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THE SCIENCE OF DEDUCTION [email protected] • @DanielleSucher DEBUGGING Friday, November 15, 13

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"People write programs without any expectation that they will be right the first time." - David Lorge Parnas Friday, November 15, 13

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Friday, November 15, 13

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

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

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“Oh, I think I know what went wrong..!” Friday, November 15, 13

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NO. Friday, November 15, 13

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READ THE ERROR MESSAGE Friday, November 15, 13

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/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 ' /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

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How do you find the important stuff? Friday, November 15, 13

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Start at the top. Friday, November 15, 13

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Skim for lines that refer to your project’s codebase Friday, November 15, 13

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/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 ' /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

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Read the line number Friday, November 15, 13

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google the error (no, really, you’re not smarter than google) Friday, November 15, 13

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Unexpected Behavior Friday, November 15, 13

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

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

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“I deduce.” Friday, November 15, 13

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Formulate a hypothesis. Try to disprove it. Repeat as needed. Friday, November 15, 13

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

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The Streetlight Effect searching where the keys fell vs searching where the light is Friday, November 15, 13

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

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First test the theories you can disprove the fastest Friday, November 15, 13

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

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

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

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

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Even partial patterns can give you useful data Friday, November 15, 13

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

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Try to break things down into smaller, self-contained, pieces. Friday, November 15, 13

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

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Friday, November 15, 13

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Miller’s Law The average person can only hold 7 ± 2 items in working memory Friday, November 15, 13

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It’s okay to think of the rest of the system as a black box Friday, November 15, 13

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Break apart methods so you can test smaller chunks of logic separately Friday, November 15, 13

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Comment out parts of your code so you can focus on one piece at a time Friday, November 15, 13

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Refactoring and adding tests (or even just fixing typos) keeps you focused and improves your understanding Friday, November 15, 13

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With really gnarly code write tests to document existing behavior Friday, November 15, 13

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"...what are the facts, and what is the truth that the facts bear out?" - Bertrand Russell Friday, November 15, 13

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Is the method you’re looking at even being called at all? Friday, November 15, 13

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

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

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

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Maybe you think you’re getting nil, but you’re getting ‘’ or [‘’] instead Friday, November 15, 13

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Is there a rescue in the way, swallowing useful errors? Friday, November 15, 13

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

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Follow the money logs $ tail -f log/whatevs.log Friday, November 15, 13

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Does doing things in a different order lead to different results? Friday, November 15, 13

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Possible Ordering Problems Methods with side effects Rails autoload order Database left in a dirty state Friday, November 15, 13

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Clean out your environment (if at all possible) Friday, November 15, 13

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Make the problem more concrete Friday, November 15, 13

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Sketch things out Friday, November 15, 13

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Wave your hands around in the air placing pieces of your system in imaginary 3d space Friday, November 15, 13

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“I need to go to my mind palace.” Friday, November 15, 13

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Question your assumptions. (Write a list or even an informal proof, if necessary.) Friday, November 15, 13

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Rubber duck debugging Friday, November 15, 13

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Trust no one. Friday, November 15, 13

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Friday, November 15, 13

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“The only truly accurate comment I've seen was just cursing, in Swedish.” - Scott Vokes (@silentbicycle) Friday, November 15, 13

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“Without a debugger, you basically have to go to the next step; understand what the program does.” - Linus Torvalds Friday, November 15, 13

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print statement debugging puts “*” * 20 puts “foo: #{foo.inspect}” puts “*” * 20 Friday, November 15, 13

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Where is this method defined? m = Foo.new.method(:do_stuff) m.source_location => [filename, line_number] Friday, November 15, 13

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“Ideas are tested by experiment. That is the core of science.” - Zombie Richard Feynman (via xkcd) Friday, November 15, 13

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Experiment with small pieces of the code in IRB or Pry Friday, November 15, 13

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

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A few Git tricks Friday, November 15, 13

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test on a different branch or a different commit Friday, November 15, 13

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git bisect binary search through your commits to figure out when the problem started! Friday, November 15, 13

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Running git bisect manually $ git bisect start $ git bisect good SHA $ git bisect bad HEAD Friday, November 15, 13

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Running git bisect automatically $ git bisect start HEAD HEAD~10 $ git bisect run rspec spec/ Friday, November 15, 13

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git blame (not actually for blaming people!) Friday, November 15, 13

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The person who wrote this code may have been an idiot. Or they may have been a mad genius. Friday, November 15, 13

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Look at the original context $ git blame -L(range) filename $ git show SHA Friday, November 15, 13

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But what about whitespace changes? $ git blame -w filename (ignores whitespace) Friday, November 15, 13

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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

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$ git log -p Includes patches, not just commit messages. You can search with / search_string Friday, November 15, 13

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$ git log -p -S “test string” Lists only commits where that string was added or deleted (with patches). Friday, November 15, 13

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Debuggers Friday, November 15, 13

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byebug (Ruby 2.0.x) debugger (Ruby 1.9.x) ruby-debug (Ruby 1.8.x) Friday, November 15, 13

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

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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

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binding.pry if @whatevs Friday, November 15, 13

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

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

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

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Friday, November 15, 13

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WHEN THINGS GET REALLY WEIRD Friday, November 15, 13

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This shouldn’t make a difference, but let’s try it just in case... Friday, November 15, 13

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It doesn’t count if you’re right by accident Friday, November 15, 13

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

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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

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Don’t be afraid to read the source Friday, November 15, 13

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$ bundle open gem_name Friday, November 15, 13

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occasionally the compiler (Rarely! But it happens.) Friday, November 15, 13

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Friday, November 15, 13

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Friday, November 15, 13

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

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THE SCIENCE OF DEDUCTION [email protected] • @DanielleSucher DEBUGGING thank you! Friday, November 15, 13