Magic Tricks of Testing (RailsConf)

Magic Tricks of Testing (RailsConf)

Tests are supposed to save us money. How is it, then, that many times they become millstones around our necks, gradually morphing into fragile, breakable things that raise the cost of change?

We write too many tests and we test the wrong kinds of things. This talk strips away the veil and offers simple, practical guidelines for choosing what to test and how to test it. Finding the right testing balance isn't magic, it's a magic trick; come and learn the secret of writing stable tests that protect your application at the lowest possible cost.

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Sandi Metz

April 30, 2013
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Transcript

  1. @sandimetz Apr 2013 The Magic Tricks of Testing Sandi Metz

    Saturday, April 27, 13
  2. @sandimetz Apr 2013 Why do I hate my tests? Saturday,

    April 27, 13
  3. @sandimetz Apr 2013 Why do I hate my tests? Saturday,

    April 27, 13
  4. @sandimetz Apr 2013 They’re slow Saturday, April 27, 13

  5. @sandimetz Apr 2013 They’re fragile Saturday, April 27, 13

  6. @sandimetz Apr 2013 They’re expensive Saturday, April 27, 13

  7. @sandimetz Apr 2013 They are misery Saturday, April 27, 13

  8. @sandimetz Apr 2013 They are misery incarnate Saturday, April 27,

    13
  9. @sandimetz Apr 2013 It doesn’t have to be this way

    Saturday, April 27, 13
  10. @sandimetz Apr 2013 Just delete some tests Saturday, April 27,

    13
  11. @sandimetz Apr 2013 Unit Tests: Goals Saturday, April 27, 13

  12. @sandimetz Apr 2013 Thorough Unit Tests: Goals Saturday, April 27,

    13
  13. @sandimetz Apr 2013 Thorough Stable Unit Tests: Goals Saturday, April

    27, 13
  14. @sandimetz Apr 2013 Thorough Stable Fast Unit Tests: Goals Saturday,

    April 27, 13
  15. @sandimetz Apr 2013 Thorough Stable Fast Few Unit Tests: Goals

    Saturday, April 27, 13
  16. @sandimetz Apr 2013 How does your app feel? Saturday, April

    27, 13
  17. @sandimetz Apr 2013 Saturday, April 27, 13

  18. @sandimetz Apr 2013 Focus on messages Saturday, April 27, 13

  19. @sandimetz Apr 2013 Objects are simple-minded Saturday, April 27, 13

  20. @sandimetz Apr 2013 Object Under Test Saturday, April 27, 13

  21. @sandimetz Apr 2013 Saturday, April 27, 13

  22. @sandimetz Apr 2013 Object Under Test Saturday, April 27, 13

  23. @sandimetz Apr 2013 Object Under Test Received from others Saturday,

    April 27, 13
  24. @sandimetz Apr 2013 Object Under Test Received from others Saturday,

    April 27, 13
  25. @sandimetz Apr 2013 Object Under Test Incoming Saturday, April 27,

    13
  26. @sandimetz Apr 2013 Sent to others Incoming Object Under Test

    Saturday, April 27, 13
  27. @sandimetz Apr 2013 Sent to others Incoming Object Under Test

    Saturday, April 27, 13
  28. @sandimetz Apr 2013 Incoming Outgoing Object Under Test Saturday, April

    27, 13
  29. @sandimetz Apr 2013 Sent to self Incoming Outgoing Object Under

    Test Saturday, April 27, 13
  30. @sandimetz Apr 2013 Sent to self Incoming Outgoing Object Under

    Test Saturday, April 27, 13
  31. @sandimetz Apr 2013 Incoming Sent to Self Outgoing Message Origin

    Saturday, April 27, 13
  32. @sandimetz Apr 2013 Sent to self Incoming Outgoing Object Under

    Test Saturday, April 27, 13
  33. @sandimetz Apr 2013 Sent to self Incoming Outgoing Object Under

    Test Saturday, April 27, 13
  34. @sandimetz Apr 2013 Sent to self Incoming Outgoing Object Under

    Test Saturday, April 27, 13
  35. @sandimetz Apr 2013 Sent to self Incoming Outgoing Query Command

    Object Under Test Saturday, April 27, 13
  36. @sandimetz Apr 2013 Query Command Message Type Saturday, April 27,

    13
  37. @sandimetz Apr 2013 Query: Return something/ change nothing Command: Return

    nothing/ change something Type Saturday, April 27, 13
  38. @sandimetz Apr 2013 Query: Return something/ change nothing Command: Return

    nothing/ change something Type Saturday, April 27, 13
  39. @sandimetz Apr 2013 Query: Return something/ change nothing Command: Return

    nothing/ change something Type Saturday, April 27, 13
  40. @sandimetz Apr 2013 We conflate commands and queries at our

    peril Saturday, April 27, 13
  41. @sandimetz Apr 2013 but we do it all the time

    Saturday, April 27, 13
  42. Query Command Message Origin x Type Outgoing Incoming Type Origin

    @sandimetz Apr 2013 Message Sent to Self Saturday, April 27, 13
  43. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Part 1 Incoming Saturday, April 27, 13
  44. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Incoming Query Messages Incoming Saturday, April 27, 13
  45. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Incoming Wheel diameter Saturday, April 27, 13
  46. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Wheel    attr_reader  :rim,  :tire    def  initialize(rim,  tire)    #  ...    end    def  diameter        rim  +  (tire  *  2)    end    #  ... Saturday, April 27, 13
  47. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Wheel    attr_reader  :rim,  :tire    def  initialize(rim,  tire)    #  ...    end    def  diameter        rim  +  (tire  *  2)    end    #  ... Saturday, April 27, 13
  48. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Wheel    attr_reader  :rim,  :tire    def  initialize(rim,  tire)    #  ...    end    def  diameter        rim  +  (tire  *  2)    end    #  ... Saturday, April 27, 13
  49. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  WheelTest  <  MiniTest::Unit::TestCase    def  test_calculates_diameter        wheel  =  Wheel.new(26,  1.5)        assert_in_delta(29,                                        wheel.diameter,                                        0.01)    end end Saturday, April 27, 13
  50. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  WheelTest  <  MiniTest::Unit::TestCase    def  test_calculates_diameter        wheel  =  Wheel.new(26,  1.5)        assert_in_delta(29,                                        wheel.diameter,                                        0.01)    end end Saturday, April 27, 13
  51. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  WheelTest  <  MiniTest::Unit::TestCase    def  test_calculates_diameter        wheel  =  Wheel.new(26,  1.5)        assert_in_delta(29,                                        wheel.diameter,                                        0.01)    end end Saturday, April 27, 13
  52. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  53. @sandimetz Apr 2013 Test incoming query messages Rule Saturday, April

    27, 13
  54. @sandimetz Apr 2013 Test incoming query messages by making assertions

    about what they send back Rule Saturday, April 27, 13
  55. Query Command The Unit Testing Minimalist Incoming Type @sandimetz Apr

    2013 Message Sent to Self Outgoing Origin Saturday, April 27, 13
  56. Query Command Assert result The Unit Testing Minimalist Incoming Type

    @sandimetz Apr 2013 Message Sent to Self Outgoing Origin Saturday, April 27, 13
  57. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Incoming Another Incoming Query Message Saturday, April 27, 13
  58. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear gear_inches Incoming Saturday, April 27, 13
  59. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  60. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  61. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  62. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  63. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Wheel Saturday, April 27, 13
  64. @sandimetz Apr 2013 Sight along the edges of the space

    capsule Saturday, April 27, 13
  65. @sandimetz Apr 2013 Sight along the edges of the space

    capsule Saturday, April 27, 13
  66. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)    end end Saturday, April 27, 13
  67. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)    end end Saturday, April 27, 13
  68. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)    end end Saturday, April 27, 13
  69. @sandimetz Apr 2013 Test the interface Saturday, April 27, 13

