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 primarily designed for examining and manipulating large and complicated runtime state.

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

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

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 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)> 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 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
• cat <var> Calls inspect on <var>
• cd <var> Starts a Pry session on the variable . E.g cd @x
• 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 -- 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.

Customizing Pry

Pry supports customization of the input, the output, the commands, the hooks, the prompt, and 'print' (the "P" in REPL).

Global customization, which applies to all Pry sessions, is done through invoking class accessors on the Pry class, the accessors are:

• Pry.input=
• Pry.output=
• Pry.commands=
• Pry.hooks=
• Pry.prompt=
• Pry.print=

Local customization (applied to a single Pry session) is done by passing config hash options to Pry.start() or to Pry.new(); also the same accessors as described above for the Pry class exist for a Pry instance so that customization can occur during runtime.

Input:

For input Pry accepts any object that implements the readline method. This includes IO objects, StringIO, Readline and custom objects. Pry initially defaults to using Readline for input.

Example: Setting global input

Setting Pry's global input causes all subsequent Pry instances to use this input by default:

Pry.input = StringIO.new("@x = 10\nexit")
Object.pry

Object.instance_variable_get(:@x) #=> 10

The above will execute the code in the StringIO non-interactively. It gets all the input it needs from the StringIO and then exits the Pry session. Note it is important to end the session with 'exit' if you are running non-interactively or the Pry session will hang as it loops indefinitely awaiting new input.

Example: Setting input for a specific session

The settings for a specific session override the global settings (discussed above). There are two ways to set input for a specific pry session: At the point the session is started, or within the session itself (at runtime):

At session start

Pry.start(Object, :input => StringIO.new("@x = 10\nexit"))
Object.instance_variable_get(:@x) #=> 10

At runtime

If you want to set the input object within the session itself you use the special _pry_ local variable which represents the Pry instance managing the current session; inside the session we type:

_pry_.input = StringIO.new("@x = 10\nexit")

Note we can also set the input object for the parent Pry session (if the current session is nested) like so:

_pry_.parent.input = StringIO.new("@x = 10\nexit")

Output

For output Pry accepts any object that implements the puts method. This includes IO objects, StringIO and custom objects. Pry initially defaults to using $stdout for output.

Example: Setting global output

Setting Pry's global output causes all subsequent Pry instances to use this output by default:

Pry.output = StringIO.new

Example: Setting output for a specific session

As per Input, given above, we set the local output as follows:

At session start

Pry.start(Object, :output => StringIO.new("@x = 10\nexit"))

At runtime

_pry_.output = StringIO.new

Commands

Pry commands are not methods; they are commands that are intercepted and executed before a Ruby eval takes place. Pry comes with a default command set (Pry::Commands), but these commands can be augmented or overriden by user-specified ones.

A valid Pry command object must inherit from Pry::CommandBase and use the special command API:

Example: Defining a command object and setting it globally

class MyCommands < Pry::CommandBase
  command "hello", "Output hello to the user." do |name|
    opts[:output].puts "hello #{name}!"
  end
end

Pry.commands = MyCommands

Then inside a pry session:

pry(main)> hello john
hello john!
=> nil

Example: Using a command object in a specific session

As in the case of input and output:

At session start:

Pry.start(self, :commands => MyCommands)

At runtime:

_pry_.commands = MyCommands

The command API

The command API is defined by the Pry::CommandBase class (hence why all commands must inherit from it). The API works as follows:

The command method defines a new command, its parameter is the name of the command and an optional second parameter is a description of the command.

The associated block defines the action to be performed. The number of parameters in the block determine the number of parameters that will be sent to the command (from the Pry prompt) when it is invoked. Note that all parameters that are received will be strings; if a parameter is not received it will be set to nil.

command "hello" do |x, y, z|
  puts "hello there #{x}, #{y}, and #{z}!"
end

Command aliases can also be defined - simply use an array of strings for the command name - all these strings will be valid names for the command.

command ["ls", "dir"], "show a list of local vars" do
  opts[:output].puts opts[:target].eval("local_variables")
end

action block parameters

Note that in the example above we are using opts[:output] for output; this is the output object in use by the current pry session. Other hash values accesible within an action block include:

• opts[:output] - The session's output object.
• opts[:val] - The line of input that invoked the command.
• opts[:eval_string] - The cumulative lines of input for multi-line input.
• opts[:target] - The object the Pry session is currently on.
• opts[:captures] - The array of regex captures generated by the command (if any).
• opts[:nesting] - Lowlevel session nesting information.
• opts[:command_info] - Lowlevel information about all Pry commands.

