Debugging With Pry
+ +Pry is a REPL (Read Eval Print Loop) that was written as a better alternative
+to IRB. It comes with syntax highlighting, code indentation that actually works
+and several other features that make it easier to debug code. I stumbled upon
+Pry when looking for an alternative to both IRB and the way I was debugging my
+code (placing puts all over the place, I think it’s called “Puts Driven
+Development”).
Pry tries to do a lot of things and I was actually quite surprised how well it +does that. It might not stick to the Unix idea of only doing a single thing +(and doing that very well) but it makes my life (and the lifes of others) so +much easier that it’s easy to forget.
+ +Pry is primarily meant to be used as a REPL. There are a lot of things that +make Pry so much more pleasant to use than IRB. One of the things almost any +Ruby programmer will notice when using IRB is that its indentation support is a +bit clunky. Indenting itself works fine most of the time but it fails to +un-indent code properly as illustrated in the code snippet below (pasted +directly from an IRB session):
+ +ruby-1.9.3-p0 :001 > class User
+ruby-1.9.3-p0 :002?> def greet
+ruby-1.9.3-p0 :003?> puts "Hello world"
+ruby-1.9.3-p0 :004?> end
+ruby-1.9.3-p0 :005?> endLuckily Pry handles this just fine, whether you’re trying to indent a class or +a hash containing an array containing a proc and so on. Pry does this by +resetting the terminal output every time a new line is entered. The downside of +this approach is that it only works on terminals that understand ANSI escape +codes. In Pry the above example works like it should do:
+ +[1] pry(main)> class User
+[1] pry(main)* def greet
+[1] pry(main)* puts "Hello world"
+[1] pry(main)* end
+[1] pry(main)* endBesides indentation Pry does a lot more. A feature that I think is very cool is
+the ability to show documentation and source code of methods right in your REPL
+(sadly this feature doesn’t work with classes or modules at the time of
+writing). This means that you no longer have to use the ri command to
+search documentation for methods. You also don’t need to install the RDoc
+documentation as Pry pulls it directly from the source code. Showing the source
+code of a method or its documentation can be done by using the show-method
+and show-doc command. For example, invoking show-method pry in a Pry
+session would give you the following output:
[1] pry(main)> show-method pry
+
+From: /path/trimmed/for/readability/lib/pry/core_extensions.rb @ line 19:
+Number of lines: 3
+Owner: Object
+Visibility: public
+
+def pry(target=self)
+ Pry.start(target)
+endCalling show-doc pry would instead show the following:
[2] pry(main)> show-doc pry
+
+From: /path/trimmed/for/readability/lib/pry/core_extensions.rb @ line 19:
+Number of lines: 17
+Owner: Object
+Visibility: public
+Signature: pry(target=?)
+
+Start a Pry REPL.
+This method differs from Pry.start in that it does not
+support an options hash. Also, when no parameter is provided, the Pry
+session will start on the implied receiver rather than on
+top-level (as in the case of Pry.start).
+It has two forms of invocation. In the first form no parameter
+should be provided and it will start a pry session on the
+receiver. In the second form it should be invoked without an
+explicit receiver and one parameter; this will start a Pry
+session on the parameter.
+param [Object, Binding] target The receiver of the Pry session.
+example First form
+ "dummy".pry
+example Second form
+ pry "dummy"
+example Start a Pry session on current self (whatever that is)
+ pryYou can also run these commands for code that was written in C. This requires
+you to install the gem pry-doc (gem install pry-doc). Do note that this
+only works for core C code, currently Pry does not support this for third party
+extensions.
Another very cool feature is that Pry can be used as a debugging tool for your
+code without having to manually jump into a session. By loading Pry, which can
+be done by writing require "pry" or by using the option -r pry when
+invoking Ruby you gain access to everything Pry has to offer. The most useful
+tool is binding.pry. This method starts a Pry session and pauses the
+script.
Lets say you have the following script and want to see the values of the +variables:
+ +language = 'Ruby'
+number = 10
+
+# Do something awesome with the above variables.The typical approach would be to insert a puts statement above the comment +followed by an exit statement. Pry in a way can do a similar thing, it just +makes it a lot more awesome. If you modify the script as following you can +truly debug your code like a boss:
+ +language = 'Ruby'
+number = 10
+
+binding.pry
+
+# Do something awesome with the above variables.If you now run the script by calling ruby -r pry file.rb you get a fancy
+Pry session:
[yorickpeterse@Wifi-Ninja in ~]$ ruby -r pry file.rb
+
+From: file.rb @ line 4 in Object#N/A:
+
+ 1: language = 'Ruby'
+ 2: number = 10
+ 3:
+ => 4: binding.pry
+ 5:
+ 6: # Do something awesome with the above variables.
+[1] pry(main)>A nice thing about starting Pry this way is that it starts in the context of
+the call to binding.pry meaning you get access to data such as the local
+variables. These can be displayed by calling ls or by simply typing their
+name.
[yorickpeterse@Wifi-Ninja in ~]$ ruby -r pry file.rb
+
+From: file.rb @ line 4 in Object#N/A:
+
+ 1: language = 'Ruby'
+ 2: number = 10
+ 3:
+ => 4: binding.pry
+ 5:
+ 6: # Do something awesome with the above variables.
+[1] pry(main)> ls
+self methods: include private public to_s
+locals: _ _dir_ _ex_ _file_ _in_ _out_ _pry_ language number
+[2] pry(main)> number
+=> 10
+[3] pry(main)>Moving out of the “breakpoint” (or moving to the next one if you have multiple +ones defined) can be done by hitting ^D (Ctrl+D usually).
+ +Besides the features mentioned in this article Pry has several more. For
+example, long output is piped to Less. This can be quite useful if you’re
+trying to display a big hash using pp. The full list of features can be
+found on the Pry website as well as by invoking the help
+command inside a Pry session. If you’re in need of help or have any suggestions
+you can join the IRC channel #pry on the Freenode network (irc.freenode.net).
+The source code of Pry is hosted on Github.