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group :development do gem ‘pry-rails’ gem ‘pry-plus’ end

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What’s  Pry?   •   “like  irb,  but  be4er”     •  Everything  you  need  to  program  in  Ruby   •  (apart  from  a  text  editor)     •  60%  of  your  Fme  is  spent  debugging!        

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Brief  history   •  2010:  John  Mair  (@banisterfiend)  created  pry   •  2011:  Ryan  Fitzgerald  (@rrff__)  and  I  joined  in   •  2012:  reached  1,000,000  downloads   –  Kyrylo  Silin  (@kyrylosilin)  and  rking   (@sharpsawDOTorg)  started  contribuFng   •  2013:  reached  2,000,000  downloads   –  71  disFnct  contributors  

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Debugging  a  blog  engine   •  IntroducFon   •  How  do  I  use  the  Base64  library?   •  Why  doesn’t  my  method  work?   •  Where  did  that  nil  come  from?   •  Further  reading  

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64

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How  do  I  use  the  Base64  library?     $ pry [1] pry(main)> require ‘base64’ => true [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)>

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> (“hi”) => “aGk=\n” [4] pry(main)> Base64.decode64(_) => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64(“hi”) => “aGk=\n” [4] pry(main)> Base64.decode64(_) => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.enc4(“hi”) => “aGk=\n” [4] pry(main)> Base64.decode64(_) => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64(“hi”) => “aGk=\n” [4] pry(main)> Base64.decode64(_) => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64 “hi”) => “aGk=\n” [4] pry(main)> Base64.decode64(_) => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64 “hi” => “aGk=\n” [4] pry(main)> Base64.decode64 _ => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64 “hi” => “aGk=\n” [4] pry(main)> Base64.decode64 _ => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64 “hi” => “aGk=\n” [4] pry(main)> Base64.decode64 _ => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [3] pry(main)> Base64.encode64 “hi” => “aGk=\n” [4] pry(main)> Base64.decode64 _ => “hi”

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [5] pry(main)>

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How  do  I  use  the  Base64  library?   [2] pry(main)> ls Base64 Base64.methods: decode64 encode64 strict_decode64 strict_encode64 urlsafe_decode64 urlsafe_encode64 [5] pry(main)> ? Base64.strict_encode64

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How  do  I  use  the  Base64  library?   [5] pry(main)> ? Base64.strict_encode64 From: ~/ruby-1.9.3/lib/ruby/base64.rb Owner: # Visibility: public Signature: strict_encode64(bin) Returns the Base64-encoded version of bin. This method complies with RFC 4648. No line feeds are added.

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How  do  I  use  the  Base64  library?   [5] pry(main)> ? Base64.encode64 From: ~/ruby-1.9.3/lib/ruby/base64.rb Owner: # Visibility: public Signature: encode64(bin) Returns the Base64-encoded version of bin. This method complies with RFC 2045. Line feeds are added every 60 characters.

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How  do  I  use  the  Base64  library?   •  Evaluate  ruby  code.   •  Colour   •  Tab-­‐compleFon   •  _  (the  last  output)   •  ls  (list  methods)   •  ?  (show-­‐doc)  

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Debugging  a  blog  engine   •  IntroducFon   •  How  do  I  use  the  Base64  library?   •  Why  doesn’t  my  method  work?   •  Where  did  that  nil  come  from?   •  Further  reading  

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb require ‘base64’ module BasicAuth def self.encode(username, password) s = [username, password].join(“:”) “Basic #{Base64.strict_encode64 s}” end def self.decode(header) base64 = header[/Basic (.*)/, 1] Base64.decode64 base64.split(“:”) end end

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb require ‘base64’ module BasicAuth def self.encode(username, password) s = [username, password].join(“:”) “Basic #{Base64.strict_encode64 s}” end def self.decode(header) base64 = header[/Basic (.*)/, 1] Base64.decode64 base64.split(“:”) end end

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb [2] pry(main)> require _file_ => true [3] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [4] pry(main)> wtf? 0’

