Runtime invocation

banister edited this page Mar 20, 2012 · 27 revisions

Runtime invocation

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Aside from using Pry as an IRB-alternative it also has the ability to function as a developer console and lightweight debugger. To use Pry in this capacity it must be invoked at runtime in the middle of a running program. This is (as you will see) very easy to do and enables all sorts of interesting possibilities.

Invoking on a binding

The standard (and recommended) way to invoke Pry at runtime is to use binding.pry. Starting Pry this way ensures that all local variables and other relevant state (such as default definee and cref) are inherited by the session. It also causes the whereami command to be invoked automatically - and so the surounding context of the session (a few lines either side of the invocation line) are displayed for the user.

Note that we can put binding.pry anywhere in our program at the point we want the session to start, and the self of the session will be the self at that point (of course, we can also invoke Pry on any Binding object, not just the current one returned by the binding method).

Also note that we are not limited to remaining in the scope where the binding.pry call was invoked - using Pry's state navigation abilities we can navigate to any part of the program we wish and examine the state there (see this screencast for more information).

When the session ends (by typing exit or exit-all) the program continues with any modifications you made to it.



# test.rb
require 'pry'

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

a =

# start a REPL session

# 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.

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Invoking on an object

A Pry session does not have to be invoked on a Binding; in fact we can start a Pry session on any Ruby object, including immediate values. The syntax for starting a session on an object is obj.pry.

When we invoke pry on an object a fresh Binding for that session will be generated enabling us to get and set locals. Note that this Binding is just for convenience inside the session and has no effect on the locals available outside the session. If you want your changes to locals to persist outside the session then consider invoking pry on a Binding object instead.



require 'pry'


Pry session:

pry(5)> self
=> 5
pry(5)> self + 10
=> 15

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Customizing your runtime session

As discussed in the Customization and configuration section, Pry can be customized to suit your needs. Some common customizations for runtime invocation include turning off .pryrc loading, and turning off history before invoking binding.pry


Pry.config.should_load_rc = false
Pry.config.history.should_save = false
Pry.config.history.should_load = false


We can also perform deeper customizations, such as using a different command set, or changing the prompt. To perform these customizations we need to invoke Pry slightly differently so that we can pass these parameters in:



command_set = do
  command "greet" do |name|
    output.puts "hello #{name}"

Pry.start binding, :commands => command_set

Pry session:

pry(main)> help

Command List:
install           Install a disabled command.
help              This menu.
greet             No description.
pry(main)> greet John
hello John

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whereami command

When a runtime session is invoked on a Binding the whereami command is automatically executed (as discussed above). However, the command can also be invoked explicitly by typing whereami in the REPL.

You can also pass a single numeric parameter to whereami, this number indicates how much context to show. Passing in the parameter "10" will display 10 lines before and 10 lines after the invocation line, for example.


pry(#<Bacon::Context:0x101136508>)> whereami 5

From: test/test_pry.rb @ line 95 in Bacon::Context#N/A:

    90:       it 'should suppress output if input ends in a ";" and is an Exception object (single line)' do
    91:         o =
    92:         str_output =
    94:         pry_tester = =>";"), :output => str_output)
 => 95:         binding.pry
    96:         pry_tester.rep(o)
    97:         str_output.string.should == ""
    98:       end
   100:       it 'should suppress output if input ends in a ";" (single line)' do

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The pry-backtrace command

Show the backtrace for the position in the code where Pry was started. This can be used to infer the behavior of the program immediately before it entered Pry, just like the backtrace property of an exception.

crow:play john$ ruby gamma.rb 

From: gamma.rb @ line 12 in Object#gamma:

     7: def beta
     8:   gamma("hello")
     9: end
    11: def gamma(x)
 => 12:   binding.pry
    13: end  
    15: alpha

[1] (pry) main: 0> pry-backtrace

gamma.rb:12:in `gamma'
gamma.rb:8:in `beta'
gamma.rb:4:in `alpha'
gamma.rb:15:in `<main>'
[2] (pry) main: 0> 

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Runtime use cases

There are a variety of use cases for runtime invocation of a Pry session, some include:

Developer Console:

Having Pry run in its own thread giving you interactive access to your application while it is running.


Inserting a binding.pry at the point in your program you want to debug, making all state at that point available for interactive inspection (including locals).

REPL-oriented development and hot-patching:

Modifying the program while it is running to provide new features.

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