Building a Python Cake Testing: The Layers of Your Application

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Matt Behrens PyConUK 2016 Building a Python Cake Testing The Layers of Your Application

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Yelp’s Mission Connecting people with great local businesses.

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Yelp Stats As of Q2 2016 92M 32 72% 108M

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Matt Behrens @askedrelic Who Am I?

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“It’s done, I just have to write the tests” - Every Engineer

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Tests Are Important!

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[FAIL CAKE]

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● The Testing Layer Cake ● Python Recipes ● Lessons and Tips Agenda

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The Testing Layer Cake Smoke Tests

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● Fast and consistent validation of one unit of code. ● Build a solid foundation. Unit Tests Don’t: - network calls - database calls - side effects Do: - mocks

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● AKA fakes, doubles, stubs, the fondant of testing ● A tool for helping test one thing at a time. ● Was the mock called? How many times? What was it called with? Mocks

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@mock.patch('pybakery.oven') def test_oven_turns_on(mock_oven): mock_oven.turn_on = True the_oven = Oven() assert(the_oven.turn_on() == True)

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● Test that your units connect together correctly. Test the relationships between your components. ● Are you testing more than one class? ● Mocks okay. ○ Mock data. Mock network or DB calls! Integration Tests

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● AKA system test, end to end tests ● Test your application end to end, with real data, in a production like environment. ● Try not to use mocks. Acceptance Tests

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● Sometimes you can’t or won’t test something 100% ○ What tradeoffs do you make to ship? ○ How can you make it easier to test? ● A chance to test in different environments for edge cases ○ Used in Yelp deployment ■ Verify in stage ■ Verify in production with % of live traffic Manual Tests

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● Simple check that everything is working and testing can continue ○ Is the homepage on fire? ● Generally blackbox and high level ● Related: distributed monitoring is hard ○ See Steve Yegge post about Amazon service architecture breakup Smoke Tests

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Python Recipes

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● Pytest ○ Fixtures ○ Parameterized test data ○ Plugins ● Mock ○ Python 3.3+ builtin How To Unit and Integration Test

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● Test one thing ○ Anti-pattern is to expand your tests to cover multiple situations ● Think about structure of a test ○ Setup ○ Test ○ Validate How To Write Good Unit Tests

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@pytest.fixture def sprinkles(): return ["°*°*°", '*~*~*~*~', '♥•**•**•♥'] def test_cake(sprinkles): cake = build_cake() # setup cake.bake(sprinkles) # test assert cake.tasty == True # validate

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● Use as needed ○ Don’t mock too much or too little ○ Anti-pattern is having too many mocks ● Check correct mock call_count ○ Anti-pattern is using mock assert_called_once_with ○ See Yelp Engineer blog post for more info How To Write Good Mocks

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@pytest.yield_fixture def mock_bake(): with mock.patch( 'pybakery.cake.logic.oven.bake' ) as mock_fn: yield mock_fn def test_cake(mock_bake): ...

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● Use a mirror structure pybakery/cake/logic/sprinkles.py tests/cake/logic/test_sprinkles.py How To Organize Unit Tests . ├─pybakery/cake/logic/oven.py └────tests/cake/logic/test_oven.py

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● Docker ○ Tool for running processes/environments in isolation ○ Real MySQL process, real SQL data ● Good ○ Relatively easy to setup ● Not so good ○ Young project, changing rapidly ○ Difficult to understand sometimes How To Acceptance Test

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Lessons and Tips

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● How much of your program is executed, when your (unit/integration) tests run ● You probably don’t need 100% coverage! ● Always good to know and improve on Code Coverage

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● Is a global input! ● Don’t forget to make it a function argument! You will want to override it. ● Freezegun is library to mock time ○ Supports Python 2, 3 Time

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● Pattern for building a specific types of objects ● Invest in making your testing easier def create(sprinkles=True, **kwargs): options.update(kwargs) Cake(**options) Factories def create(sprinkles=True, **kwargs): default_cake_options = {‘sprinkles’: sprinkles} default_cake_options.update(kwargs) Cake(**default_cake_options)

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● Super easy to test ○ Just inputs and outputs. Eliminate side effects! ● Think about what layers you are building ● Python Builtins ○ import itertools, functools ○ map(), filter(), reduce(), any(), all() ○ List comprehensions Functional Programming

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1. Write a test first 2. Watch it fail 3. Write the logic to make it succeed 4. Refactor 5. Repeat ● You know when you are done ● You solve the right problem Test Driven Development

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Each unit should have limited knowledge about other units. ● Depend on interfaces, not objects. ● Make classes easy to swap out. Law Of Demeter

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How can you change the structure of your code to make it easier to test? Final Thoughts

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Questions? @askedrelic [email protected] @YelpEngineering