Test Doubles are Not To Be Mocked

Test doubles (which you may know under the alias “mock objects”) are the most misunderstood and misused testing tools we've got. Starting with the fact that nobody can agree on what to call them. Contrary to what you may have heard, test doubles do not inherently lead to fragile tests. What they do is hold up a harsh mirror to the assumptions in our code. They are the light saber of testing tools: a more elegant weapon for a more civilized age. But be careful not to cut your code in half, so to speak. Herein: a guide to using test doubles without losing your sanity.

Transcript

1. This is a Mock Slide Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
2. Hi! Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
3. Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
4. This is the talk about test doubles Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
5. Not the talk by the head of Test Double Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
6. Justin isn't here, so I can mock him... Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
7. expect(:justin).to recieve(:this_talk) .and_return(:polite_dissent) Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
8. So, I wrote a book Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
9. And I wrote some tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
10. it "calculates price where tickets have discounts" do ticket_one = spy(price_check: 1000) ticket_two = spy(price_check: 1000) discount = spy() calculator = PriceCalculator.new([ticket_one, ticket_two], [discount]) calculator.calculate! expect(calculator.price_cents.to eq(2000)) expect(ticket_one).to have_received(:price_check).with(discount) expect(ticket_two).to have_received(:price_check).with(discount) end Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
11. Then I un-wrote some tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
12. it "calculates price where tickets have discounts" do ticket_one = Ticket.create(price_cents: 2000) ticket_two = Ticket.create(price_cents: 2000) discount = DiscountCode.create(discount_percentage: 25) calculator = PriceCalculator.new([ticket_one, ticket_two], [discount]) calculator.calculate! expect(calculator.price_cents.to eq(3000)) end Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
13. Why did I do that? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
14. I've been ambivalent about test doubles for years Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
15. "I have to say, as much as I love using mocks and stubs to cover hard-to-reach objects and states, my own history with very strict behavior-based mock test structures hasn’t been great. My experience was that writing all the mocks around a given object tended to be a drag on the test process. But I’m wide open to the possibility that this method works better for others or that I’m not doing it right." Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
16. Three years later Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
17. "My opinion about the best way to use mock objects changes every few months. I’ll try some mocks, they’ll work well, I’ll start using more mocks, they’ll start getting in the way, I’ll back off, and then I’ll think, “Let’s try some mocks.” This cycle has been going for years and I have no reason to think it’s going to change anytime soon." Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
18. Why does this keep happening to me? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
19. What can I do about it? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
20. Test Doubles are not to be Mocked Noel Rappin, Table XI (@noelrap) Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
21. I'm not talking about this: Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
22. it "handles a stripe failure" do token = instance_double(StripeCharge, success: false) allow(StripeCharge).to receive(:create).and return(token) workflow.run expect(workflow.status).to eq(:failure) end Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
23. Non controversial usage replacing heavyweight objects specifying failure states Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
24. What makes a test "good"? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
25. SWIFT Straightforward, Well-Defined, Independent, Fast, Truthful Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
26. Long term priorities might be different Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
27. A good test Leads to well-designed code Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
28. What is "well-designed code"? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
29. Clear Easy to change Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
30. How? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
31. Domain discovery Behavior validation Safety net Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
32. A Tale of Two Tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
33. Calculate the total price of multiple tickets, given a discount Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
34. State test it "calculates price where tickets have discounts" do ticket_one = Ticket.create(price_cents: 2000) ticket_two = Ticket.create(price_cents: 2000) discount = DiscountCode.create(discount_percentage: 25) calculator = PriceCalculator.new([ticket_one, ticket_two], [discount]) calculator.calculate! expect(calculator.price_cents.to eq(3000)) end Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
35. Spy Test (!) it "calculates price where tickets have discounts" do ticket_one = spy(price_check: 1000) ticket_two = spy(price_check: 1000) discount = spy() calculator = PriceCalculator.new([ticket_one, ticket_two], [discount]) calculator.calculate! expect(calculator.price_cents.to eq(2000)) expect(ticket_one).to have_received(:price_check).with(discount) expect(ticket_two).to have_received(:price_check).with(discount) end Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
36. State: uses real objects, expectation on state Spy !: uses test doubles, expectations on behavior Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
37. Straightforward: State Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
38. Well-defined Tie Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
39. Independent Spy Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
40. Fast Spy Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
41. Truthful Let's examine... Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
42. If the Calculator has a bug: State fails Spy fails Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
43. If the Ticket has a bug State fails Spy passes Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
44. If the Ticket doesn't exist State fails Spy passes Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
45. So far, this seems bad for the Spy test Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
46. The code can be wrong and the double test will pass Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
47. This is a matter of perspective Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
48. And this is not the only test in the suite Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
49. Let's say the discount logic gets more complex Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
50. If the ticket API changes: Team State fails Team Double passes Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
51. The state test fails even though neither the code or the test changed Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
52. The test can fail even though the test is right Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
53. Design goal: A failure state causes exactly one test to fail Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
54. Doubles test the design and the behavior Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
55. The double encourages the creation of additional tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
56. This is good because Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
57. It encourages good design practices Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
58. This is bad because Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
59. The design might change Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
60. It is hard to drive the next failing test Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
61. This implies a lot of test and code isolation Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
62. More isolation that many people are comfortable committing to up front Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
63. Test doubles are design canaries Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
64. Lot of setup might mean poorly factored design Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
65. This level of isolation is hard on a team if you are the only one who cares Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
66. A third-party framework can make this hard Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
67. So what happened? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
68. I wrote the spy tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
69. I was unwilling or unable to make the code isolated enough to keep the spies Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
70. I re-wrote them Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
71. This is easy But it breaks encapsulation But it's easy Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
72. Takeaways Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
73. Conway's law applies to tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
74. Think about what will make your tests fail Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
75. Try doubles where behavior is more important than state Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
76. Try writing Rails tests with isolation Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
77. Using an integration test can force additional unit tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
78. Noel Rappin (@noelrap) (http://www.noelrappin.com) Table XI https://tinyletter.com/noelrap http://pragprog.com/book/nrwebpay http://pragprog.com/book/nrtest2 Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap
79. Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap