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README.markdown
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.
- 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:
- Install rubygems-test:
gem install rubygems-test
- Run the test:
gem test pry
- 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; andPry#repl
for the loop (Pry.start
simply wrapsPry.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 aBinding
object as parameter.Pry.start()
is implemented asPry.new.repl()
-
obj.pry
andpry(obj)
may also be used as alternative syntax toPry.start(obj)
.However there are some differences.
obj.pry
opens a Pry session on the receiver whereasPry.start
(with no parameter) will start a Pry session on top-level. The other form of thepry
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 formpry(binding)
orbinding.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 asPry#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 asPry#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.gPry.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 informationhelp
shows the list of session commands with brief explanations.exit
orquit
orback
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. Seels --help
for more info.cat VAR
Callsinspect
onVAR
cd VAR
Starts aPry
session on the variable VAR. E.gcd @x
(usecd ..
to go back).show-method [OPTIONS] METH
Displays the sourcecode for the methodMETH
. e.gshow-method hello
. Seeshow-method --help
for more info.show-doc [OPTIONS] METH
Displays comments forMETH
. Seeshow-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)
orobj.pry
to nest another Pry session within the current one withobj
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 functionalityexample_input.rb
- Demonstrates how to set theinput
object.example_output.rb
- Demonstrates how to set theoutput
object.example_hooks.rb
- Demonstrates how to set thehooks
hash.example_print.rb
- Demonstrates how to set theprint
object.example_prompt.rb
- Demonstrates how to set theprompt
.example_input2.rb
- An advancedinput
example.example_commands.rb
- Implementing a mathematical command set.example_commands_override.rb
- An advancedcommands
example.example_image_edit.rb
- A simple image editor using a Pry REPL (requiresGosu
andTexPlay
gems).
Customizing Pry
Pry allows a large degree of customization.
Read how to customize Pry here.
Contact
Problems or questions contact me at github