Test Doubles are Not To Be Mocked

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This is a Mock Slide Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Hi! Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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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

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So, I wrote a book Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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And I wrote some tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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Then I un-wrote some tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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Why did I do that? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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"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

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Three years later Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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"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

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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

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What can I do about it? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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I'm not talking about this: Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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What makes a test "good"? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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Long term priorities might be different Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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What is "well-designed code"? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Clear Easy to change Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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How? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Domain discovery Behavior validation Safety net Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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A Tale of Two Tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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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

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Straightforward: State Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Well-defined Tie Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Independent Spy Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Fast Spy Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Truthful Let's examine... Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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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

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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

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This is a matter of perspective Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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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

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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

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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

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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

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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

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This is good because Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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It encourages good design practices Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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This is bad because Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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The design might change Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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Test doubles are design canaries Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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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

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So what happened? Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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I wrote the spy tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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I re-wrote them Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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Takeaways Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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Conway's law applies to tests Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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Try writing Rails tests with isolation Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap

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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

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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

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Noel Rappin | Test Doubles are not to be Mocked | http://www.noelrappin.com | http://www.tablexi.com | @noelrap