CSC/ECE 517 Fall 2014/ch1a 26 gn

Debugging in Rails using Pry

Pry - A powerful IRB alternative and runtime developer console for Ruby.

Ruby on Rails is a Web application framework for Ruby. It was first released to the public in July 2004. Within months, it was a widely used development environment. Many multinational corporations are using it to create Web applications. It is the standard Web-development framework for Ruby. Models are objects that represent the components of an application that perform information processing in the problem domain. Views are objects that display some aspect of the model. They are the output mechanism for the models. Controllers are objects that control how user actions are interpreted. They are the input mechanism for the views and models.

Since a Rails application goes beyond the model view controller, like Mailers, Routes (REST, HTTP-Verbs, Constraints), Environments and Initializers, Caching (Redis, Memcached), Assets (CSS, SASS,, JavaScript, CoffeeScript, Pipelining), Bundler and dependency management, Tests (RSpec, Capybara), gems, plugins and engines used in the application, one needs to know a lot of stuff to master debugging their own application. Debugging is one of the things a developer has to do most and there exists debuggers for Rails that are suitable for browsers and lack the ability to allow moving around in the call-stack and inspecting objects at runtime.

Pry in literal sense means to look or inquire closely, curiously, or impertinently. Pry is a powerful alternative to the standard IRB shell for Ruby. It not only for debugging but also (and mainly) for inspecting objects. It features syntax highlighting, a flexible plugin architecture, runtime invocation and source and documentation browsing. It enables the user to browse source code, inspect objects, eliminates all the iteration- and startup time when running red/green cycles and is especially useful when you don’t know exactly how something should function in the first place.

Pry: super-fast, painless, debugging for the (ruby) masses.

Pry aims to be more than an IRB replacement; it is an attempt to bring REPL driven programming to the Ruby language. Pry is also fairly flexible and allows significant user customization making it a good choice for implementing custom shells. The pry console gives you access to the method that raised the exception, you can use it to inspect the values of variables (no more print statements!), the source code of methods (no more flapping around with a text editor!), and even move up and down the call stack (like a real debugger!).

Because the shell opens as though the exception were just about to be raised, you don't even need to re-run your program from the beginning in order to start debugging. It's optimized for the "exceptions-happen" workflow that you need when developing code.

Features

Pry is written from scratch to provide a number of advanced features, some of these include : <ref>http://pryrepl.org</ref>

1. Source code browsing (including core C source with the pry-doc gem)
2. Navigation around state (cd, ls and friends)
3. Rubinius core source browsing
4. Documentation browsing
5. Live help system
6. Open methods in editors (edit-method Class#method)
7. Syntax highlighting
8. Command shell integration (start editors, run git, and rake from within Pry)
9. Gist integration
10. Runtime invocation (use Pry as a developer console or debugger)
11. Exotic object support (BasicObject instances, IClasses, ...)
12. A powerful and flexible command system
13. Ability to view and replay history
14. Many convenience commands inspired by IPython, Smalltalk and other advanced REPLs

Installation

The steps you have to do are simple:

$ gem install pry
$ pry

You could install pry on Rails by adding the pry-rails gem to your gem file:

group :development do
   gem 'pry'
   ...
 end

Once you have got that settled, change back into the sample_app directory and install all the necessary dependencies:

bundle install

Setting up the database:

rake db:schema:load

Note that some of you might need to prefix the above command with bundle exec.

One final thing before we proceed to the actual stuff: Create .pryrc in your home directory and fill it in with the editor of your choice:

% touch ~/.pryrc
Pry.config.editor = 'vim'

To launch pry through the terminal you simply write:

rails c

and you’re greeted with a pry console:

% rails c
Loading development environment (Rails 3.2.16)
Frame number: 0/3
[1] sample_app »

Usage and Example

Usage

After you install Pry using the above mentioned install command, you run your program using rescue foo.rb instead of ruby foo.rb. If not, just wrap chunks of your program in Pry::rescue{ }. Any exceptions that are unhandled within that block will be rescued by pry on your behalf.

Whenever pry opens because of an exception you have two choices:

1. Hit ctrl-d to let the exception bubble on its way
2. Do some debugging.

If you choose the second option, then you can use the full power of pry to fix the problem; and then try-again to verify the fix worked.

Example

Fr example, lets assume the following piece of code:

def find_capitalized(a)
   a.select do |name|
     name.chars.first == name.chars.first.upcase
   end
 end

Now if you run rescue foo.rb, you would get a output which would look like this:

From: rescue.rb @ line 2 Object#find_capitalized:

    2: def find_capitalized(a)
    3:   a.select do |name|
 => 4:     name.chars.first == name.chars.first.upcase
    5:   end
    6: end

NoMethodError: undefined method `chars' for :direction:Symbol
from rescue.rb:4:in `block in find_capitalized'
[1] pry(main)>

Well, clearly it's gone wrong. Let's move up the stack and see if we can find which code is calling this method with symbols:

[1] pry(main)> up
From: rescue.rb @ line 8 Object#extract_people:

     8: def extract_people(opts)
 =>  9:   name_keys = find_capitalized(opts.keys)
    10: 
    11:   name_keys.each_with_object({}) do |name, o|
    12:     o[name] = opts.delete name
    13:   end
    14: end

[2] pry(main)> opts
=> {"Arthur"=>"Dent", :direction=>:left}

Let's see if there's a better way of implementing find_capitalized:

[3] pry(main)> down
[4] pry(main)> name.first
NoMethodError: undefined method `first' for :direction
from (pry):9:in `block in find_capitalized'
[5] pry(main)> name.capitalize
=> :Direction

Let's edit-method to fix the code, and try-again to verify the fix worked:

[6] pry(main)> edit-method
[7] pry(main)> whereami
From: rescue.rb @ line 2 Object#find_capitalized:

    2: def find_capitalized(a)
 => 3:   a.select do |name|
    4:     name.capitalize == name
    5:   end
    6: end

[8] pry(main)> try-again
Arthur Dent moves left

Pry Commands

There are a myriad of pry commands that are useful while debugging. Follwing is the list of some important list of commands that can be used while debugging using Pry.

1. ls <Object>

Description: It show all of the available methods that can be called by an object

1. _

Description: Last output

1. ? <Object>

Description: Shows more information (doc) about an object, or method

1. cat <File>

Description: Display the content of a file

1. _file_

Description: Represent the last file Pry touched

1. wtf?

Description: Print the stack trace, same as _ex_.backtrace

1. $

Description: Show source, shortcut for show-source

1. edit <Method>

Description: Open file in $EDITOR, change file are auto reloaded

1. <ctrl+r>

Description: Search history

1. _out_

Description: Array of all outputs values, also _in_

1. cd

Description: Step into an object, change the value of self

1. cd ..

Description: Take out of a level

1. binding.pry

Description: Breakpoint

1. edit --ex

Description: Edit the file where the last exception was thrown

1. .<Shell>

Description: Runs the <Shell> command

1. whereami

Description: Print the context where the debugger is stopped

1. ;

Description: Would mute the return output by Ruby

1. play -l

Description: Execute the line in the current debugging context

The best thing to do is just type help once you've installed pry.

References

http://pryrepl.org

http://www.sitepoint.com/pry-friends-rails

http://cirw.in/blog/pry-to-the-rescue

http://www.jackkinsella.ie/2014/06/06/debugging-rails-with-pry-debugger.html

https://github.com/ConradIrwin/pry-rescue