A talk on using Mocks in Python. Presented at Django District in Feb 2015

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…

5. Testing Journey

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!
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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…
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16. • Eliminates dependencies in the CUT
(class under test)
– Isolated Unit Tests
Problems Mocks Solve
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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
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20. Ok, I’m sold…
Show me how to actually do this…
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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

32. Patching
Module.attribute
Mock method

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…

41. Use side_effect
41

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

44. Problem:
Matching in “Docker”

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!!!

47. Behavior Verification
47

48. What about checking
parameters???
48

49. Verifying Parameters
49

50. Shouldn’t we check the other
methods too?
50
We’d need to have multiple mocks to do that!!!

51. Multiple Mocks
51

52. Patch Multiple
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53. Testing Multiple Calls…
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54. Multiple calls
54

55. Whew!!!
Almost Done!
There will be beer soon!
Sweet, delicious beer!
55

56. Mocking built-ins
and Exceptions
56

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
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59. • Let’s just create a sample file!
How do I test open()
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60. Can you even mock a builtin?
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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

63. What about error
handling?
63

64. What about error handling?
64

65. What about ValueError?

66. Wrap Up:
Key Take-Aways
66

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!!!
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70. Questions?
70