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Pry

(C) John Mair (banisterfiend) 2011

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.

In some sense it is the opposite of IRB in that you bring a REPL session to your code (with Pry) instead of bringing your code to a REPL session (as with IRB).

It is not based on the IRB codebase, and implements some unique REPL commands such as show_method and show_doc

Pry is also fairly flexible and allows significant user customization. It is trivial to set it to read from any object that has a readline method and write to any object that has a puts method - many other aspects of Pry are also configurable.

Pry also has rubygems-test support; to participate, first install Pry, then:

  1. Install rubygems-test: gem install rubygems-test
  2. Run the test: gem test pry
  3. Finally choose 'Yes' to upload the results.

Example: Interacting with an object at runtime

With the Object#pry 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

Test.pry

# 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 Pry.start(obj) or pry(obj) syntax to start a pry session on obj. e.g

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

OR

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

Example: Pry sessions can nest

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)> Hello.pry
Beginning Pry session for Hello
pry(Hello):1> instance_variables
=> [:@x]
pry(Hello):1> @x.pry
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.
  • Use _pry_ to reference the Pry instance managing the current session.
  • Pry supports tab completion.
  • Pry has multi-line support built in.
  • Pry has special commands not found in many other Ruby REPLs: show_method, show_doc jump_to, ls, cd, cat
  • Pry gives good control over nested sessions (important when exploring complicated runtime state)
  • Pry is not based on the IRB codebase.
  • Pry allows significant customizability.
  • Pry uses the method_source gem; so this functionality is available to a Pry session.
  • Pry uses RubyParser to validate expressions in 1.8, and Ripper for 1.9.
  • 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 simply wraps Pry.new.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
  • Pry's show_method and show_doc commands do not work in Ruby 1.8.

Commands

The Pry API:

  • Pry.start() Starts a Read-Eval-Print-Loop on the object it receives as a parameter. In the case of no parameter it operates on top-level (main). It can receive any object or a Binding object as parameter. Pry.start() is implemented as Pry.new.repl()
  • obj.pry and pry(obj) may also be used as alternative syntax to Pry.start(obj).

    However there are some differences. obj.pry opens a Pry session on the receiver whereas Pry.start (with no parameter) will start a Pry session on top-level. The other form of the pry method: pry(obj) will also start a Pry session on its parameter.

    The pry method invoked by itself, with no explict receiver and no parameter will start a Pry session on the implied receiver. It is perhaps more useful to invoke it in this form pry(binding) or binding.pry so as to get access to locals in the current context.

    Another difference is that Pry.start() accepts a second parameter that is a hash of configuration options (discussed further, below).

  • If, for some reason you do not want to 'loop' then use Pry.new.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).
  • ls returns a list of local variables and instance variables in the current scope
  • ls_methods List all methods defined on immediate class of receiver.
  • ls_imethods List all instance methods defined on receiver.
  • cat <var> Calls inspect on <var>
  • cd <var> Starts a Pry session on the variable . E.g cd @x (use cd .. to go back).
  • show_method <methname> Displays the sourcecode for the method . E.g show_method hello
  • show_imethod <methname> Displays the sourcecode for the instance method . E.g show_imethod goodbye
  • show_doc <methname> Displays comments for <methname>
  • show_idoc <methname> Displays comments for instance method <methname>
  • exit_program or quit_program will end the currently running program.
  • nesting Shows Pry nesting information.
  • !pry Starts a Pry session on the implied receiver; this can be used in the middle of an expression in multi-line input.
  • jump_to <nest_level> Unwinds the Pry stack (nesting level) until the appropriate nesting level is reached.
  • 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.

Example Programs

Pry comes bundled with a few example programs to illustrate some features, see the examples/ directory.

  • example_input.rb - Demonstrates how to set the input object.
  • example_output.rb - Demonstrates how to set the output object.
  • example_hooks.rb - Demonstrates how to set the hooks hash.
  • example_print.rb - Demonstrates how to set the print object.
  • example_prompt.rb - Demonstrates how to set the prompt.
  • example_input2.rb - An advanced input example.
  • example_commands.rb - Implementing a mathematical command set.
  • example_commands_override.rb - An advanced commands example.
  • example_image_edit.rb - A simple image editor using a Pry REPL (requires Gosu and TexPlay gems).

Customizing Pry

Pry allows a large degree of customization.

Read how to customize Pry here.

Contact

Problems or questions contact me at github

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