Pry

(C) John Mair (banisterfiend) 2010

attach an irb-like session to any object at runtime

Pry is a simple Ruby REPL (Read-Eval-Print-Loop) that specializes in the interactive manipulation of objects during the running of a program.

It is not based on the IRB codebase and is small, at around 260 LOC.

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

example: Interacting with an object at runtime

With the Pry.start() 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 current thread is halted for the duration of the session.

require 'pry'

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

Pry.start(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

Alternative Syntax

You can also use the obj.pry or pry(obj) syntax to start a pry session on obj. e.g

5.pry
Beginning Pry session for 5
pry(5)>

OR

pry 6
beginning Pry session for 6
pry(6)>

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.start() without parameters begins a Pry session on top-level (main)
Pry.start
Beginning Pry session for main
pry(main)> class Hello
pry(main)*   @x = 20
pry(main)* end
=> 20
pry(main)> Pry.start Hello
Beginning Pry session for Hello
pry(Hello):1> instance_variables
=> [:@x]
pry(Hello):1> Pry.start @x
Beginning Pry session for 20
pry(20:2)> self + 10
=> 30
pry(20:2)> exit
Ending Pry session for 20
pry(Hello):1> exit
Ending Pry session for Hello
pry(main)> exit
Ending Pry session for main

The number after the : in the pry prompt indicates the nesting level. To display more information about nesting, use the nesting command. E.g

pry("friend":3)> nesting
Nesting status:
0. main (Pry top level)
1. Hello
2. 100
3. "friend"
=> nil

We can then jump back to any of the previous nesting levels by using the jump_to command:

pry("friend":3)> jump_to 1
Ending Pry session for "friend"
Ending Pry session for 100
=> 100
pry(Hello):1>

If we just want to go back one level of nesting we can of course use the quit or exit or back commands.

To break out of all levels of Pry nesting and return immediately to the calling process use exit_all:

pry("friend":3)> exit_all
Ending Pry session for "friend"
Ending Pry session for 100
Ending Pry session for Hello
Ending Pry session for main
=> main

# program resumes here

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.

Features:

• 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' or 'back'
• Use _ to recover last result.
• Pry has multi-line support built in.
• Pry gives good control over nested sessions (important when exploring complicated runtime state)
• Pry is not based on the IRB codebase.
• Pry is small; around 260 LOC.
• 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.start is simply an alias for Pry.repl). You can invoke any of these methods directly depending on exactly what aspect of the functionality you need.

Limitations:

• Pry does not pretend to be a replacement for irb, and so does not have an executable. It is designed to be used by other programs, not on its own. For a full-featured irb replacement see ripl
• Although Pry works fine in Ruby 1.9, only Ruby 1.8 syntax is supported. This is because Pry uses the RubyParser gem internally to validate expressions, and RubyParser, as yet, only parses Ruby 1.8 code. In practice this usually just means you cannot use the new hash literal syntax (this: syntax) or the 'stabby lambda' syntax (->).

Commands

The Pry API:

• Pry.start() and Pry.into() and Pry.repl() are all aliases of oneanother. They all start a Read-Eval-Print-Loop on the object they receive as a parameter. In the case of no parameter they operate on top-level (main). They can receive any object or a Binding object as parameter.
• obj.pry and pry(obj) may also be used as alternative syntax to Pry.start(obj)
• If, for some reason you do not want to 'loop' then use Pry.rep(); it only performs the Read-Eval-Print section of the REPL - it ends the session after just one line of input. It takes the same parameters as Pry.repl()
• Likewise Pry.re() only performs the Read-Eval section of the REPL, it returns the result of the evaluation or an Exception object in case of error. It also takes the same parameters as Pry.repl()
• Similarly Pry.r() only performs the Read section of the REPL, only returning the Ruby expression (as a string). It takes the same parameters as all the others.

Session commands

Pry supports a few commands inside the session itself. These commands are not methods and must start at the beginning of a line, with no whitespace in between.

If you want to access a method of the same name, prefix the invocation by whitespace.

• Typing ! on a line by itself will refresh the REPL - useful for getting you out of a situation if the parsing process goes wrong.
• status shows status information about the current session.
• help shows the list of session commands with brief explanations.
• exit or quit or back will end the current Pry session and go back to the calling process or back one level of nesting (if there are nested sessions).
• exit_program or quit_program will end the currently running program.
• nesting shows Pry nesting information.
• jump_to <nest_level> unwinds the Pry stack (nesting level) until the appropriate nesting level is reached -- as per the output of nesting
• exit_all breaks out of all Pry nesting levels and returns to the calling process.
• You can type Pry.start(obj) or obj.pry to nest another Pry session within the current one with obj as the receiver of the new session. Very useful when exploring large or complicated runtime state.

Contact

Problems or questions contact me at github