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
These methods are also part of the matcher protocol, but are optional:
does_not_match?(actual) failure_message_when_negated description supports_block_expectations?
## 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.
Note that RSpec does not provide composable aliases for these dynamic predicate matchers. You can easily define your own aliases, though:
RSpec::Matchers.alias_matcher :a_user_who_is_an_admin, :be_an_admin expect(user_list).to include(a_user_who_is_an_admin)
## 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 do |player| # generate and return the appropriate string. end failure_message_when_negated 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 and failure_message_when_negated) 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 "expected #{@target.inspect} to be in Zone #{@expected}" end def failure_message_when_negated "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):
RSpec::configure do |config| config.include(CustomGameMatchers) end
### Making custom matchers composable
RSpec’s built-in matchers are designed to be composed, in expressions like:
expect(["barn", 2.45]).to contain_exactly( a_value_within(0.1).of(2.5), a_string_starting_with("bar") )
Custom matchers can easily participate in composed matcher expressions like these. Include {RSpec::Matchers::Composable} in your custom matcher to make it support being composed (matchers defined using the DSL have this included automatically). Within your matcher’s `matches?` method (or the `match` block, if using the DSL), use `values_match?(expected, actual)` rather than `expected == actual`. Under the covers, `values_match?` is able to match arbitrary nested data structures containing a mix of both matchers and non-matcher objects. It uses `===` and `==` to perform the matching, considering the values to match if either returns `true`. The `Composable` mixin also provides some helper methods for surfacing the matcher descriptions within your matcher’s description or failure messages.
RSpec’s built-in matchers each have a number of aliases that rephrase the matcher from a verb phrase (such as `be_within`) to a noun phrase (such as `a_value_within`), which reads better when the matcher is passed as an argument in a composed matcher expressions, and also uses the noun-phrase wording in the matcher’s `description`, for readable failure messages. You can alias your custom matchers in similar fashion using {RSpec::Matchers.alias_matcher}.
Defines a matcher alias. The returned matcher’s `description` will be overriden to reflect the phrasing of the new name, which will be used in failure messages when passed as an argument to another matcher in a composed matcher expression.
@param new_name [Symbol] the new name for the matcher @param old_name [Symbol] the original name for the matcher @yield [String] optional block that, when given is used to define the overriden
description. The yielded arg is the original description. If no block is provided, a default description override is used based on the old and new names.
@example
RSpec::Matchers.alias_matcher :a_list_that_sums_to, :sum_to sum_to(3).description # => "sum to 3" a_list_that_sums_to(3).description # => "a list that sums to 3"
@example
RSpec::Matchers.alias_matcher :a_list_sorted_by, :be_sorted_by do |description| description.sub("be sorted by", "a list sorted by") end be_sorted_by(:age).description # => "be sorted by age" a_list_sorted_by(:age).description # => "a list sorted by age"
@!macro [attach] alias_matcher
@!parse alias $1 $2
# File lib/rspec/matchers.rb, line 252 def self.alias_matcher(new_name, old_name, &description_override) description_override ||= lambda do |old_desc| old_desc.gsub(Pretty.split_words(old_name), Pretty.split_words(new_name)) end define_method(new_name) do |*args, &block| matcher = __send__(old_name, *args, &block) AliasedMatcher.new(matcher, description_override) end end
@api private Used by rspec-core to clear the state used to generate descriptions after an example.
# File lib/rspec/matchers/generated_descriptions.rb, line 11 def self.clear_generated_description self.last_matcher = nil self.last_expectation_handler = nil end
Delegates to {RSpec::Expectations.configuration}. This is here because rspec-core’s `expect_with` option looks for a `configuration` method on the mixin (`RSpec::Matchers`) to yield to a block. @return [RSpec::Expectations::Configuration] the configuration object
# File lib/rspec/matchers.rb, line 886 def self.configuration Expectations.configuration end
@api private Generates an an example description based on the last expectation. Used by rspec-core’s one-liner syntax.
# File lib/rspec/matchers/generated_descriptions.rb, line 19 def self.generated_description return nil if last_expectation_handler.nil? "#{last_expectation_handler.verb} #{last_description}" end
Passes if actual all expected objects pass. This works for any enumerable object.
@example
expect([1, 3, 5]).to all be_odd expect([1, 3, 6]).to all be_odd # fails
@note The negative form `not_to all` is not supported. Instead
use `not_to include` or pass a negative form of a matcher as the argument (e.g. `all exclude(:foo)`).
@note You can also use this with compound matchers as well.
