Mocking in Python
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Excella's Dan Davis presented "Mocking Strategies" at the Django District Meetup group on February 10, 2015. The video of the presentation can be found here: http://youtu.be/zW0f4ZRYF5M
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Mocking in Python
- 1. Mocking Strategies Please don’t mock me after this presentation… Daniel Davis
- 2. • Daniel Davis • Software Developer for 8 years • Fun Fact: – I ran the “Jingle All The Way” 5k dressed as a Giant Gingerbread man Who Am I?
- 3. 3
- 4. Really though, who are you? • Came from Java world • Python developer for 2 years • DevOps – Lots of work with automation and quality • Passionate about quality! – Doesn’t happen overnight…
- 6. • Became a Certified Scrum Developer (CSD) about 2 years ago • Finally learned about craftsmanship and writing better tests – Basically black magic • Learned in Java… I want to be a better developer
- 7. • Switching to Python was jarring • Kept wondering about writing unit tests • Dealing with complicated frameworks like Django Finding Python 7
- 8. • Asked colleagues – Many had the same questions • Decided to learn… • Wanted to share with the community Unit testing? Mocks? 8
- 9. Let’s Talk About Unit Tests…
- 10. Unit – 70% Integration – 20% UI 10% Testing Types
- 11. • “They’re good when the problem is easy” – A rabbit hole of testing • “I spend too much time writing lots of code to test, so I give up” • “There’s just some stuff you can’t unit test” Great Unit Testing Myths
- 12. Mocking makes unit testing easier! 12
- 13. What Are Mocks? 13 Test Double Dummy Object Test Stub Test Spy Mock Object Fake Object Commonly referred to as “Mocks”
- 14. • Stubs – Provide a canned response to method calls • Spy – Real objects that behave like normal except when a specific condition is met • Mocks – Verifies behavior (calls) to a method What Are Mocks? 14
- 15. Blah Blah Blah professor Dan… 15
- 16. • Eliminates dependencies in the CUT (class under test) – Isolated Unit Tests Problems Mocks Solve 16 foo(x) bar(x)
- 17. • Tests methods that have no return value Problems Mocks Solve 17 How do we know that bar(x) has been called?
- 18. • Tests error handling Problems Mocks Solve 18 How do we generate this exception???
- 19. • Eliminate dependency on database calls – Speed up testing! • Reduce test complexity – Don’t have to write complex logic to handle behavior of methods not under test • Don’t have to wait to implement other methods Other Reasons Mocks Are Important 19
- 20. Ok, I’m sold… Show me how to actually do this… 20
- 21. • Mock (MagicMock) – Most robust, popular – Built-in as of Python 3.3! • flexmock – Based on Ruby’s flexmock • mox – Similar to Java’s EasyMock • Mocker • dingus – “record then assert” mocking library • fudge – Similar to Mockito • MiniMock – Simple mocking with DocTest What Are The Python Options? 21
- 22. Sample Problem 22 Valentine’s Day Edition
- 23. Problem: Tinder Competitor 23 Get yo’ container on…
- 24. • Create a method to return a new, random victim candidate – Must not show the same person – Must not show someone the user has already “swiped” on Problem: “Docker” dating app
- 25. Easy enough… 25 “Surely no one could have seen EVERYONE in the database!!!” - The Intern get_next_person() get_random_person()
- 26. Write a Unit Test… 26
- 27. It works!!! 27
- 29. 29
- 30. Easy enough… 30 What if knew the result of get_random_person()???
- 31. 31
- 33. It works EVERY SINGLE TIME!!! 33
- 34. Variations on a theme 34
- 35. Variations on a theme 35
- 36. Variations on a theme 36
- 37. Variations on a theme 37 OMG, ContextManagers! WHAT???
- 38. But what if we call it multiple times??? 38
- 39. • What if I want to test the while loop? Different results on multiple calls
- 40. Uh…umm… 40 UMM…
- 42. • Use patching / mocks to bring certainty to method calls • Eliminates dependencies on other code – Even unfinished code!!! • Lots of ways to do it, pick your favorite Recap: What did we learn? 42
- 43. Mocking to Verify Behavior 43
- 45. • When a user swipes right… • If the other user “likes” them: – Send them both a message with contact info • If the other user “dislikes” them: – Let the user down gently… • If the other user hasn’t evaluated yet: – Display the “give it time” message Problem: “Docker” matches
- 46. Implementation 46 How do we test this??? No return values!!!
- 50. Shouldn’t we check the other methods too? 50 We’d need to have multiple mocks to do that!!!
- 55. Whew!!! Almost Done! There will be beer soon! Sweet, delicious beer! 55
- 57. Simple JSON reader How do we test something like this???
- 58. • Test parsing a valid file • Test an IOError (i.e. file missing) • Test a ValueError (i.e invalid json) Testing JSON reader 58
- 59. • Let’s just create a sample file! How do I test open() 59
- 60. Can you even mock a builtin? 60
- 61. • open() returns a File object • open(filename).read() • So we really need to mock File.read() – But it’s an instance!!! Oh no! What? Why Not??? 61 Have you tried solving it with Mocks???
- 62. Mocks returning Mocks? WAT??? 62
- 64. What about error handling? 64
- 67. • Mocking makes writing unit tests simpler – Eliminates dependencies – Verifies behavior – Tests error handling • You just need some practice! Remember This!
- 68. • http://mock.readthedocs.org/en/latest/ • Pip Install Mock • Create a simple class, then write tests! Try It On Your Own 68
- 69. Let’s Go Write Some Tests!!! 69
- 70. Questions? 70