  70. @sandimetz Apr 2013 Test the interface (not the implementation) Saturday,

    April 27, 13
  71. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Incoming Command Messages Incoming Saturday, April 27, 13
  72. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear set_cog Incoming Saturday, April 27, 13
  73. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear  attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  74. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear  attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  75. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear  attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)        #  ...    end    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  76. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_set_cog        gear  =  Gear.new        gear.set_cog(27)        assert(27,  gear.cog)    end end Saturday, April 27, 13
  77. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_set_cog        gear  =  Gear.new        gear.set_cog(27)        assert(27,  gear.cog)    end end Saturday, April 27, 13
  78. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_set_cog        gear  =  Gear.new        gear.set_cog(27)        assert(27,  gear.cog)    end end Send the message Saturday, April 27, 13
  79. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_set_cog        gear  =  Gear.new        gear.set_cog(27)        assert(27,  gear.cog)    end end Assert the side effect Saturday, April 27, 13
  80. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  81. @sandimetz Apr 2013 Test incoming command messages Rule Saturday, April

    27, 13
  82. @sandimetz Apr 2013 Test incoming command messages by making assertions

    about direct public side effects Rule Saturday, April 27, 13
  83. Query Command Assert result The Unit Testing Minimalist Incoming Type

    @sandimetz Apr 2013 Message Sent to Self Outgoing Origin Saturday, April 27, 13
  84. Query Command Assert result The Unit Testing Minimalist Incoming Type

    @sandimetz Apr 2013 Message Assert direct public side effects Sent to Self Outgoing Origin Saturday, April 27, 13
  85. @sandimetz Apr 2013 DRY It Out Saturday, April 27, 13

  86. @sandimetz Apr 2013 Receiver of incoming message has sole responsibility

    DRY It Out Saturday, April 27, 13
  87. @sandimetz Apr 2013 Receiver of incoming message has sole responsibility

    for asserting the result direct public side effects DRY It Out Saturday, April 27, 13
  88. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Part 2 to Self Incoming Saturday, April 27, 13
  89. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Messages Sent to Self to Self Incoming Saturday, April 27, 13
  90. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  91. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Anti-Pattern to Self Incoming Saturday, April 27, 13
  92. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_ratio        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(4.7,                                        gear.ratio,                                        0.01)    end Saturday, April 27, 13
  93. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_ratio        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(4.7,                                        gear.ratio,                                        0.01)    end Test the private method? Saturday, April 27, 13
  94. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_ratio        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(4.7,                                        gear.ratio,                                        0.01)    end Saturday, April 27, 13
  95. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_ratio        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(4.7,                                        gear.ratio,                                        0.01)    end Saturday, April 27, 13
  96. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_ratio        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(4.7,                                        gear.ratio,                                        0.01)    end Redundant: #gear_inches test is proof enough Saturday, April 27, 13
  97. Saturday, April 27, 13

  98. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Anti-Pattern to Self Incoming Saturday, April 27, 13
  99. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  100. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  101. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  Gear      #  ...    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  102. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)    end end Saturday, April 27, 13
  103. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)        gear.expect(:ratio)        gear.verify    end Saturday, April 27, 13
  104. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)        gear.expect(:ratio)        gear.verify    end Expect to send the private method? Saturday, April 27, 13
  105. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)        gear.expect(:ratio)        gear.verify    end Over Speci ed: Adds no safety yet breaks with every change Saturday, April 27, 13
  106. Saturday, April 27, 13

  107. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  108. @sandimetz Apr 2013 Do not test private methods Rule Saturday,