@example
expect([1, 3, 5]).to all( be_odd.and be_an(Integer) )
# File lib/rspec/matchers.rb, line 599 def all(expected) BuiltIn::All.new(expected) end
@example
expect(actual).to be_truthy expect(actual).to be_falsey 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 303 def be(*args) args.empty? ? Matchers::BuiltIn::Be.new : equal(*args) end
passes if target.kind_of?(klass)
# File lib/rspec/matchers.rb, line 309 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 334 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 321 def be_an_instance_of(expected) BuiltIn::BeAnInstanceOf.new(expected) end
Passes if actual.between?(min, max). Works with any Comparable object, including String, Symbol, Time, or Numeric (Fixnum, Bignum, Integer, Float, Complex, and Rational).
By default, `be_between` is inclusive (i.e. passes when given either the max or min value), but you can make it `exclusive` by chaining that off the matcher.
@example
expect(5).to be_between(1, 10) expect(11).not_to be_between(1, 10) expect(10).not_to be_between(1, 10).exclusive
# File lib/rspec/matchers.rb, line 352 def be_between(min, max) BuiltIn::BeBetween.new(min, max) end
Passes if actual is falsey (false or nil)
# File lib/rspec/matchers.rb, line 270 def be_falsey BuiltIn::BeFalsey.new end
Passes if actual is nil
# File lib/rspec/matchers.rb, line 278 def be_nil BuiltIn::BeNil.new end
Passes if actual is truthy (anything but false or nil)
# File lib/rspec/matchers.rb, line 264 def be_truthy BuiltIn::BeTruthy.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 363 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(..).to` or `expect(…).not_to` method.
You can chain any of the following off of the end to specify details about the change:
`by`
`by_at_least`
`by_at_most`
`from`
`to`
@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 }.from("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` supports the form that specifies `from` (which specifies what you expect the starting, unchanged value to be) but does not support forms with subsequent calls to `by`, `by_at_least`, `by_at_most` or `to`.
# File lib/rspec/matchers.rb, line 445 def change(receiver=nil, message=nil, &block) BuiltIn::Change.new(receiver, message, &block) 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 contain_exactly(other_array)` is not supported.
@example
expect([1, 2, 3]).to contain_exactly(1, 2, 3) expect([1, 2, 3]).to contain_exactly(1, 3, 2)
@see match_array
# File lib/rspec/matchers.rb, line 467 def contain_exactly(*items) BuiltIn::ContainExactly.new(items) 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
### Warning |
Ruby >= 1.9 only |
# File lib/rspec/matchers.rb, line 485 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 501 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 517 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 532 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 547 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 557 def exist(*args) BuiltIn::Exist.new(*args) 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 575 def include(*expected) BuiltIn::Include.new(*expected) end
Given a `Regexp` or `String`, passes if `actual.match(pattern)` Given an arbitrary nested data structure (e.g. arrays and hashes), matches if `expected === actual` || `actual == expected` for each pair of elements.
@example
expect(email).to match(/^([^\s]+)((?:[-a-z0-9]+\.)+[a-z]{2,})$/i) expect(email).to match("@example.com")
@example
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) } } )
@note The `match_regex` alias is deprecated and is not recommended for use.
It was added in 2.12.1 to facilitate its use from within custom matchers (due to how the custom matcher DSL was evaluated in 2.x, `match` could not be used there), but is no longer needed in 3.x.
# File lib/rspec/matchers.rb, line 636 def match(expected) BuiltIn::Match.new(expected) end
An alternate form of `contain_exactly` that accepts the expected contents as a single array arg rather that splatted out as individual items.
@example
expect(results).to contain_exactly(1, 2) # is identical to: expect(results).to match_array([1, 2])
@see contain_exactly
# File lib/rspec/matchers.rb, line 655 def match_array(items) contain_exactly(*items) end
With no arg, passes if the block outputs `to_stdout` or `to_stderr`. With a string, passes if the blocks outputs that specific string `to_stdout` or `to_stderr`. With a regexp or matcher, passes if the blocks outputs a string `to_stdout` or `to_stderr` that matches.
@example
expect { print 'foo' }.to output.to_stdout expect { print 'foo' }.to output('foo').to_stdout expect { print 'foo' }.to output(/foo/).to_stdout expect { do_something }.to_not output.to_stdout expect { warn('foo') }.to output.to_stderr expect { warn('foo') }.to output('foo').to_stderr expect { warn('foo') }.to output(/foo/).to_stderr expect { do_something }.to_not output.to_stderr
@note This matcher works by temporarily replacing `$stdout` or `$stderr`,
so it's not able to intercept stream output that explicitly uses `STDOUT`/`STDERR` or that uses a reference to `$stdout`/`$stderr` that was stored before the matcher is used.
# File lib/rspec/matchers.rb, line 681 def output(expected=nil) BuiltIn::Output.new(expected) 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 701 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 721 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 740 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 756 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 779 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 803 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 875 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 853 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 822 def yield_with_no_args BuiltIn::YieldWithNoArgs.new end
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