Debugging: The Science of Deduction

Debugging: The Science of Deduction

Tips for debugging code in Ruby.

Dc9f215c22985887d9929b85b02c49ef?s=128

Danielle Sucher

November 14, 2013
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  1. THE SCIENCE OF DEDUCTION dsucher@gmail.com • @DanielleSucher DEBUGGING Friday, November

    15, 13
  2. "People write programs without any expectation that they will be

    right the first time." - David Lorge Parnas Friday, November 15, 13
  3. Friday, November 15, 13

  4. Let’s start simple. Friday, November 15, 13

  5. When you get an error message Friday, November 15, 13

  6. “Oh, I think I know what went wrong..!” Friday, November

    15, 13
  7. NO. Friday, November 15, 13

  8. READ THE ERROR MESSAGE Friday, November 15, 13

  9. /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/attribute_assigner.rb:14:in `tap' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/attribute_assigner.rb:14:in `object' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/evaluation.rb:12:in `object' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/

    lib/factory_girl/strategy/build.rb:9:in `result' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory.rb:42:in `run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory_runner.rb:23:in `block in run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/activesupport-4.0.1/ lib/active_support/notifications.rb:161:in `instrument' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory_runner.rb:22:in `run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/strategy_syntax_method_registrar.rb:19:in `block in define_singular_strategy_method' /Users/stan/projects/rails4/spec/models/project_spec.rb:21:in `block (4 levels) in <top (required)>' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `block (2 levels) in let' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `fetch' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `block in let' Friday, November 15, 13
  10. How do you find the important stuff? Friday, November 15,

    13
  11. Start at the top. Friday, November 15, 13

  12. Skim for lines that refer to your project’s codebase Friday,

    November 15, 13
  13. /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/attribute_assigner.rb:14:in `tap' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/attribute_assigner.rb:14:in `object' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/evaluation.rb:12:in `object' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/

    lib/factory_girl/strategy/build.rb:9:in `result' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory.rb:42:in `run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory_runner.rb:23:in `block in run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/activesupport-4.0.1/ lib/active_support/notifications.rb:161:in `instrument' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/factory_runner.rb:22:in `run' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/ lib/factory_girl/strategy_syntax_method_registrar.rb:19:in `block in define_singular_strategy_method' /Users/stan/projects/rails4/spec/models/project_spec.rb:21:in `block (4 levels) in <top (required)>' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `block (2 levels) in let' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `fetch' /Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/ lib/rspec/core/memoized_helpers.rb:199:in `block in let' /Users/stan/projects/rails4/spec/models/project_spec.rb:24:in `block Friday, November 15, 13
  14. Read the line number Friday, November 15, 13

  15. google the error (no, really, you’re not smarter than google)

    Friday, November 15, 13
  16. Unexpected Behavior Friday, November 15, 13

  17. “I don’t guess. I observe.” Friday, November 15, 13

  18. “And once I’ve observed...” Friday, November 15, 13

  19. “I deduce.” Friday, November 15, 13

  20. Formulate a hypothesis. Try to disprove it. Repeat as needed.

    Friday, November 15, 13
  21. Where do you start looking? Friday, November 15, 13

  22. The Streetlight Effect searching where the keys fell vs searching

    where the light is Friday, November 15, 13
  23. Start looking where the light is Friday, November 15, 13

  24. First test the theories you can disprove the fastest Friday,

    November 15, 13
  25. To test your hypotheses, you need to reproduce the bug

    Friday, November 15, 13
  26. If you can’t reproduce the bug, what’s different about your

    environment? Friday, November 15, 13
  27. Watch somebody else reproduce the problem. Friday, November 15, 13

  28. Sometimes it’s all in the timing Friday, November 15, 13

  29. Even partial patterns can give you useful data Friday, November

    15, 13
  30. the 500 mile bug http://www.ibiblio.org/harris/ 500milemail.html Friday, November 15, 13

  31. Try to break things down into smaller, self-contained, pieces. Friday,

    November 15, 13
  32. Don’t even try to understand everything Friday, November 15, 13

  33. Friday, November 15, 13

  34. Miller’s Law The average person can only hold 7 ±

    2 items in working memory Friday, November 15, 13
  35. It’s okay to think of the rest of the system

    as a black box Friday, November 15, 13
  36. Break apart methods so you can test smaller chunks of

    logic separately Friday, November 15, 13
  37. Comment out parts of your code so you can focus

    on one piece at a time Friday, November 15, 13
  38. Refactoring and adding tests (or even just fixing typos) keeps

    you focused and improves your understanding Friday, November 15, 13
  39. With really gnarly code write tests to document existing behavior

    Friday, November 15, 13
  40. "...what are the facts, and what is the truth that

    the facts bear out?" - Bertrand Russell Friday, November 15, 13
  41. Is the method you’re looking at even being called at

    all? Friday, November 15, 13
  42. def might_be_called puts “IN MIGHT_BE_CALLED” puts caller # ... end

    Friday, November 15, 13
  43. “What they don’t confess to is almost always more interesting.”

    Friday, November 15, 13
  44. What are the actual inputs your method is receiving? Friday,

    November 15, 13
  45. Maybe you think you’re getting nil, but you’re getting ‘’

    or [‘’] instead Friday, November 15, 13
  46. Is there a rescue in the way, swallowing useful errors?