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb [2] pry(main)> require _file_ => true [3] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [4] pry(main)> wtf? 0’

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb [2] pry(main)> require _file_ => true [3] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [4] pry(main)> wtf? 0’

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb [2] pry(main)> require _file_ => true [3] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [5] pry(main)> wtf? 0’

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Why  doesn’t  my  method  work?   [1] pry(main)> cat basic_auth.rb [2] pry(main)> require _file_ => true [3] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [5] pry(main)> wtf? 0’

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Why  doesn’t  my  method  work?   [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [5] pry(main)> wtf? 0: ~/ruby-1.9.3/lib/ruby/base64.rb:58 in `decode’ 1: ~/basic_auth.rb:10 in `decode’ 2: (pry):3 in `__pry__’

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Why  doesn’t  my  method  work?   [4] pry(main)> BasicAuth.decode _ NoMethodError: undefined method `unpack’ for [“aGk6bXVt”]:Array [5] pry(main)> wtf? 0: ~/ruby-1.9.3/lib/ruby/base64.rb:58 in `decode64’ 1: ~/basic_auth.rb:10 in `decode’ 2: (pry):3 in `__pry__’ [6] pry(main)>

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Why  doesn’t  my  method  work?   [5] pry(main)> wtf? 0: ~/ruby-1.9.3/lib/ruby/base64.rb:58 in `decode64’ 1: ~/basic_auth.rb:10 in `decode’ 2: (pry):3 in `__pry__’ [6] pry(main)> $ BasicAuth.decode

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Why  doesn’t  my  method  work?   [5] pry(main)> wtf? 0: ~/ruby-1.9.3/lib/ruby/base64.rb:58 in `decode64’ 1: ~/basic_auth.rb:10 in `decode’ 2: (pry):3 in `__pry__’ [6] pry(main)> show-source BasicAuth.decode

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Why  doesn’t  my  method  work?   [6] pry(main)> $ BasicAuth.decode From: ~/basic_auth.rb Owner: # Visibility: public Number of lines: 4 def self.decode(header) base64 = header[/Basic (.*)/, 1] Base64.decode64 base64.split(“:”) end

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Why  doesn’t  my  method  work?   [5] pry(main)> wtf? 0: ~/ruby-1.9.3/lib/ruby/base64.rb:58 in `decode64’ 1: ~/basic_auth.rb:10 in `decode’ 2: (pry):3 in `__pry__’ [6] pry(main)> $ BasicAuth.decode [7] pry(main)> edit

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Why  doesn’t  my  method  work?   [6] pry(main)> $ BasicAuth.decode [7] pry(main)> edit

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Why  doesn’t  my  method  work?   [6] pry(main)> $ BasicAuth.decode [7] pry(main)> edit (on Linux) _ (on a Mac)

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Why  doesn’t  my  method  work?   [6] pry(main)> $ BasicAuth.decode [7] pry(main)> edit BasicAuth.decode

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)>

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)>

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)> (reverse-i-search)`’:

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ (reverse-i-search)`enc’

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [9] pry(main)>

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [9] pry(main)> BasicAuth.decode _ => [“hi”, “mum”] [10] pry(main)>

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Why  doesn’t  my  method  work?   [7] pry(main)> edit BasicAuth.decode [8] pry(main)> BasicAuth.encode ‘hi’, ‘mum’ => “Basic aGk6bXVt” [9] pry(main)> BasicAuth.decode _ => [“hi”, “mum”] [10] pry(main)>

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Why  doesn’t  my  method  work?   •  wd?  (show  backtrace,  also  _ex_.backtrace)   •  $  (show-­‐source)   •  edit  (uses  $EDITOR)   •  ,  _  (copy  last  word  down)   •   (search  history)   •  _file_  (last  shown  file,  also  _dir_)   •  _out_  (array  of  all  output  values,  also  _in_)  

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Debugging  a  blog  engine   •  IntroducFon   •  How  do  I  use  the  Base64  library?   •  Why  doesn’t  my  method  work?   •  Where  did  that  nil  come  from?   •  Further  reading  

