Pry

(C) John Mair (banisterfiend) 2010

attach an irb-like session to any object

Pry is a simple Ruby REPL that specializes in the interactive manipulation of objects during the running of a program.

• Install the gem: gem install pry
• Read the documentation
• See the source code

example: prying on an object at runtime

With the Pry.into() method we can pry (open an irb-like session) on an object. In the example below we open a Pry session for the Test class and execute a method and add an instance variable. The program is halted for the duration of the session.

require 'pry'

class Test
  def self.hello() "hello world" end
end

Pry.into(Test)

# Pry session begins on stdin
Beginning Pry session for Test
pry(Test)> self
=> Test
pry(Test)> hello
=> "hello world"
pry(Test)> @y = 20
=> 20
pry(Test)> exit
Ending Pry session for Test

# program resumes here

If we now inspect the Test object we can see our changes have had effect:

Test.instance_variable_get(:@y) #=> 20

example: Pry sessions can nest arbitrarily deep so we can pry on objects inside objects:

Here we will begin Pry at top-level, then pry on a class and then on an instance variable inside that class:

Pry.into
Beginning Pry session for main
pry(main)> class Hello
pry(main)*   @x = 20
pry(main)* end
=> 20
pry(main)> Pry.into Hello
Beginning Pry session for Hello
pry(Hello)> instance_variables
=> [:@x]
pry(Hello)> Pry.into @x
Beginning Pry session for 20
pry(20)> self + 10
=> 30
pry(20)> exit
Ending Pry session for 20
pry(Hello)> exit
Ending Pry session for Hello
pry(main)> exit
Ending Pry session for main

# program resumes here

example: Spawn a separate thread so you can use Pry to manipulate an object without halting the program.

If we embed our Pry.into method inside its own thread we can examine and manipulate objects without halting the program.

# Pry.into() without parameters opens up the top-level (main)
Thread.new { Pry.into }

Features and limitations

Pry is an irb-like clone with an emphasis on interactively examining and manipulating objects during the running of a program.

Its primary utility is probably in debugging, though it may have other uses (such as implementing a quake-like console for games, for example). Here is a list of Pry's features along with some of its limitations given at the end.

• Pry can be invoked at any time and on any object in the running program.
• Pry sessions can nest arbitrarily deeply -- to go back one level of nesting type 'exit' or 'quit'
• Pry has multi-line support built in.
• Pry implements all the methods in the REPL chain separately: Pry.r for reading; Pry.re for eval; Pry.rep for printing; and Pry.repl for the loop (Pry.into is simply an alias for Pry.repl)

Contact

Problems or questions contact me at github