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Feature: predicate matchers
Ruby objects commonly provide predicate methods:
7.zero? # => false
0.zero? # => true
[1].empty? # => false
[].empty? # => true
{ :a => 5 }.has_key?(:b) # => false
{ :b => 5 }.has_key?(:b) # => true
You could use a basic equality matcher to set expectations on these:
7.zero?.should == true # fails with "expected true, got false (using ==)"
...but RSpec provides dynamic predicate matchers that are more readable and
provide better failure output.
For any predicate method, RSpec gives you a corresponding matcher. Simply
prefix the method with "be_" and remove the question mark. Examples:
7.should_not be_zero # calls 7.zero?
[].should be_empty # calls [].empty?
x.should be_multiple_of(3) # calls x.multiple_of?(3)
Alternately, for a predicate method that begins with "has_" like Hash#has_key?,
RSpec allows you to use an alternate form since "be_has_key" makes no sense.
hash.should have_key(:foo) # calls hash.has_key?(:foo)
array.should_not have_odd_values # calls array.has_odd_values?
In either case, RSpec provides nice, clear error messages, such as:
expected zero? to return true, got false
Any arguments passed to the matcher will be passed on to the predicate method.
Scenario: should be_zero (based on Fixnum#zero?)
Given a file named "should_be_zero_spec.rb" with:
"""
describe 0 do
it { should be_zero }
end
describe 7 do
it { should be_zero } # deliberate failure
end
"""
When I run `rspec should_be_zero_spec.rb`
Then the output should contain "2 examples, 1 failure"
And the output should contain "expected zero? to return true, got false"
Scenario: should_not be_empty (based on Array#empty?)
Given a file named "should_not_be_empty_spec.rb" with:
"""
describe [1, 2, 3] do
it { should_not be_empty }
end
describe [] do
it { should_not be_empty } # deliberate failure
end
"""
When I run `rspec should_not_be_empty_spec.rb`
Then the output should contain "2 examples, 1 failure"
And the output should contain "expected empty? to return false, got true"
Scenario: should have_key (based on Hash#has_key?)
Given a file named "should_have_key_spec.rb" with:
"""
describe Hash do
subject { { :foo => 7 } }
it { should have_key(:foo) }
it { should have_key(:bar) } # deliberate failure
end
"""
When I run `rspec should_have_key_spec.rb`
Then the output should contain "2 examples, 1 failure"
And the output should contain "expected #has_key?(:bar) to return true, got false"
Scenario: should_not have_all_string_keys (based on custom #has_all_string_keys? method)
Given a file named "should_not_have_all_string_keys_spec.rb" with:
"""
class Hash
def has_all_string_keys?
keys.all? { |k| String === k }
end
end
describe Hash do
context 'with symbol keys' do
subject { { :foo => 7, :bar => 5 } }
it { should_not have_all_string_keys }
end
context 'with string keys' do
subject { { 'foo' => 7, 'bar' => 5 } }
it { should_not have_all_string_keys } # deliberate failure
end
end
"""
When I run `rspec should_not_have_all_string_keys_spec.rb`
Then the output should contain "2 examples, 1 failure"
And the output should contain "expected #has_all_string_keys? to return false, got true"
Scenario: matcher arguments are passed on to the predicate method
Given a file named "predicate_matcher_argument_spec.rb" with:
"""
class Fixnum
def multiple_of?(x)
(self % x).zero?
end
end
describe 12 do
it { should be_multiple_of(3) }
it { should_not be_multiple_of(7) }
# deliberate failures
it { should_not be_multiple_of(4) }
it { should be_multiple_of(5) }
end
"""
When I run `rspec predicate_matcher_argument_spec.rb`
Then the output should contain "4 examples, 2 failures"
And the output should contain "expected multiple_of?(4) to return false, got true"
And the output should contain "expected multiple_of?(5) to return true, got false"
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