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Where  did  that  nil  come  from?   $ ruby post.rb post.rb:11:in `make_safe': undefined method `gsub' for nil:NilClass (NoMethodError) from post.rb:7:in `safe_title' from post.rb:19:in `’ $

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Where  did  that  nil  come  from?   $ ruby post.rb post.rb:11:in `make_safe': undefined method `gsub' for nil:NilClass (NoMethodError) from post.rb:7:in `safe_title' from post.rb:19:in `’ $ subl post.rb

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Where  did  that  nil  come  from?   $ subl post.rb $  

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Where  did  that  nil  come  from?   $ subl post.rb $ ruby post.rb  

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Where  did  that  nil  come  from?   $ subl post.rb $ ruby post.rb From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [1] pry(main)>

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Where  did  that  nil  come  from?   $ subl post.rb $ ruby post.rb From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [1] pry(main)> play –l 19

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Where  did  that  nil  come  from?   $ subl post.rb $ ruby post.rb From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [1] pry(main)> puts new_post.safe_title

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Where  did  that  nil  come  from?   $ subl post.rb $ ruby post.rb From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [1] pry(main)> play –l 19 NoMethodError: undefined method `gsub' for nil

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)>

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post [3] pry(#):1>

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post [3] pry(#):1> ls

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post [3] pry(#):1> ls Post#methods: make_safe safe_title instance variables: @params locals: _ _dir_ _ex_ _file_ _in_ _out_ _pry_ [4] pry(#):1>

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post [3] pry(#):1> ls Post#methods: make_safe safe_title instance variables: @params locals: _ _dir_ _ex_ _file_ _in_ _out_ _pry_ [4] pry(#):1> $ safe_title

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Where  did  that  nil  come  from?   [1] pry(main)> puts new_post.safe_title NoMethodError: undefined method `gsub' for nil [2] pry(main)> cd new_post [3] pry(#):1> ls Post#methods: make_safe safe_title instance variables: @params locals: _ _dir_ _ex_ _file_ _in_ _out_ _pry_ [4] pry(#):1> $ Post#safe_title

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Where  did  that  nil  come  from?   [4] pry(#):1> $ safe_title From: post.rb @ line 7: Number of lines: 3 Owner: Post Visibility: public def safe_title make_safe @params[:title] end [5] pry(#):1>

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Where  did  that  nil  come  from?   [4] pry(#):1> $ safe_title From: post.rb @ line 7: Number of lines: 3 Owner: Post Visibility: public def safe_title make_safe @params[:title] end [5] pry(#):1> @params

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Where  did  that  nil  come  from?   [4] pry(#):1> $ safe_title From: post.rb @ line 7: Number of lines: 3 Owner: Post Visibility: public def safe_title make_safe @params[:title] end [5] pry(#):1> @params {“title”=>”new post”, “body”=>”your text here”} [6] pry(#):1>

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Where  did  that  nil  come  from?   [4] pry(#):1> $ safe_title From: post.rb @ line 7: Number of lines: 3 Owner: Post Visibility: public def safe_title make_safe @params[:title] end [5] pry(#):1> @params {“title”=>”new post”, “body”=>”your text here”} [6] pry(#):1> edit --ex

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Where  did  that  nil  come  from?   [5] pry(#):1> @params {“title”=>”new post”, “body”=>”your text here”} [6] pry(#):1> edit --ex [7] pry(#):1>

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Where  did  that  nil  come  from?   [5] pry(#):1> @params {“title”=>”new post”, “body”=>”your text here”} [6] pry(#):1> edit --ex [7] pry(#):1> .git diff

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Where  did  that  nil  come  from?   [7] pry(#):1> .git diff diff --git a/post.rb b/post.rb index d0ed356..057d37c 100644 --- a/post.rb +++ b/post.rb @@ -4, 7 +4,7 @@ class Post end def safe_title - make_safe @params[:title] + make_safe @params[‘title’] end [8] pry(#):1>

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Where  did  that  nil  come  from?   [7] pry(#):1> .git diff diff --git a/post.rb b/post.rb index d0ed356..057d37c 100644 --- a/post.rb +++ b/post.rb @@ -4, 7 +4,7 @@ class Post end def safe_title - make_safe @params[:title] + make_safe @params[‘title’] end [8] pry(#):1> cd ..

