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Project: RSpec Expectations 3.5

Publisher: RSpec

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:

  expect(7.zero?).to eq 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:

  expect(7).not_to be_zero       # calls 7.zero?
  expect([]).to be_empty         # calls [].empty?
  expect(x).to 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.

  expect(hash).to have_key(:foo)       # calls hash.has_key?(:foo)
  expect(array).not_to 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

Calling private methods will also fail:

expected private_method? to return true but it's a private method

Any arguments passed to the matcher will be passed on to the predicate method.

Scenarios
  1. should be_zero (based on Fixnum#zero?)
  2. should_not be_empty (based on Array#empty?)
  3. should have_key (based on Hash#has_key?)
  4. should_not have_all_string_keys (based on custom #has_all_string_keys? method)
  5. matcher arguments are passed on to the predicate method
  6. calling private method with be_predicate causes error
  7. calling private method with have_predicate causes error
should be_zero (based on Fixnum#zero?)
Given
a file named "should_be_zero_spec.rb" with: 
RSpec.describe 0 do
  it { is_expected.to be_zero }
end

RSpec.describe 7 do
  it { is_expected.to 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 7.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: 
RSpec.describe [1, 2, 3] do
  it { is_expected.not_to be_empty }
end

RSpec.describe [] do
  it { is_expected.not_to 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: 
RSpec.describe Hash do
  subject { { :foo => 7 } }
  it { is_expected.to have_key(:foo) }
  it { is_expected.to 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

RSpec.describe Hash do
  context 'with symbol keys' do
    subject { { :foo => 7, :bar => 5 } }
    it { is_expected.not_to have_all_string_keys }
  end

  context 'with string keys' do
    subject { { 'foo' => 7, 'bar' => 5 } }
    it { is_expected.not_to 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

RSpec.describe 12 do
  it { is_expected.to be_multiple_of(3) }
  it { is_expected.not_to be_multiple_of(7) }

  # deliberate failures
  it { is_expected.not_to be_multiple_of(4) }
  it { is_expected.to 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 12.multiple_of?(4) to return false, got true"
And
the output should contain "expected 12.multiple_of?(5) to return true, got false"
calling private method with be_predicate causes error
Given
a file named "attempting_to_match_private_method_spec.rb" with: 
class WithPrivateMethods
  def secret?
    true
  end
  private :secret?
end

RSpec.describe 'private methods' do
  subject { WithPrivateMethods.new }

  # deliberate failure
  it { is_expected.to be_secret }
end
When
I run rspec attempting_to_match_private_method_spec.rb
Then
the output should contain "1 example, 1 failure"
And
the output should contain "secret? is a private method"
calling private method with have_predicate causes error
Given
a file named "attempting_to_match_private_method_spec.rb" with: 
class WithPrivateMethods
  def has_secret?
    true
  end
  private :has_secret?
end

RSpec.describe 'private methods' do
  subject { WithPrivateMethods.new }

  # deliberate failure
  it { is_expected.to have_secret }
end
When
I run rspec attempting_to_match_private_method_spec.rb
Then
the output should contain "1 example, 1 failure"
And
the output should contain "has_secret? is a private method"

Last published over 5 years ago by myronmarston.