Clarity on Test Doubles in Ruby Mocks, Stubs, Fakes, Dummies, Spies and such. Presented at Boston.rb, Philly.rb and NYC.rb.

1. DOUBLETROUBLE
Clarity on test doubles.

2. TESTING PHASES
exercise
verify
teardown
let(:sox) { Team.new “Boston” }
let(:drays) { Team.new “Tampa” }
before { sox.play(drays) }
it “should set standings” do
expect(drays).to be_losing
end
after { standings.delete_all }
setup

3. TESTING PHASES
setup
exercise
verify
teardown
→
let(:sox) { Team.new “Boston” }
let(:drays) { Team.new “Tampa” }
before { sox.play(drays) }
it “should set standings” do
expect(drays).to be_losing
end
after { standings.delete_all }

4. TESTING PHASES
excercise
setup
verify
teardown
→
let(:sox) { Team.new “Boston” }
let(:drays) { Team.new “Tampa” }
before { sox.play(drays) }
it “should set standings” do
expect(drays).to be_losing
end
after { standings.delete_all }

5. TESTING PHASES
verify
setup
exercise
teardown
→
let(:sox) { Team.new “Boston” }
let(:drays) { Team.new “Tampa” }
before { sox.play(drays) }
it “should set standings” do
expect(drays).to be_losing
end
after { standings.delete_all }

6. TESTING PHASES
teardown
setup
exercise
verify
→
let(:sox) { Team.new “Boston” }
let(:drays) { Team.new “Tampa” }
before { sox.play(drays) }
it “should set standings” do
expect(drays).to be_losing
end
after { standings.delete_all }

7. WHAT IS ATEST DOUBLE?

8. WHAT IS ATEST DOUBLE?
SUT

9. DOC
WHAT IS ATEST DOUBLE?
SUT

10. DOC
WHAT IS ATEST DOUBLE?
SUT
Indirect Output

11. DOC
WHAT IS ATEST DOUBLE?
SUT
Indirect Output
Indirect Input

12. DOCTest
Double
WHAT IS ATEST DOUBLE?
SUT
Indirect Output
Indirect Input

13. TEST DOUBLE PATTERNS
• Dummy Object
• Fake Object
• Mock Object
• Test Stub
• Test Spy

14. DUMMY OBJECT
A placeholder that is passed to the SUT and never used
let(:side_a) { 1 }
let(:side_b) { 2 }
let(:dummy) { Object.new }
subject { HighSchoolTrig.hypotenuse(a, b, dummy) }
it { should eq 2.236 }

15. FAKE OBJECT
An object which replaces the real DOC with an alternate
implementation of the same functionality
class FakePiCalc
def pi; 3.14159; end
end
let(:radius) { 2 }
before { MyGeo.pi_calculator = FakePiCalc }
subject { MyGeo.circumference(radius) }
it { should eq 13 }

16. MOCKS, STUBS & SPIES
An example:
class User < ActiveRecord::Base
before_create :enqueue_welcome_message
def enqueue_welcome_message
queue = Application.config.email_queue
raise(“Failed to queue”) unless queue.push(email, “Welcome”)
end
end

17. NO DOUBLES
let(:email) { “tom@crui.se” }
subject { User.create(email: email) }
it { should be_persisted }
its(:username) { should eq email }

18. MOCK OBJECT
An object which replaces the real DOC that can verify indirect
output from the SUT with expectations
let(:mock_queue) { double() }
let(:email) { “tom@crui.se” }
before do
Application.config.email_queue = mock_queue
expect(mock_queue).to receive(:push).with(email, “Welcome”)
end
subject { User.create(email: email) }
it { should be_persisted }
its(:username) { should eq email }

19. TEST STUB
An object which replaces the real DOC to control indirect
input to the SUT
let(:stub_queue) { double(push: true) }
let(:email) { “tom@crui.se” }
before do
Application.config.email_queue = stub_queue
end
subject { User.create(email: email) }
it { should be_persisted }
its(:username) { should eq email }

