pry-rescue

Super-fast debugging for Ruby. (See Pry to the rescue!)

Introduction

pry-rescue is an implementation of "break on unhandled exception" for Ruby. Whenever an exception is raised, but not rescued, pry-rescue will automatically open Pry for you:

$ rescue examples/example2.rb
From: /home/conrad/0/ruby/pry-rescue/examples/example2.rb @ line 19 Object#beta:

    17: def beta
    18:   y = 30
 => 19:   gamma(1, 2)
    20: end

ArgumentError: wrong number of arguments (2 for 1)
from /home/conrad/0/ruby/pry-rescue/examples/example2.rb:22:in `gamma`
[1] pry(main)>

Installation

You can install pry-rescue with RubyGems as normal, and I strongly recommend you also install pry-stack_explorer. See Known bugs for places that won't work.

gem install pry-rescue pry-stack_explorer

If you're using Bundler, you can add it to your Gemfile in the development group:

group :development do
  gem 'pry-rescue'
  gem 'pry-stack_explorer'
end

Usage

For simple Ruby scripts, just run them with the rescue executable instead of the ruby executable.

rescue <script.rb> [arguments..]

Rails

For Rails, use rescue rails in place of rails, for example:

rescue rails server

If you're using bundle exec the rescue should go after the exec:

bundle exec rescue rails server

Then whenever an unhandled exception happens inside Rails, a Pry console will open on stdout. This is the same terminal that you see the Rails logs on, so if you're using something like pow then you will run into difficulties.

If you are using non-default http servers like Unicorn or Thin, you can also trigger this behavior via (after including pry-rescue in your Gemfile):

PRY_RESCUE_RAILS=1 bundle exec unicorn

You might also be interested in better_errors which opens consoles in your browser on unhandled exceptions, and pry-rails which adds some Rails specific helpers to Pry, and replaces rails console by Pry.

RSpec

If you're using RSpec or respec, you can open a Pry session on every test failure using rescue rspec or rescue respec:

$ rescue rspec
From: /home/conrad/0/ruby/pry-rescue/examples/example_spec.rb @ line 9 :

     6:
     7: describe "Float" do
     8:   it "should be able to add" do
 =>  9:     (0.1 + 0.2).should == 0.3
    10:   end
    11: end

RSpec::Expectations::ExpectationNotMetError: expected: 0.3
     got: 0.30000000000000004 (using ==)
[1] pry(main)>

Unfortunately using edit -c to edit _spec.rb files does not yet reload the code in a way that the try-again command can understand. You can still use try-again if you edit code that is not in spec files.

Minitest

Add the following to your test_helper.rb or to the top of your test file.

require 'minitest/autorun'
require 'pry-rescue/minitest'

Then, when you have a failure, you can use edit, edit -c, and edit-method, then try-again to re-run the tests.

Rack

If you're using Rack, you should use the middleware instead (though be careful to only include it in development!):

use PryRescue::Rack if ENV["RACK_ENV"] == 'development'

Pry commands

pry-rescue adds two commands to Pry. cd-cause and try-again. In combination with edit --method these can let you fix the problem with your code and verify that the fix worked without restarting your program.

cd-cause

If you've run some code in Pry, and an exception was raised, you can use the cd-cause command:

[1] pry(main)> foo
RuntimeError: two
from a.rb:4:in `rescue in foo`
[2] pry(main)> cd-cause
From: a.rb @ line 4 Object#foo:

    1: def foo
    2:   raise "one"
    3: rescue => e
 => 4:   raise "two"
    5: end

[3] pry(main)>

If that exception was in turn caused by a previous exception you can use cd-cause again to move to the original problem:

[3] pry(main)> cd-cause
From: examples/example.rb @ line 4 Object#test:

    4: def test
 => 5:   raise "foo"
    6: rescue => e
    7:   raise "bar"
    8: end

RuntimeError: foo
from examples/example.rb:5:in `test`
[4] pry(main)>

To get back from cd-cause you can either type <ctrl+d> or cd ...

try-again

Once you've used Pry's edit or command to fix your code, you can issue a try-again command to re-run your code. For Rails and rack, this re-runs the request, for minitest and rspec, it re-runs the current test, for more advanced users this re-runs the Pry::rescue{ } block.

