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 negative_failure_message #optional description #optional
See Spec::Expectations to learn how to use these as Expectation Matchers. See Spec::Mocks to learn how to use them as Mock Argument Constraints.
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.
For example, 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")
To do this, you would need to write a class like this:
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 negative_failure_message "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 is true, false or nil (respectively).
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? "this string".should be_an_intance_of(String) 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 202 202: def be(*args) 203: Matchers::Be.new(*args) 204: end
Passes if actual == expected +/- delta
result.should be_close(3.0, 0.5)
# File lib/spec/matchers/be_close.rb, line 33 33: def be_close(expected, delta) 34: Matchers::BeClose.new(expected, delta) 35: 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) 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 be, 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 117 117: def change(target=nil, message=nil, &block) 118: Matchers::Change.new(target, message, &block) 119: 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 39 39: def eql(expected) 40: Matchers::Eql.new(expected) 41: 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 39 39: def equal(expected) 40: Matchers::Equal.new(expected) 41: 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 113 113: def have(n) 114: Matchers::Have.new(n) 115: end
Exactly like have() with >=.
should_not have_at_least is not supported
# File lib/spec/matchers/have.rb, line 126 126: def have_at_least(n) 127: Matchers::Have.new(n, :at_least) 128: end
Exactly like have() with <=.
should_not have_at_most is not supported
# File lib/spec/matchers/have.rb, line 138 138: def have_at_most(n) 139: Matchers::Have.new(n, :at_most) 140: 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 66 66: def include(*expected) 67: Matchers::Include.new(*expected) 68: end
Given a Regexp, passes if actual =~ regexp
email.should match(/^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i)
# File lib/spec/matchers/match.rb, line 37 37: def match(regexp) 38: Matchers::Match.new(regexp) 39: 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.
lambda { do_something_risky }.should raise_error lambda { do_something_risky }.should raise_error(PoorRiskDecisionError) lambda { do_something_risky }.should raise_error(PoorRiskDecisionError, "that was too risky") lambda { do_something_risky }.should raise_error(PoorRiskDecisionError, /oo 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, /oo ri/)
# File lib/spec/matchers/raise_error.rb, line 96 96: def raise_error(error=Exception, message=nil) 97: Matchers::RaiseError.new(error, message) 98: 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 43: def satisfy(&block) 44: Matchers::Satisfy.new(&block) 45: end
Given a Symbol argument, matches if a proc throws the specified Symbol.
Given no argument, matches if a proc throws any Symbol.
lambda { do_something_risky }.should throw_symbol lambda { do_something_risky }.should throw_symbol(:that_was_risky) lambda { do_something_risky }.should_not throw_symbol lambda { do_something_risky }.should_not throw_symbol(:that_was_risky)
# File lib/spec/matchers/throw_symbol.rb, line 68 68: def throw_symbol(sym=nil) 69: Matchers::ThrowSymbol.new(sym) 70: end