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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, show-doc, ls and cd (type help to get a full list).

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 making it a good choice for implementing custom shells.

Pry now comes with an executable so it can be invoked at the command line. Just enter pry to start. A .pryrc file in the user's home directory will be loaded if it exists. Type pry --help at the command line for more information.

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)> cd Hello
Beginning Pry session for Hello
pry(Hello):1> instance_variables
=> [:@x]
pry(Hello):1> cd @x
Beginning Pry session for 20
pry(20:2)> self + 10
=> 30
pry(20:2)> cd ..
Ending Pry session for 20
pry(Hello):1> cd ..
Ending Pry session for Hello
pry(main)> cd ..
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.
  • Use ^d (control-d) to quickly break out of a session.
  • 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:

  • Some Pry commands (e.g show-command) do not work in Ruby 1.8.
  • 1.9 support requires Ripper - some implementations may not support this.

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.
  • Pry.run_command COMMAND enables you to invoke Pry commands outside of a session, e.g Pry.run_command "ls -m", :context => MyObject. See docs for more info.

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 clear the input buffer - useful for getting you out of a situation where the parsing process goes wrong and you get stuck in an endless read loop.
  • status shows status information about the current session.
  • version Show Pry version information
  • help shows the list of session commands with brief explanations.
  • exit or quit or back or ^d (control-d) 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 [OPTIONS] [VAR] returns a list of local variables, instance variables, and methods, etc. Highly flexible. See ls --help for more info.
  • cat VAR Calls inspect on VAR
  • cd VAR Starts a Pry session on the variable VAR. E.g cd @x (use cd .. to go back).
  • show-method [OPTIONS] METH Displays the sourcecode for the method METH. e.g show-method hello. See show-method --help for more info.
  • show-doc [OPTIONS] METH Displays comments for METH. See show-doc --help for more info.
  • show-command COMMAND Displays the sourcecode for the given Pry command. e.g: show-command cd
  • 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.

Bindings and objects

Pry ultimately operates on Binding objects. If you invoke Pry with a Binding object it uses that Binding. If you invoke Pry with anything other than a Binding, Pry will generate a Binding for that object and use that.

If you want to open a Pry session on the current context and capture the locals you should use: binding.pry. If you do not care about capturing the locals you can simply use pry (which will generate a fresh Binding for the receiver).

Top-level is a special case; you can start a Pry session on top-level and capture locals by simply using: pry. This is because Pry automatically uses TOPLEVEL_BINDING for the top-level object (main).

Example Programs

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

  • example_basic.rb - Demonstrate basic Pry functionality
  • 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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