20. TEST SPY
A more capableTest Stub allowing verification of indirect output
from the SUT
let(:spy_queue) { double(push: true) }
let(:email) { “tom@crui.se” }
before do
Application.config.email_queue = spy_queue
end
subject { User.create(email: email) }
it { should be_persisted }
its(:username) { should eq email }
it “should enqueue welcome message” do
expect(spy_queue).to have_received(:push).with(email, “Ohai”)
end

21. DESIGNING FOR DOUBLES
• Dependency Lookup
• Dependency Injection

22. class Buddy
def good_friend?; on_tap.craft?; end
def on_tap
Fridge.cold_one
end
end
describe Buddy, “serving coors” do
# TODO control indirect input to the SUT
it “should not be a good friend” do
expect(subject).not_to be_good_friend
end

23. DEPENDENCY LOOKUP
class Buddy
def good_friend?; on_tap.craft?; end
def on_tap
Fridge.cold_one
end
end
describe Buddy, “serving coors” do
let(:coors) { double(craft?: false) }
before { Fridge.stubs(:cold_one) { coors } }
it “should not be a good friend” do
expect(subject).not_to be_good_friend
end

24. class Buddy
attr_accessor :fridge
def good_friend?; on_tap.craft?; end
def on_tap
@fridge.cold_one
end
end
describe Buddy, “serving coors” do
let(:coors) { double(craft?: false) }
let(:stub_fridge) { double(cold_one: coors) }
before { subject.fridge = stub_fridge }
it “should not be a good friend” do
expect(subject).not_to be_good_friend
end
DEPENDENCY INJECTION

25. RETROFITTING
• Test-Specific Subclasses
• Test Hooks

26. class Buddy
attr_reader :supermarket
def make_breakfast(request=“Steak & eggs”)
ingredients = supermarket.find(request)
prepare(ingredients)
end
end

27. TEST-SPECIFIC SUBCLASSES
class Buddy
attr_reader :supermarket
def make_breakfast(request=“Steak & eggs”)
ingredients = supermarket.find(request)
prepare(ingredients)
end
end
class TestBuddy < Buddy
attr_writer :supermarket
end

28. TEST HOOKS
class Buddy
if ENV != “TEST”
attr_reader :supermarket
else
attr_accessor :supermarket
end
def make_breakfast(request=“Steak & eggs”)
ingredients = supermarket.find(request)
prepare(ingredients)
end
end

29. ARETHEY FORYOU?

30. “MOCKIST”TDD / BDD
• Uses mocks for all DOCs
• Likes the test writing process to inform design decisions
• Tests in strict isolation

31. CLASSICTDD
• Uses test doubles only for awkward DOCs, favoring “real”
objects
• Minimizes coupling between tests and implementation
• Tests small clusters of components, not isolated units

32. CLASSICTDDERS CONSIDER
USING ATEST DOUBLE IF:
• The behavior of the DOC cannot be changed/observed
• Use of the DOC could cause unwanted side-effects
• The DOC is too slow
• The DOC doesn’t exist yet

33. OVERUSE CAN LEADTO:
• Over specified tests of the SUT’s process, not its result
• Fragile tests that break when implementation changes
• Untested integration
• Less time on Hacker News while your build runs

34. MORE
xUnit Test Patterns: xunitpatterns.com
Mocks aren’t Stubs by Martin Fowler: martinfowler.com/articles/mocksArentStubs.html
A case against a case against mocking and stubbing by David Chelimsky:
blog.davidchelimsky.net/2008/12/11/a-case-against-a-case-against-mocking-and-stubbing/
Timecop for testing time-dependent code: github.com/travisjeffery/timecop
RSpec: rspec.info
MiniTest: ruby-doc.org/stdlib
Mocha: github.com/freerange/mocha

35. greg@promptworks.com