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Where  did  that  nil  come  from?   [8] pry(#):1> cd .. [9] pry(main)>

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Where  did  that  nil  come  from?   [8] pry(#):1> cd .. [9] pry(main)> whereami

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Where  did  that  nil  come  from?   [8] pry(#):1> cd .. [9] pry(main)> whereami From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [10] pry(main)>

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Where  did  that  nil  come  from?   [8] pry(#):1> cd .. [9] pry(main)> whereami From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [10] pry(main)> puts new_post.safe_title

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Where  did  that  nil  come  from?   [9] pry(main)> whereami From: post.rb @ line 18 : 15: new_post = Post.new( 16: 'title' => 'new post', 17: 'body' => 'your text here') => 18: binding.pry # Added for debugging 19: puts new_post.safe_title [10] pry(main)> puts new_post.safe_title new-post => nil [11] pry(main)>

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Where  did  that  nil  come  from?   [10] pry(main)> puts new_post.safe_title new-post => nil [11] pry(main)> puts new_post.safe_title;

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Where  did  that  nil  come  from?   [10] pry(main)> puts new_post.safe_title new-post => nil [11] pry(main)> puts new_post.safe_title; new-post [12] pry(main)>

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Where  did  that  nil  come  from?   [10] pry(main)> puts new_post.safe_title new-post => nil [11] pry(main)> puts new_post.safe_title; new-post [12] pry(main)>

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Where  did  that  nil  come  from?   [10] pry(main)> puts new_post.safe_title new-post => nil [11] pry(main)> puts new_post.safe_title; new-post [12] pry(main)> new-post $

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Where  did  that  nil  come  from?   •  binding.pry  (open  pry  right  here,  right  now)   •  cd  (change  self)   •  whereami   •  edit  -­‐-­‐ex   •  play  -­‐l   •  .  (run’s  shell  commands)   •   (cd  ..  or  exit)   •  ;  (at  end  of  line,  hides  output)  

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Debugging  a  blog  engine   •  IntroducFon   •  How  do  I  use  the  Base64  library?   •  Why  doesn’t  my  method  work?   •  Where  did  that  nil  come  from?   •  Further  reading  

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pry-­‐rescue/pry-­‐stack_explorer   •  pry-­‐rescue   – AutomaFcally  opens  pry  wherever  you  have  an   unhandled  excepFon  or  test  failure.   – cd-­‐cause  lets  you  pry  into  any  excepFon  that   happens  inside  the  console       •  pry-­‐stack_explorer   – lets  you  move  up/down  the  callstack  as  though   you  had  a  binding.pry  at  every  level    

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be4er_errors   •  BetterErrors.use_pry! •  Opens  a  web  page  on  unhandled  excepFons   •  Lets  you  debug  your  program  amer  it  crashes  

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pry-­‐debugger   •  break   – Adds  a  breakpoint   •  step/next   – Step  into  or  over  the  next  line     •  conFnue   – Stop  stepping  through    

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Recommended  plugins   •  pry-­‐plus   – pry-­‐debugger,  pry-­‐rescue,  pry-­‐stack_explorer   – pry-­‐doc,  documentaFon  for  C  methods   – pry-­‐docmore,  documentaFon  for  ruby  syntax   – bond,  tab  compleFon  for  Hash#[],  etc.   – jist,  IntegraFon  with  gist.github.com   •  pry-­‐rails   – makes  “rails  c”  use  pry!  

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• 60%  of  your  Fme  is  spent   debugging   • use  binding.pry  

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QuesFons?!   •  Pry  (@pryrepl)   – h4p://pryrepl.org   – irc://irc.freenode.net/#pry   – type  “help”  in  pry  :)   •  Conrad  Irwin  (@ConradIrwin)   – h4p://cirw.in   – irc://irc.freenode.net/cirwin