(C) John Mair (banisterfiend) 2011

Get to the code

Pry is a powerful alternative to the standard IRB shell for Ruby. It is written from scratch to provide a number of advanced features, some of these include:

• Source code browsing (including core C source with the pry-doc gem)
• Documentation browsing
• Live help system
• Open methods in editors (edit-method Class#method)
• Syntax highlighting
• Command shell integration (start editors, run git, and rake from within Pry)
• Gist integration
• Navigation around state (cd, ls and friends)
• Runtime invocation (use Pry as a developer console or debugger)
• Exotic object support (BasicObject instances, IClasses, ...)
• A Powerful and flexible command system
• Ability to view and replay history
• Many convenience commands inspired by IPython and other advanced REPLs

Pry also aims to be more than an IRB replacement; it is an attempt to bring REPL driven programming to the Ruby language. It is currently not nearly as powerful as tools like SLIME for lisp, but that is the general direction Pry is heading.

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 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.

Try gem install pry-doc for additional documentation on Ruby Core methods. The additional docs are accessed through the show-doc and show-method commands.

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

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.

Commands

Nearly every piece of functionality in a Pry session is implemented as a command. Commands are not methods and must start at the beginning of a line, with no whitespace in between. Commands support a flexible syntax and allow 'options' in the same way as shell commands, for example the following Pry command will show a list of all private instance methods (in scope) that begin with 'pa'

pry(YARD::Parser::SourceParser):5> ls -Mp --grep pa
[:parser_class, :parser_type=, :parser_type_for_filename]

Navigating around state

Pry allows us to pop in and out of different scopes (objects) using the cd command. This enables us to explore the run-time view of a program or library. To view which variables and methods are available within a particular scope we use the versatile ls command.

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

pry(main)> class Hello
pry(main)*   @x = 20
pry(main)* end
=> 20
pry(main)> cd Hello
pry(Hello):1> ls -i
=> [:@x]
pry(Hello):1> cd @x
pry(20:2)> self + 10
=> 30
pry(20:2)> cd ..
pry(Hello):1> cd ..
pry(main)> cd ..

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>

Runtime invocation

Pry can be invoked in the middle of a running program. It opens a Pry session at the point it's called and makes all program state at that point available. It can be invoked on any object using the my_object.pry syntax or on the current binding (or any binding) using binding.pry. The Pry session will then begin within the scope of the object (or binding). When the session ends the program continues with any modifications you made to it.

This functionality can be used for such things as: debugging, implementing developer consoles and applying hot patches.

code:

# test.rb
require 'pry'

class A
  def hello() puts "hello world!" end
end

a = A.new

# start a REPL session
binding.pry

# program resumes here (after pry session)
puts "program resumes here."

Pry session:

pry(main)> a.hello
hello world!
=> nil
pry(main)> def a.goodbye
pry(main)*   puts "goodbye cruel world!"
pry(main)* end
=> nil
pry(main)> a.goodbye
goodbye cruel world!
=> nil
pry(main)> exit

program resumes here.

Command Shell Integration

A line of input that begins with a '.' will be forwarded to the command shell. This enables us to navigate the file system, spawn editors, and run git and rake directly from within Pry.

Further, we can use the shell-mode command to incorporate the present working directory into the Pry prompt and bring in (limited at this stage, sorry) file name completion. We can also interpolate Ruby code directly into the shell by using the normal #{} string interpolation syntax.

In the code below we're going to switch to shell-mode and edit the .pryrc file in the home directory. We'll then cat its contents and reload the file.

pry(main)> shell-mode
pry main:/home/john/ruby/projects/pry $ .cd ~
pry main:/home/john $ .emacsclient .pryrc
pry main:/home/john $ .cat .pryrc
def hello_world
  puts "hello world!"
end
pry main:/home/john $ load ".pryrc"
=> true
pry main:/home/john $ hello_world
hello world!

We can also interpolate Ruby code into the shell. In the example below we use the shell command cat on a random file from the current directory and count the number of lines in that file with wc:

pry main:/home/john $ .cat #{Dir['*.*'].sample} | wc -l
44

Code Browsing

show-method

You can browse method source code with the show-method command. Nearly all Ruby methods (and some C methods, with the pry-doc gem) can have their source viewed. Code that is longer than a page is sent through a pager (such as less), and all code is properly syntax highlighted (even C code).