(see commands.rb for examples of how some of these options are used)

The help command

The Pry::CommandBase class automatically defines a help command for you. Typing help in a Pry session will show a list of commands to the user followed by their descriptions. Passing a parameter to help with the command name will just return the description of that specific command.

Hooks

Currently Pry supports just two hooks: before_session and after_session. These hooks are invoked before a Pry session starts and after a session ends respectively. The default hooks used are stored in the Pry::DEFAULT_HOOKS and just output the text "Beginning Pry session for <obj>" and "Ending Pry session for <obj>".

Example: Setting global hooks

All subsequent Pry instances will use these hooks as default:

Pry.hooks = {
  :before_session => proc { |out, obj| out.puts "Opened #{obj}" },
  :after_session => proc { |out, obj| out.puts "Closed #{obj}" }
}

5.pry

Inside the session:

Opened 5
pry(5)> exit
Closed 5

Note that the before_session and after_session procs receive the current session's output object and session receiver as parameters.

Example: Setting hooks for a specific session

Like all the other customization options, the global default (as explained above) can be overriden for a specific session, either at session start or during runtime.

At session start

Pry.start(self, :hooks => { :before_session => proc { puts "hello world!" },
                            :after_session => proc { puts "goodbye world!" }
                          })

At runtime

_pry_.hooks = { :before_session => proc { puts "puts "hello world!" } }

Prompts

The Pry prompt is used by Readline and other input objects that accept a prompt. Pry can accept two prompt-types for every prompt; the 'main prompt' and the 'wait prompt'. The main prompt is always used for the first line of input; the wait prompt is used in multi-line input to indicate that the current expression is incomplete and more lines of input are required. The default Prompt used by Pry is stored in the Pry::DEFAULT_PROMPT constant.

A valid Pry prompt is either a single Proc object or a two element array of Proc objects. When an array is used the first element is the 'main prompt' and the last element is the 'wait prompt'. When a single Proc object is used it will be used for both the main prompt and the wait prompt.

Example: Setting global prompt

The prompt Proc objects are passed the receiver of the Pry session and the nesting level of that session as parameters (they can simply ignore these if they do not need them).

# Using one proc for both main and wait prompts
Pry.prompt = proc { |obj, nest_level| "#{obj}:#{nest_level}> " }

# Alternatively, provide two procs; one for main and one for wait
Pry.prompt = [ proc { "ENTER INPUT> " }, proc { "MORE INPUT REQUIRED!* " }]

Example: Setting the prompt for a specific session

At session start

Pry.start(self, :prompt => [proc { "ENTER INPUT> " },
                            proc { "MORE INPUT REQUIRED!* " }])

At runtime

_pry_.prompt = [proc { "ENTER INPUT> " },
                proc { "MORE INPUT REQUIRED!* " }]

Print

The Print phase of Pry's READ-EVAL-PRINT-LOOP can be customized. The default action is stored in the Pry::DEFAULT_PRINT constant and it simply outputs the value of the current expression preceded by a => (or the first line of the backtrace if the value is an Exception object.)

The print object should be a Proc and the parameters passed to the Proc are the output object for the current session and the 'value' returned by the current expression.

Example: Setting global prompt

Let's define a print object that displays the full backtrace of any exception and precedes the output of a value by the text "Output is: ":

Pry.print = proc do |output, value|
              case value
              when Exception
                output.puts value.backtrace
              else
                output.puts "Output is: #{value}"
              end
            end

Example: Setting the prompt for a specific session

At session start

Pry.start(self, :print => proc do |output, value|
                            case value
                            when Exception
                              output.puts value.backtrace
                            else
                              output.puts "Output is: #{value.inspect}"
                            end
                          end)

At runtime

_pry_.print =  proc do |output, value|
                 case value
                 when Exception
                   output.puts value.backtrace
                 else
                   output.puts "Output is: #{value.inspect}"
                 end
               end

Contact

Problems or questions contact me at github