    April 27, 13
  109. @sandimetz Apr 2013 Do not test private methods Do not

    make assertions about their result Rule Saturday, April 27, 13
  110. @sandimetz Apr 2013 Do not test private methods Do not

    make assertions about their result Do not expect to send them Rule Saturday, April 27, 13
  111. @sandimetz Apr 2013 Break rule if it saves $$$ during

    development Caveat Saturday, April 27, 13
  112. Query Command Assert result Assert direct public side-effects The Unit

    Testing Minimalist Incoming Type @sandimetz Apr 2013 Message Sent to Self Outgoing Origin Saturday, April 27, 13
  113. Query Command Assert result Assert direct public side-effects The Unit

    Testing Minimalist Incoming Type @sandimetz Apr 2013 Ignore Message Sent to Self Outgoing Origin Saturday, April 27, 13
  114. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Part 3 to Self Incoming Outgoing Saturday, April 27, 13
  115. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Outgoing Query Messages to Self Incoming Outgoing Saturday, April 27, 13
  116. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Spoiler Alert Same rules as ‘Sent to Self’ to Self Incoming Outgoing Saturday, April 27, 13
  117. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    diameter Gear gear_inches Wheel to Self Incoming Outgoing Saturday, April 27, 13
  118. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)          #  ...    end    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Saturday, April 27, 13
  119. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)          #  ...    end    def  gear_inches        ratio  *  wheel.diameter    end      private    def  ratio        chainring  /  cog.to_f    end      #  ... end Wheel Saturday, April 27, 13
  120. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear Gear Outgoing Wheel diameter Incoming Query gear_inches to Self Incoming Outgoing Saturday, April 27, 13
  121. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Anti-Pattern to Self Incoming to Self Incoming Outgoing Saturday, April 27, 13
  122. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)    end end Saturday, April 27, 13
  123. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          assert_in_delta(29,                                        gear.wheel.diameter,                                        0.01)    end end Saturday, April 27, 13
  124. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          assert_in_delta(29,                                        gear.wheel.diameter,                                        0.01)    end end Assert result of outgoing query? Saturday, April 27, 13
  125. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          assert_in_delta(29,                                        gear.wheel.diameter,                                        0.01)    end end Redundant: Duplicates Wheel’s tests Saturday, April 27, 13
  126. Saturday, April 27, 13

  127. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Anti-Pattern to Self Incoming to Self Incoming Outgoing Saturday, April 27, 13
  128. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          gear.wheel.expect(:diameter)        gear.verify    end end Saturday, April 27, 13
  129. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          gear.wheel.expect(:diameter)        gear.verify    end end Expect to send outgoing query? Saturday, April 27, 13
  130. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_calculates_gear_inches        gear  =    Gear.new(                            chainring:  52,                            cog:              11,                            wheel:          Wheel.new(26,  1.5))        assert_in_delta(137.1,                                        gear.gear_inches,                                        0.01)          gear.wheel.expect(:diameter)        gear.verify    end end Over Speci ed: Adds costs but not bene ts Saturday, April 27, 13
  131. Saturday, April 27, 13

  132. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  133. @sandimetz Apr 2013 Rule Do not test outgoing query messages

    Saturday, April 27, 13
  134. @sandimetz Apr 2013 Rule Do not test outgoing query messages

    Do not make assertions about their result Saturday, April 27, 13
  135. @sandimetz Apr 2013 Rule Do not test outgoing query messages

    Do not make assertions about their result Do not expect to send them Saturday, April 27, 13
  136. @sandimetz Apr 2013 If a message has no visible side-effects

    Saturday, April 27, 13
  137. @sandimetz Apr 2013 If a message has no visible side-effects

    the sender should not test it Saturday, April 27, 13
  138. Query Command Assert result Assert direct public side effects The

    Unit Testing Minimalist Incoming Type @sandimetz Apr 2013 Message Ignore Sent to Self Outgoing Origin Saturday, April 27, 13
  139. Query Command Assert result Assert direct public side effects The

