_error(error=Exception, message=nil, &block) BuiltIn::RaiseError.new(error, message, &block) end
Matches if the target object responds to all of the names provided. Names can be Strings or Symbols.
@example
expect(“string”).to #respond_to(:length)
# File lib/rspec/matchers.rb, line 541 def respond_to(*names) BuiltIn::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.
@example
expect(5).to satisfy { |n| n > 3 }
# File lib/rspec/matchers.rb, line 558 def satisfy(&block) BuiltIn::Satisfy.new(&block) end
Matches if the actual value starts with the expected value(s). In the case of a string, matches against the first `expected.length` characters of the actual string. In the case of an array, matches against the first `expected.length` elements of the actual array.
@example
expect("this string").to start_with "this s" expect([0, 1, 2, 3, 4]).to start_with 0 expect([0, 2, 3, 4, 4]).to start_with 0, 1
# File lib/rspec/matchers.rb, line 572 def start_with(*expected) BuiltIn::StartWith.new(*expected) 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.
@example
expect { do_something_risky }.to throw_symbol expect { do_something_risky }.to throw_symbol(:that_was_risky) expect { do_something_risky }.to throw_symbol(:that_was_risky, 'culprit') expect { do_something_risky }.not_to throw_symbol expect { do_something_risky }.not_to throw_symbol(:that_was_risky) expect { do_something_risky }.not_to throw_symbol(:that_was_risky, 'culprit')
# File lib/rspec/matchers.rb, line 592 def throw_symbol(expected_symbol=nil, expected_arg=nil) BuiltIn::ThrowSymbol.new(expected_symbol, expected_arg) end
Passes if the method called in the expect block yields, regardless of whether or not arguments are yielded.
@example
expect { |b| 5.tap(&b) }.to yield_control expect { |b| "a".to_sym(&b) }.not_to yield_control
@note Your expect block must accept a parameter and pass it on to
the method-under-test as a block.
@note This matcher is not designed for use with methods that yield
multiple times.
# File lib/rspec/matchers.rb, line 608 def yield_control BuiltIn::YieldControl.new end
Designed for use with methods that repeatedly yield (such as iterators). Passes if the method called in the expect block yields multiple times with arguments matching those given.
Argument matching is done using `===` (the case match operator) and `==`. If the expected and actual arguments match with either operator, the matcher will pass.
@example
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]) expect { |b| [1, 2, 3].each(&b) }.not_to yield_successive_args(1, 2)
@note Your expect block must accept a parameter and pass it on to
the method-under-test as a block.
# File lib/rspec/matchers.rb, line 674 def yield_successive_args(*args) BuiltIn::YieldSuccessiveArgs.new(*args) end
Given no arguments, matches if the method called in the expect block yields with arguments (regardless of what they are or how many there are).
Given arguments, matches if the method called in the expect block yields with arguments that match the given arguments.
Argument matching is done using `===` (the case match operator) and `==`. If the expected and actual arguments match with either operator, the matcher will pass.
@example
expect { |b| 5.tap(&b) }.to yield_with_args # because #tap yields an arg expect { |b| 5.tap(&b) }.to yield_with_args(5) # because 5 == 5 expect { |b| 5.tap(&b) }.to yield_with_args(Fixnum) # because Fixnum === 5 expect { |b| File.open("f.txt", &b) }.to yield_with_args(/txt/) # because /txt/ === "f.txt" expect { |b| User.transaction(&b) }.not_to yield_with_args # because it yields no args expect { |b| 5.tap(&b) }.not_to yield_with_args(1, 2, 3)
@note Your expect block must accept a parameter and pass it on to
the method-under-test as a block.
@note This matcher is not designed for use with methods that yield
multiple times.
# File lib/rspec/matchers.rb, line 654 def yield_with_args(*args) BuiltIn::YieldWithArgs.new(*args) end
Passes if the method called in the expect block yields with no arguments. Fails if it does not yield, or yields with arguments.
@example
expect { |b| User.transaction(&b) }.to yield_with_no_args expect { |b| 5.tap(&b) }.not_to yield_with_no_args # because it yields with `5` expect { |b| "a".to_sym(&b) }.not_to yield_with_no_args # because it does not yield
@note Your expect block must accept a parameter and pass it on to
the method-under-test as a block.
@note This matcher is not designed for use with methods that yield
multiple times.
# File lib/rspec/matchers.rb, line 625 def yield_with_no_args BuiltIn::YieldWithNoArgs.new end
# File lib/rspec/matchers/method_missing.rb, line 6 def method_missing(method, *args, &block) return Matchers::BuiltIn::BePredicate.new(method, *args, &block) if method.to_s =~ /^be_/ return Matchers::BuiltIn::Has.new(method, *args, &block) if method.to_s =~ /^have_/ super end