The show-method command accepts two syntaxes, the typical ri Class#method syntax and also simply the name of a method that's in scope. You can optionally pass the -l option to show-method to include line numbers in the output.

In the following example we will enter the Pry class, list the instance methods beginning with 're' and display the source code for the rep method:

pry(main)> cd Pry
pry(Pry):1> ls -M --grep ^re
[:re, :readline, :rep, :repl, :repl_epilogue, :repl_prologue, :retrieve_line]
pry(Pry):1> show-method rep -l

From: /home/john/ruby/projects/pry/lib/pry/pry_instance.rb @ line 143:
Number of lines: 6

143: def rep(target=TOPLEVEL_BINDING)
144:   target = Pry.binding_for(target)
145:   result = re(target)
146:
147:   show_result(result) if should_print?
148: end

Note that we can also view C methods (from Ruby Core) using the pry-doc gem; we also show off the alternate syntax for show-method:

pry(main)> show-method Array#select

From: array.c in Ruby Core (C Method):
Number of lines: 15

static VALUE
rb_ary_select(VALUE ary)
{
    VALUE result;
    long i;

    RETURN_ENUMERATOR(ary, 0, 0);
    result = rb_ary_new2(RARRAY_LEN(ary));
    for (i = 0; i < RARRAY_LEN(ary); i++) {
    if (RTEST(rb_yield(RARRAY_PTR(ary)[i]))) {
        rb_ary_push(result, rb_ary_elt(ary, i));
    }
    }
    return result;
}

Special locals

Some commands such as show-method, show-doc, show-command, stat and cat update the _file_ and _dir_ local variables after they run. These locals contain the full path to the file involved in the last command as well as the directory containing that file.

You can then use these special locals in conjunction with shell commands to do such things as change directory into the directory containing the file, open the file in an editor, display the file using cat, and so on.

In the following example we wil use Pry to fix a bug in a method:

pry(main)> greet "john"
hello johnhow are you?=> nil
pry(main)> show-method greet

From: /Users/john/ruby/play/bug.rb @ line 2:
Number of lines: 4

def greet(name)
  print "hello #{name}"
  print "how are you?"
end
pry(main)> .emacsclient #{_file_}
pry(main)> load _file_
pry(main)> greet "john"
hello john
how are you?
=> nil
pry(main)> show-method greet

From: /Users/john/ruby/play/bug.rb @ line 2:
Number of lines: 4

def greet(name)
  puts "hello #{name}"
  puts "how are you?"
end

Documentation Browsing

One use-case for Pry is to explore a program at run-time by cd-ing in and out of objects and viewing and invoking methods. In the course of exploring it may be useful to read the documentation for a specific method that you come across. Like show-method the show-doc command supports two syntaxes - the normal ri syntax as well as accepting the name of any method that is currently in scope.

The Pry documentation system does not rely on pre-generated rdoc or ri, instead it grabs the comments directly above the method on demand. This results in speedier documentation retrieval and allows the Pry system to retrieve documentation for methods that would not be picked up by rdoc. Pry also has a basic understanding of both the rdoc and yard formats and will attempt to syntax highlight the documentation appropriately.

Nonetheless The ri functionality is very good and has an advantage over Pry's system in that it allows documentation lookup for classes as well as methods. Pry therefore has good integration with ri through the ri command. The syntax for the command is exactly as it would be in command-line - so it is not necessary to quote strings.

In our example we will enter the Gem class and view the documentation for the try_activate method:

pry(main)> cd Gem
pry(Gem):1> show-doc try_activate

From: /Users/john/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/site_ruby/1.9.1/rubygems.rb @ line 201:
Number of lines: 3

Try to activate a gem containing path. Returns true if
activation succeeded or wasn't needed because it was already
activated. Returns false if it can't find the path in a gem.
pry(Gem):1>

We can also use ri in the normal way:

pry(main) ri Array#each
----------------------------------------------------------- Array#each
     array.each {|item| block }   ->   array
------------------------------------------------------------------------
     Calls _block_ once for each element in _self_, passing that element
     as a parameter.

        a = [ "a", "b", "c" ]
        a.each {|x| print x, " -- " }

     produces:

        a -- b -- c --

History

Readline history can be viewed and replayed using the hist command. When hist is invoked with no arguments it simply displays the history (passing the output through a pager if necessary)) when the --replay option is used a line or a range of lines of history can be replayed.

