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More about adding a collaboratorpredicate 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.
- Scenarios
-
- should be_zero (based on Fixnum#zero?)
- should_not be_empty (based on Array#empty?)
- should have_key (based on Hash#has_key?)
- should_not have_all_string_keys (based on custom #has_all_string_keys? method)
- matcher arguments are passed on to the predicate method
- 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"
- 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"
- 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"
- 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"
- 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"
Last published over 7 years ago by myronmarston.