    Unit Testing Minimalist Incoming Type @sandimetz Apr 2013 Message Ignore Sent to Self Outgoing Origin Saturday, April 27, 13
  140. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Outgoing Command Messages to Self Incoming Outgoing Saturday, April 27, 13
  141. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    diameter gear_inches set_cog changed Wheel Gear Obs to Self Incoming Outgoing Saturday, April 27, 13
  142. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)              #  ...      end    def  set_cog(new_cog)        @cog  =  new_cog    end end Change #set_cog to add observer Saturday, April 27, 13
  143. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear    attr_reader  :chainring,  :cog,  :wheel    def  initialize(args)              #  ...      end    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  144. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear    attr_reader  :chainring,  :cog,  :wheel,  :observer    def  initialize(args)              #  ...        @observer    =  args[:observer]    end    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  145. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear      #  ...    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  146. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear      #  ...    def  set_cog(new_cog)        @cog  =  new_cog    end end Saturday, April 27, 13
  147. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear      #  ...    def  set_cog(new_cog)        @cog  =  new_cog        changed        @cog    end    def  changed        observer.changed(chainring,  cog)    end end Saturday, April 27, 13
  148. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear      #  ...    def  set_cog(new_cog)        @cog  =  new_cog        changed        @cog    end    def  changed        observer.changed(chainring,  cog)    end end Saturday, April 27, 13
  149. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear set_cog changed Obs to Self Incoming Outgoing Saturday, April 27, 13
  150. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear set_cog changed Obs Side Effects to Self Incoming Outgoing Saturday, April 27, 13
  151. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    changed Gear set_cog Obs Incoming Command Side Effects to Self Incoming Outgoing Saturday, April 27, 13
  152. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Gear Outgoing set_cog Obs changed Incoming Command Side Effects to Self Incoming Outgoing Saturday, April 27, 13
  153. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear        #  ...    def  set_cog(new_cog)        @cog  =  new_cog        changed        @cog    end    def  changed        observer.changed(chainring,  cog)    end end Saturday, April 27, 13
  154. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  Gear        #  ...    def  set_cog(new_cog)        @cog  =  new_cog        changed        @cog    end    def  changed        observer.changed(chainring,  cog)    end end This message MUST get sent Saturday, April 27, 13
  155. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Outgoing Incoming

    Anti-Pattern to Self Incoming Outgoing Saturday, April 27, 13
  156. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Saturday, April 27, 13
  157. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Saturday, April 27, 13
  158. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Saturday, April 27, 13
  159. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Saturday, April 27, 13
  160. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Depends on the distant side effect Saturday, April 27, 13
  161. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Is this Gear’s responsibility? Saturday, April 27, 13
  162. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end Is this Gear’s responsibility? Saturday, April 27, 13
  163. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_saves_changed_cog_in_db        @observer  =  Obs.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)          @gear.set_cog(27)          #  assert  something  about  the  state  of  the  db    end end This is an integration test Saturday, April 27, 13
  164. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  165. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  166. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  167. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  168. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  169. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Saturday, April 27, 13
  170. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Depends on the interface Saturday, April 27, 13
  171. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end Gear is responsible for sending #changed to @observer Saturday, April 27, 13
  172. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  173. @sandimetz Apr 2013 Expect to send outgoing command messages Rule

    Saturday, April 27, 13
  174. @sandimetz Apr 2013 Break rule if side effects are stable

    and cheap Caveat Saturday, April 27, 13
  175. Query Command Assert result Assert direct public side effects Ignore

    The Unit Testing Minimalist Incoming Type @sandimetz Apr 2013 Message Ignore Sent to Self Outgoing Origin Saturday, April 27, 13
  176. Query Command Assert result Assert direct public side effects Ignore

    Expect to send The Unit Testing Minimalist Incoming Type @sandimetz Apr 2013 Message Ignore Sent to Self Outgoing Origin Saturday, April 27, 13
  177. @sandimetz Apr 2013 But Saturday, April 27, 13

