(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:

• Syntax highlighting
• Navigation around state (cd, ls and friends)
• Runtime invocation (use Pry as a developer console or debugger)
• Command shell integration
• Source code browsing (including core C source with the pry-doc gem)
• Documentation browsing
• 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 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.

Navigating around state

Pry allows us to pop in and out of different scopes (objects) using the cd command. 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. 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 ~
=> /home/john
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, cat out the entire file, 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

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.
• 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.
• 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.
• method_source functionality does not work in JRuby.
• 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.
• whereami AROUND shows the code context of the session. Shows AROUND lines either side of the current line.
• version Show Pry version information
• help shows the list of session commands with brief explanations.
• toggle-color turns on and off syntax highlighting.
• simple-prompt toggles the simple prompt mode.
• 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.

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.

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