[4] pry(main)> edit --method
[5] pry(main)> whereami
From: examples/example.rb @ line 4 Object#test:

    4: def test
 => 5:   puts "foo"
    6: rescue => e
    7:   raise "bar"
    8: end
[6] pry(main)> try-again
foo

Advanced usage

Block form

If you want more fine-grained control over which parts of your code are rescued, you can also use the block form:

require 'pry-rescue'

def test
  raise "foo"
rescue => e
  raise "bar"
end

Pry.rescue do
  test
end

This will land you in a pry-session:

From: examples/example.rb @ line 4 Object#test:

    4: def test
    5:   raise "foo"
    6: rescue => e
 => 7:   raise "bar"
    8: end

RuntimeError: bar
from examples/example.rb:7:in `rescue in test`
[1] pry(main)>

Rescuing an exception

Finally. If you're doing your own exception handling, you can ask Pry to open on an exception that you've caught. For this to work you must be inside a Pry::rescue{ } block.

def test
  raise "foo"
rescue => e
  Pry::rescued(e)
end

Pry::rescue{ test }

Peeking

Sometimes bugs in your program don't cause exceptions. Instead your program just gets stuck. Examples include infinite loops, slow network calls, or tests that take a surprisingly long time to run.

In this case it's useful to be able to open a Pry console when you notice that your program is not going anywhere. To do this, send your process a SIGQUIT using <ctrl+\>.

cirwin@localhost:/tmp/pry $ ruby examples/loop.rb
^\
Preparing to peek via pry!
Frame number: 0/4

From: ./examples/loop.rb @ line 10 Object#r
    10: def r
    11:   some_var = 13
    12:   loop do
 => 13:     x = File.readlines('lib/pry-rescue.rb')
    14:   end
    15: end
pry (main)>

Advanced peeking

You can configure which signal pry-rescue listens for by default by exporting the PRY_PEEK environment variable that suits your use-case best:

export PRY_PEEK=""    # don't autopeek at all
export PRY_PEEK=INT   # peek on SIGINT (<ctrl+c>)
export PRY_PEEK=QUIT  # peek on SIGQUIT
export PRY_PEEK=USR1  # peek on SIGUSR1
export PRY_PEEK=USR2  # peek on SIGUSR2
export PRY_PEEK=EXIT  # peek on program exit

If it's only important for one program, then you can also set the environment variable in Ruby before requiring pry-rescue:

ENV['PRY_PEEK'] = '' # disable SIGQUIT handler
require "pry-rescue"

Finally, you can enable peeking into programs that do not include pry-rescue by configuring Ruby to always load one (or several) of these files:

export RUBYOPT=-rpry-rescue/peek/int   # peek on SIGINT (<ctrl-c>)
export RUBYOPT=-rpry-rescue/peek/quit  # peek on SIGQUIT (<ctrl-\>)
export RUBYOPT=-rpry-rescue/peek/usr1  # peek on SIGUSR1
export RUBYOPT=-rpry-rescue/peek/usr2  # peek on SIGUSR2
export RUBYOPT=-rpry-rescue/peek/exit  # peek on program exit

These last examples relies on having pry-rescue in the load path (i.e. at least in the gemset, or Gemfile of the program). If that is not true, you can use absolute paths. The hook files do not require the whole of pry-rescue, nor is any of Pry itself loaded until you trigger the signal.

export RUBYOPT=-r/home/cirwin/src/pry-rescue/lib/pry-rescue/peek/usr2

Known bugs

• Ruby 2.0, 1.9.3, 1.9.2 – no known bugs
• Ruby 1.9.1 — not supported
• Ruby 1.8.7 — occasional incorrect values for self
• REE 1.8.7 — no known bugs
• JRuby 1.7 (1.8 mode and 1.9 mode) — no known bugs
• JRuby 1.6 (1.8 mode and 1.9 mode) — incorrect value for self in NoMethodErrors
• Rubinius (1.8 mode and 1.9 mode) – does not catch some low-level errors (e.g. ZeroDivisionError)

Meta-fu

Released under the MIT license, see LICENSE.MIT for details. Contributions and bug-reports are welcome.