In the example below we will enter a few lines in a Pry session and then view history; we will then replay one of those lines:

pry(main)> hist
0: hist -h
1: ls
2: ls
3: show-method puts
4: x = rand
5: hist
pry(main)> hist --replay 3

From: io.c in Ruby Core (C Method):
Number of lines: 8

static VALUE
rb_f_puts(int argc, VALUE *argv, VALUE recv)
{
    if (recv == rb_stdout) {
    return rb_io_puts(argc, argv, recv);
    }
    return rb_funcall2(rb_stdout, rb_intern("puts"), argc, argv);
}

In the next example we will replay a range of lines in history. Note that we replay to a point where a class definition is still open and so we can continue to add instance methods to the class:

pry(main)> hist
0: class Hello
1:   def hello_world
2:     puts "hello world!"
3:   end
4: end
5: hist
pry(main)> hist --replay 0..3
pry(main)* def goodbye_world
pry(main)*   puts "goodbye world!"
pry(main)* end
pry(main)* end
=> nil
pry(main)> Hello.new.goodbye_world;
goodbye world!
pry(main)>

Also note that in the above the line Hello.new.goodbye_world; ends with a semi-colon which causes expression evaluation output to be suppressed.

Gist integration

If the gist gem is installed then method source or documentation can be gisted to github with the gist-method command. The gist-method command accepts the same two syntaxes as show-method. In the example below we will gist the C source code for the Symbol#to_proc method to github:

pry(main)> gist-method Symbol#to_proc
https://gist.github.com/5332c38afc46d902ce46
pry(main)>

You can see the actual gist generated here: https://gist.github.com/5332c38afc46d902ce46

Edit methods

You can use edit-method Class#method or edit-method my_method (if the method is in scope) to open a method for editing directly in your favorite editor. Pry has knowledge of a few different editors and will attempt to open the file at the line the method is defined.

You can set the editor to use by assigning to the Pry.editor accessor. Pry.editor will default to $EDITOR or failing that will use nano as the backup default. The file that is edited will be automatically reloaded after exiting the editor - reloading can be suppressed by passing the --no-reload option to edit-method

In the example below we will set our default editor to "emacsclient" and open the Pry#repl method for editing:

pry(main)> Pry.editor = "emacsclient"
pry(main)> edit-method Pry#repl

Live Help System

Many other commands are available in Pry; to see the full list type help at the prompt. A short description of each command is provided with basic instructions for use; some commands have a more extensive help that can be accessed via typing command_name --help. A command will typically say in its description if the --help option is avaiable.

Other Features and limitations

Other Features:

• Pry can be invoked both at the command-line and used as a more powerful alternative to IRB or it can be invoked at runtime and used as a developer consoler / debugger.
• Additional documentation and source code for Ruby Core methods are supported when the pry-doc gem is installed.
• Pry sessions can nest arbitrarily deeply -- to go back one level of nesting type 'exit' or 'quit' or 'back'
• Pry comes with syntax highlighting on by default just use the toggle-color command to turn it on and off.
• Use _ to recover last result.
• Use _pry_ to reference the Pry instance managing the current session.
• Use _ex_ to recover the last exception.
• Use _file_ and _dir_ to refer to the associated file or directory containing the definition for a method.
• A trailing ; on an entered expression suppresses the display of the evaluation output.
• 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.
• 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.
• JRuby not officially supported due to currently too many quirks and strange behaviour. Nonetheless most functionality should still work OK in JRuby. Full JRuby support coming in a future version.
• method_source functionality does not work in JRuby with Ruby 1.8
• Color support does not work in JRuby with Ruby 1.9 (due to a limitation in JRuby's regex).
• Tab completion is currently a bit broken/limited this will have a major overhaul in a future version.

Syntax Highlighting

Syntax highlighting is on by default in Pry. You can toggle it on and off in a session by using the toggle-color command. Alternatively, you can turn it off permanently by putting the line Pry.color = false in your ~/.pryrc file.

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.

Future Directions

Many new features are planned such as:

• Much improved tab completion (using Bond)
• Improved JRuby support
• Support for viewing source-code of binary gems and C stdlib
• git integration
• Much improved documentation system, better support for YARD
• A proper plugin system
• Get rid of . prefix for shell commands in shell-mode
• Better support for code and method reloading
• Extended and more sophisticated command system, allowing piping between commands and running commands in background

Contact

Problems or questions contact me at github

Contributors

The Pry team consists of:

• banisterfiend
• epitron
• injekt
• Mon_Ouie