    Friday, November 15, 13
  47. What is the state of the universe? Friday, November 15,

    13
  48. Follow the money logs $ tail -f log/whatevs.log Friday, November

    15, 13
  49. Does doing things in a different order lead to different

    results? Friday, November 15, 13
  50. Possible Ordering Problems Methods with side effects Rails autoload order

    Database left in a dirty state Friday, November 15, 13
  51. Clean out your environment (if at all possible) Friday, November

    15, 13
  52. Make the problem more concrete Friday, November 15, 13

  53. Sketch things out Friday, November 15, 13

  54. Wave your hands around in the air placing pieces of

    your system in imaginary 3d space Friday, November 15, 13
  55. “I need to go to my mind palace.” Friday, November

    15, 13
  56. Question your assumptions. (Write a list or even an informal

    proof, if necessary.) Friday, November 15, 13
  57. Rubber duck debugging Friday, November 15, 13

  58. Trust no one. Friday, November 15, 13

  59. Friday, November 15, 13

  60. “The only truly accurate comment I've seen was just cursing,

    in Swedish.” - Scott Vokes (@silentbicycle) Friday, November 15, 13
  61. “Without a debugger, you basically have to go to the

    next step; understand what the program does.” - Linus Torvalds Friday, November 15, 13
  62. print statement debugging puts “*” * 20 puts “foo: #{foo.inspect}”

    puts “*” * 20 Friday, November 15, 13
  63. Where is this method defined? m = Foo.new.method(:do_stuff) m.source_location =>

    [filename, line_number] Friday, November 15, 13
  64. “Ideas are tested by experiment. That is the core of

    science.” - Zombie Richard Feynman (via xkcd) Friday, November 15, 13
  65. Experiment with small pieces of the code in IRB or

    Pry Friday, November 15, 13
  66. Focus on what’s changed since the problem started Friday, November

    15, 13
  67. A few Git tricks Friday, November 15, 13

  68. test on a different branch or a different commit Friday,

    November 15, 13
  69. git bisect binary search through your commits to figure out

    when the problem started! Friday, November 15, 13
  70. Running git bisect manually $ git bisect start $ git

    bisect good SHA $ git bisect bad HEAD Friday, November 15, 13
  71. Running git bisect automatically $ git bisect start HEAD HEAD~10

    $ git bisect run rspec spec/ Friday, November 15, 13
  72. git blame (not actually for blaming people!) Friday, November 15,

    13
  73. The person who wrote this code may have been an

    idiot. Or they may have been a mad genius. Friday, November 15, 13
  74. Look at the original context $ git blame -L(range) filename

    $ git show SHA Friday, November 15, 13
  75. But what about whitespace changes? $ git blame -w filename

    (ignores whitespace) Friday, November 15, 13
  76. But what about lines that were just copied or moved?

    $ git blame -wCCC filename -w ignores whitespace -C detects moved/copied lines -CCC is like -C, but tries harder Friday, November 15, 13
  77. $ git log -p Includes patches, not just commit messages.

    You can search with / search_string Friday, November 15, 13
  78. $ git log -p -S “test string” Lists only commits

    where that string was added or deleted (with patches). Friday, November 15, 13
  79. Debuggers Friday, November 15, 13

  80. byebug (Ruby 2.0.x) debugger (Ruby 1.9.x) ruby-debug (Ruby 1.8.x) Friday,

    November 15, 13
  81. Pry https://github.com/pry/pry https://github.com/pry/pry/ wiki/Available-plugins Friday, November 15, 13

  82. ls - list methods on an object cd - into

    objects wtf? - backtrace show-doc - prints docs show-method - prints source whereami - no really Pry Friday, November 15, 13
  83. binding.pry if @whatevs Friday, November 15, 13

  84. Pry-remote https://github.com/mon-ouie/ pry-remote Friday, November 15, 13

  85. binding.pry_remote $ pry-remote Friday, November 15, 13

  86. Better Errors https://github.com/charliesome/ better_errors Friday, November 15, 13

  87. Friday, November 15, 13

  88. WHEN THINGS GET REALLY WEIRD Friday, November 15, 13

  89. This shouldn’t make a difference, but let’s try it just

    in case... Friday, November 15, 13
  90. Friday, November 15, 13

  91. It doesn’t count if you’re right by accident Friday, November

    15, 13
  92. “That’s clever. Is it clever?” “...why is it clever?” Friday,

    November 15, 13
  93. If you don’t understand the cause of the problem, it’s

    not fixed, because you can’t predict when it might come up again. Friday, November 15, 13
  94. Don’t be afraid to read the source Friday, November 15,

    13
  95. $ bundle open gem_name Friday, November 15, 13

  96. occasionally the compiler (Rarely! But it happens.) Friday, November 15,

    13
  97. Friday, November 15, 13

  98. Friday, November 15, 13

  99. “You know my methods, Watson.” Friday, November 15, 13

  100. THE SCIENCE OF DEDUCTION dsucher@gmail.com • @DanielleSucher DEBUGGING thank you!

    Friday, November 15, 13