RSpec::Matchers provides a number of useful matchers we use to compose 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 `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.
## 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.
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:
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 should
or should_not
).
### 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
# 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
@example
actual.should be_true actual.should be_false actual.should be_nil actual.should be_[arbitrary_predicate](*args) actual.should_not be_nil actual.should_not 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/be.rb, line 220 def be(*args) args.empty? ? Matchers::Be.new : equal(*args) end
passes if target.kind_of?(klass)
# File lib/rspec/matchers/be.rb, line 226 def be_a(klass) be_a_kind_of(klass) end
Passes if actual.kind_of?(expected)
@example
5.should be_kind_of(Fixnum) 5.should be_kind_of(Numeric) 5.should_not be_kind_of(Float)
# File lib/rspec/matchers/be_kind_of.rb, line 18 def be_a_kind_of(expected) BeAKindOf.new(expected) end
Passes if actual.instance_of?(expected)
@example
5.should be_instance_of(Fixnum) 5.should_not be_instance_of(Numeric) 5.should_not be_instance_of(Float)
# File lib/rspec/matchers/be_instance_of.rb, line 18 def be_an_instance_of(expected) 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})", "be_within(#{delta}).of(#{expected})") be_within(delta).of(expected) end
Passes if actual is falsy (false or nil)
# File lib/rspec/matchers/be.rb, line 27 def be_false BeFalse.new end
Passes if actual is nil
# File lib/rspec/matchers/be.rb, line 48 def be_nil BeNil.new end
Passes if actual is truthy (anything but false or nil)
# File lib/rspec/matchers/be.rb, line 14 def be_true BeTrue.new end
Passes if actual == expected +/- delta
@example
result.should be_within(0.5).of(3.0) result.should_not be_within(0.5).of(3.0)
# File lib/rspec/matchers/be_within.rb, line 43 def be_within(delta) 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 should
or
should_not
method.
@example
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") lambda { doctor.leave_office }.should change(doctor, :sign).from(%ris in/).to(%ris out/) user = User.new(:type => "admin") lambda { user.symbolize_type }.should change(user, :type).from(String).to(Symbol)
Evaluates receiver.message
or block
before and
after it evaluates the proc object (generated by the lambdas in the
examples above).
should_not change
only supports the form with no subsequent
calls to by
, by_at_least
,
by_at_most
, to
or from
.
# File lib/rspec/matchers/change.rb, line 193 def change(receiver=nil, message=nil, &block) Matchers::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
(1..10).should cover(5) (1..10).should cover(4, 6) (1..10).should cover(4, 6, 11) # will fail (1..10).should_not cover(11) (1..10).should_not cover(5) # will fail
Ruby >= 1.9 only
# File lib/rspec/matchers/cover.rb, line 32 def cover(*values) Cover.new(*values) end
Passes if actual == expected
.
See www.ruby-doc.org/core/classes/Object.html#M001057 for more information about equality in Ruby.
@example
5.should eq(5) 5.should_not eq(3)
# File lib/rspec/matchers/eq.rb, line 31 def eq(expected) 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
5.should eql(5) 5.should_not eql(3)
# File lib/rspec/matchers/eql.rb, line 31 def eql(expected) 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
5.should equal(5) # Fixnums are equal "5".should_not equal("5") # Strings that look the same are not the same object
# File lib/rspec/matchers/equal.rb, line 54 def equal(expected) Equal.new(expected) end
Passes if `actual.exist?` or `actual.exists?`
@example
File.should exist("path/to/file")
# File lib/rspec/matchers/exist.rb, line 30 def exist(*args) Exist.new(*args) end
Extends the submitted block with aliases to and to_not for should and should_not.
@example
expect { this_block }.to change{this.expression}.from(old_value).to(new_value) expect { this_block }.to raise_error
# File lib/rspec/matchers/block_aliases.rb, line 15 def expect(&block) block.extend BlockAliases 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 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 ['a', 'b', 'c'].count == 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/rspec/matchers/have.rb, line 133 def have(n) Matchers::Have.new(n) end
Exactly like have() with >=.
@example
"this".should have_at_least(3).letters
### Warning:
`should_not #have_at_least` is not supported
# File lib/rspec/matchers/have.rb, line 146 def have_at_least(n) Matchers::Have.new(n, :at_least) end
Exactly like have() with <=.
@example
should have_at_most(number).items
### Warning:
`should_not #have_at_most` is not supported
# File lib/rspec/matchers/have.rb, line 158 def have_at_most(n) Matchers::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
[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/rspec/matchers/include.rb, line 62 def include(*expected) Include.new(*expected) end
Given a Regexp or String, passes if actual.match(pattern)
@example
email.should match(/^([^\s]+)((?:[-a-z0-9]+\.)+[a-z]{2,})$/i) email.should match("@example.com")
# File lib/rspec/matchers/match.rb, line 17 def match(expected) Match.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
lambda { do_something_risky }.should raise_error lambda { do_something_risky }.should raise_error(PoorRiskDecisionError) lambda { do_something_risky }.should raise_error(PoorRiskDecisionError) { |error| error.data.should == 42 } lambda { do_something_risky }.should raise_error(PoorRiskDecisionError, "that was too risky") lambda { do_something_risky }.should raise_error(PoorRiskDecisionError, %roo ri/) lambda { do_something_risky }.should_not raise_error lambda { do_something_risky }.should_not raise_error(PoorRiskDecisionError) lambda { do_something_risky }.should_not raise_error(PoorRiskDecisionError, "that was too risky") lambda { do_something_risky }.should_not raise_error(PoorRiskDecisionError, %roo ri/)
# File lib/rspec/matchers/raise_error.rb, line 111 def raise_error(error=Exception, message=nil, &block) Matchers::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
# File lib/rspec/matchers/respond_to.rb, line 76 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.
@example
5.should satisfy { |n| n > 3 }
# File lib/rspec/matchers/satisfy.rb, line 42 def satisfy(&block) Matchers::Satisfy.new(&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.
@example
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/rspec/matchers/throw_symbol.rb, line 108 def throw_symbol(expected_symbol=nil, expected_arg=nil) Matchers::ThrowSymbol.new(expected_symbol, expected_arg) end