  178. @sandimetz GoGaRuCo 2012 @sandimetz Apr 2013 to Self Incoming Outgoing

    class  GearTest  <  MiniTest::Unit::TestCase    def  test_notifies_observers_when_cogs_change        @observer  =  MiniTest::Mock.new        @gear          =  Gear.new(                                    chainring:  52,                                    cog:              11,                                    observer:    @observer)        @observer.expect(:changed,  true,  [52,  27])        @gear.set_cog(27)        @observer.verify    end What happens if Observer stops implementing #changed? Saturday, April 27, 13
  179. @sandimetz Apr 2013 Pain Saturday, April 27, 13

  180. @sandimetz Apr 2013 API Drift Pain Saturday, April 27, 13

  181. @sandimetz Apr 2013 Rule Saturday, April 27, 13

  182. @sandimetz Apr 2013 Honor the contract Rule Saturday, April 27,

    13
  183. @sandimetz Apr 2013 Honor the contract Ensure test doubles stay

    in sync with the API Rule Saturday, April 27, 13
  184. @sandimetz Apr 2013 Automagically  minitest: requires that stubbed methods exist

     rspec: #should_receive(:msg).and_call_original  https://github.com/psyho/bogus  https://github.com/benmoss/quacky  https://github.com/xaviershay/rspec-­‐fire  https://github.com/cfcosta/minitest-­‐firemock Saturday, April 27, 13
  185. @sandimetz Apr 2013 Summary Saturday, April 27, 13

  186. @sandimetz Apr 2013 Be a minimalist Saturday, April 27, 13

  187. Query Command Assert result Assert direct public side effects Ignore

    Expect to send Incoming Type @sandimetz Apr 2013 Message Ignore Sent to Self Outgoing Origin Saturday, April 27, 13
  188. @sandimetz Apr 2013 Use good judgement Saturday, April 27, 13

  189. @sandimetz Apr 2013 Test Saturday, April 27, 13

  190. @sandimetz Apr 2013 Test. Everything Saturday, April 27, 13

  191. @sandimetz Apr 2013 Test . Everything. Once Saturday, April 27,

    13
  192. @sandimetz Apr 2013 Test the interface Saturday, April 27, 13

  193. @sandimetz Apr 2013 Trust collaborators Saturday, April 27, 13

  194. @sandimetz Apr 2013 Insist on simplicity Saturday, April 27, 13

  195. @sandimetz Apr 2013 Practice the Tricks Saturday, April 27, 13

  196. @sandimetz Apr 2013 Saturday, April 27, 13

  197. @sandimetz Apr 2013 Saturday, April 27, 13

  198. @sandimetz Apr 2013 I love my tests Saturday, April 27,

    13
  199. Thanks Sandi Metz @sandimetz Saturday, April 27, 13

  200. http://www. ickr.com/photos/phil_shirley/5452562957/ thicket http://www. ickr.com/photos/zebble/6080622/ one padlock http://www. ickr.com/photos/chrisinplymouth/5543281168/ two

    padlocks http://www. ickr.com/photos/donjohnson395/2915304926/ twins at beach http://www.med.navy.mil/sites/nmcp/Patients/pediatrics/PublishingImages/binocular_boy.gif http://upload.wikimedia.org/wikipedia/commons/e/e1/Ordinary_bicycle02.jpg ordinary bike Magic Hat (82805980) Copyright Mmaxer - Shutterstock.com Dogs in Hats (39263399) Copyright MLoiselle - Fotolia.com Garden (35521385) Copyright Beboy - Fotolia.com Space Walk - Nasa http://grin.hq.nasa.gov/copyright.html Photo Credits Saturday, April 27, 13
  201. @sandimetz Apr 2013 http://poodr.info Saturday, April 27, 13

  202. Questions? Saturday, April 27, 13

  203. @sandimetz Apr 2013 http://poodr.info Saturday, April 27, 13